text: // Copyright (c) 2009-2010 Satoshi Nakamoto
  // Copyright (c) 2009-2020 The Bitcoin Core developers
  // Distributed under the MIT software license, see the accompanying
  // file COPYING or http://www.opensource.org/licenses/mit-license.php.
  
  #include <script/interpreter.h>
  
  #include <crypto/ripemd160.h>
  #include <crypto/sha1.h>
  #include <crypto/sha256.h>
  #include <pubkey.h>
  #include <script/script.h>
  #include <uint256.h>
  
  typedef std::vector<unsigned char> valtype;
  
  namespace {
  
  inline bool set_success(ScriptError* ret)
  {
      if (ret)
          *ret = SCRIPT_ERR_OK;
      return true;
  }
  
  inline bool set_error(ScriptError* ret, const ScriptError serror)
  {
      if (ret)
          *ret = serror;
      return false;
  }
  
  } // namespace
  
  bool CastToBool(const valtype& vch)
  {
      for (unsigned int i = 0; i < vch.size(); i++)
      {
          if (vch[i] != 0)
          {
              // Can be negative zero
              if (i == vch.size()-1 && vch[i] == 0x80)
                  return false;
              return true;
          }
      }
      return false;
  }
  
  /**
   * Script is a stack machine (like Forth) that evaluates a predicate
   * returning a bool indicating valid or not.  There are no loops.
   */
  #define stacktop(i)  (stack.at(stack.size()+(i)))
  #define altstacktop(i)  (altstack.at(altstack.size()+(i)))
  static inline void popstack(std::vector<valtype>& stack)
  {
      if (stack.empty())
          throw std::runtime_error("popstack(): stack empty");
      stack.pop_back();
  }
  
  bool static IsCompressedOrUncompressedPubKey(const valtype &vchPubKey) {
      if (vchPubKey.size() < CPubKey::COMPRESSED_SIZE) {
          //  Non-canonical public key: too short
          return false;
      }
      if (vchPubKey[0] == 0x04) {
          if (vchPubKey.size() != CPubKey::SIZE) {
              //  Non-canonical public key: invalid length for uncompressed key
              return false;
          }
      } else if (vchPubKey[0] == 0x02 || vchPubKey[0] == 0x03) {
          if (vchPubKey.size() != CPubKey::COMPRESSED_SIZE) {
              //  Non-canonical public key: invalid length for compressed key
              return false;
          }
      } else {
          //  Non-canonical public key: neither compressed nor uncompressed
          return false;
      }
      return true;
  }
  
  bool static IsCompressedPubKey(const valtype &vchPubKey) {
      if (vchPubKey.size() != CPubKey::COMPRESSED_SIZE) {
          //  Non-canonical public key: invalid length for compressed key
          return false;
      }
      if (vchPubKey[0] != 0x02 && vchPubKey[0] != 0x03) {
          //  Non-canonical public key: invalid prefix for compressed key
          return false;
      }
      return true;
  }
  
  /**
   * A canonical signature exists of: <30> <total len> <02> <len R> <R> <02> <len S> <S> <hashtype>
   * Where R and S are not negative (their first byte has its highest bit not set), and not
   * excessively padded (do not start with a 0 byte, unless an otherwise negative number follows,
   * in which case a single 0 byte is necessary and even required).
   *
   * See https://bitcointalk.org/index.php?topic=8392.msg127623#msg127623
   *
   * This function is consensus-critical since BIP66.
   */
  bool static IsValidSignatureEncoding(const std::vector<unsigned char> &sig) {
      // Format: 0x30 [total-length] 0x02 [R-length] [R] 0x02 [S-length] [S] [sighash]
      // * total-length: 1-byte length descriptor of everything that follows,
      //   excluding the sighash byte.
      // * R-length: 1-byte length descriptor of the R value that follows.
      // * R: arbitrary-length big-endian encoded R value. It must use the shortest
      //   possible encoding for a positive integer (which means no null bytes at
      //   the start, except a single one when the next byte has its highest bit set).
      // * S-length: 1-byte length descriptor of the S value that follows.
      // * S: arbitrary-length big-endian encoded S value. The same rules apply.
      // * sighash: 1-byte value indicating what data is hashed (not part of the DER
      //   signature)
  
      // Minimum and maximum size constraints.
      if (sig.size() < 9) return false;
      if (sig.size() > 73) return false;
  
      // A signature is of type 0x30 (compound).
      if (sig[0] != 0x30) return false;
  
      // Make sure the length covers the entire signature.
      if (sig[1] != sig.size() - 3) return false;
  
      // Extract the length of the R element.
      unsigned int lenR = sig[3];
  
      // Make sure the length of the S element is still inside the signature.
      if (5 + lenR >= sig.size()) return false;
  
      // Extract the length of the S element.
      unsigned int lenS = sig[5 + lenR];
  
      // Verify that the length of the signature matches the sum of the length
      // of the elements.
      if ((size_t)(lenR + lenS + 7) != sig.size()) return false;
  
      // Check whether the R element is an integer.
      if (sig[2] != 0x02) return false;
  
      // Zero-length integers are not allowed for R.
      if (lenR == 0) return false;
  
      // Negative numbers are not allowed for R.
      if (sig[4] & 0x80) return false;
  
      // Null bytes at the start of R are not allowed, unless R would
      // otherwise be interpreted as a negative number.
      if (lenR > 1 && (sig[4] == 0x00) && !(sig[5] & 0x80)) return false;
  
      // Check whether the S element is an integer.
      if (sig[lenR + 4] != 0x02) return false;
  
      // Zero-length integers are not allowed for S.
      if (lenS == 0) return false;
  
      // Negative numbers are not allowed for S.
      if (sig[lenR + 6] & 0x80) return false;
  
      // Null bytes at the start of S are not allowed, unless S would otherwise be
      // interpreted as a negative number.
      if (lenS > 1 && (sig[lenR + 6] == 0x00) && !(sig[lenR + 7] & 0x80)) return false;
  
      return true;
  }
  
  uint32_t static GetHashType(const valtype &vchSig) {
      if (vchSig.size() == 0)
          return 0;
      // check IsValidSignatureEncoding()'s comment for vchSig format
      return vchSig.back();
  }
  
  bool static IsLowDERSignature(const valtype &vchSig, ScriptError* serror) {
      if (!IsValidSignatureEncoding(vchSig)) {
          return set_error(serror, SCRIPT_ERR_SIG_DER);
      }
      // https://bitcoin.stackexchange.com/a/12556:
      //     Also note that inside transaction signatures, an extra hashtype byte
      //     follows the actual signature data.
      std::vector<unsigned char> vchSigCopy(vchSig.begin(), vchSig.begin() + vchSig.size() - 1);
      // If the S value is above the order of the curve divided by two, its
      // complement modulo the order could have been used instead, which is
      // one byte shorter when encoded correctly.
      if (!CPubKey::CheckLowS(vchSigCopy)) {
          return set_error(serror, SCRIPT_ERR_SIG_HIGH_S);
      }
      return true;
  }
  
  bool static IsDefinedHashtypeSignature(const valtype &vchSig) {
      if (vchSig.size() == 0) {
          return false;
      }
      unsigned char nHashType = GetHashType(vchSig) & (~(SIGHASH_ANYONECANPAY | SIGHASH_FORKID));
      if (nHashType < SIGHASH_ALL || nHashType > SIGHASH_SINGLE)
          return false;
  
      return true;
  }
  
  bool static UsesForkId(uint32_t nHashType) {
      return nHashType & SIGHASH_FORKID;
  }
  
  bool static UsesForkId(const valtype &vchSig) {
      uint32_t nHashType = GetHashType(vchSig);
      return UsesForkId(nHashType);
  }
  
  bool static ForkIdDisabled(unsigned int flags) {
      return flags & SCRIPT_FORKID_DISABLED;
  }
  
  bool CheckSignatureEncoding(const std::vector<unsigned char> &vchSig, unsigned int flags, ScriptError* serror) {
      // Empty signature. Not strictly DER encoded, but allowed to provide a
      // compact way to provide an invalid signature for use with CHECK(MULTI)SIG
      if (vchSig.size() == 0) {
          return true;
      }
      if ((flags & (SCRIPT_VERIFY_DERSIG | SCRIPT_VERIFY_LOW_S | SCRIPT_VERIFY_STRICTENC)) != 0 && !IsValidSignatureEncoding(vchSig)) {
          return set_error(serror, SCRIPT_ERR_SIG_DER);
      } else if ((flags & SCRIPT_VERIFY_LOW_S) != 0 && !IsLowDERSignature(vchSig, serror)) {
          // serror is set
          return false;
      } else if ((flags & SCRIPT_VERIFY_STRICTENC) != 0) {
          if (!IsDefinedHashtypeSignature(vchSig)) {
              return set_error(serror, SCRIPT_ERR_SIG_HASHTYPE);
          }
  
          bool requiresForkId = !ForkIdDisabled(flags);
          bool usesForkId = UsesForkId(vchSig);
          if (requiresForkId && !usesForkId) {
              return set_error(serror, SCRIPT_ERR_SIG_HASHTYPE);
          }
      }
      return true;
  }
  
  bool static CheckPubKeyEncoding(const valtype &vchPubKey, unsigned int flags, const SigVersion &sigversion, ScriptError* serror) {
      if ((flags & SCRIPT_VERIFY_STRICTENC) != 0 && !IsCompressedOrUncompressedPubKey(vchPubKey)) {
          return set_error(serror, SCRIPT_ERR_PUBKEYTYPE);
      }
      // Only compressed keys are accepted in segwit
      if ((flags & SCRIPT_VERIFY_WITNESS_PUBKEYTYPE) != 0 && sigversion == SigVersion::WITNESS_V0 && !IsCompressedPubKey(vchPubKey)) {
          return set_error(serror, SCRIPT_ERR_WITNESS_PUBKEYTYPE);
      }
      return true;
  }
  
  bool static CheckMinimalPush(const valtype& data, opcodetype opcode) {
      // Excludes OP_1NEGATE, OP_1-16 since they are by definition minimal
      assert(0 <= opcode && opcode <= OP_PUSHDATA4);
      if (data.size() == 0) {
          // Should have used OP_0.
          return opcode == OP_0;
      } else if (data.size() == 1 && data[0] >= 1 && data[0] <= 16) {
          // Should have used OP_1 .. OP_16.
          return false;
      } else if (data.size() == 1 && data[0] == 0x81) {
          // Should have used OP_1NEGATE.
          return false;
      } else if (data.size() <= 75) {
          // Must have used a direct push (opcode indicating number of bytes pushed + those bytes).
          return opcode == data.size();
      } else if (data.size() <= 255) {
          // Must have used OP_PUSHDATA.
          return opcode == OP_PUSHDATA1;
      } else if (data.size() <= 65535) {
          // Must have used OP_PUSHDATA2.
          return opcode == OP_PUSHDATA2;
      }
      return true;
  }
  
  int FindAndDelete(CScript& script, const CScript& b)
  {
      int nFound = 0;
      if (b.empty())
          return nFound;
      CScript result;
      CScript::const_iterator pc = script.begin(), pc2 = script.begin(), end = script.end();
      opcodetype opcode;
      do
      {
          result.insert(result.end(), pc2, pc);
          while (static_cast<size_t>(end - pc) >= b.size() && std::equal(b.begin(), b.end(), pc))
          {
              pc = pc + b.size();
              ++nFound;
          }
          pc2 = pc;
      }
      while (script.GetOp(pc, opcode));
  
      if (nFound > 0) {
          result.insert(result.end(), pc2, end);
          script = std::move(result);
      }
  
      return nFound;
  }
  
  namespace {
  /** A data type to abstract out the condition stack during script execution.
   *
   * Conceptually it acts like a vector of booleans, one for each level of nested
   * IF/THEN/ELSE, indicating whether we're in the active or inactive branch of
   * each.
   *
   * The elements on the stack cannot be observed individually; we only need to
   * expose whether the stack is empty and whether or not any false values are
   * present at all. To implement OP_ELSE, a toggle_top modifier is added, which
   * flips the last value without returning it.
   *
   * This uses an optimized implementation that does not materialize the
   * actual stack. Instead, it just stores the size of the would-be stack,
   * and the position of the first false value in it.
   */
  class ConditionStack {
  private:
      //! A constant for m_first_false_pos to indicate there are no falses.
      static constexpr uint32_t NO_FALSE = std::numeric_limits<uint32_t>::max();
  
      //! The size of the implied stack.
      uint32_t m_stack_size = 0;
      //! The position of the first false value on the implied stack, or NO_FALSE if all true.
      uint32_t m_first_false_pos = NO_FALSE;
  
  public:
      bool empty() { return m_stack_size == 0; }
      bool all_true() { return m_first_false_pos == NO_FALSE; }
      void push_back(bool f)
      {
          if (m_first_false_pos == NO_FALSE && !f) {
              // The stack consists of all true values, and a false is added.
              // The first false value will appear at the current size.
              m_first_false_pos = m_stack_size;
          }
          ++m_stack_size;
      }
      void pop_back()
      {
          assert(m_stack_size > 0);
          --m_stack_size;
          if (m_first_false_pos == m_stack_size) {
              // When popping off the first false value, everything becomes true.
              m_first_false_pos = NO_FALSE;
          }
      }
      void toggle_top()
      {
          assert(m_stack_size > 0);
          if (m_first_false_pos == NO_FALSE) {
              // The current stack is all true values; the first false will be the top.
              m_first_false_pos = m_stack_size - 1;
          } else if (m_first_false_pos == m_stack_size - 1) {
              // The top is the first false value; toggling it will make everything true.
              m_first_false_pos = NO_FALSE;
          } else {
              // There is a false value, but not on top. No action is needed as toggling
              // anything but the first false value is unobservable.
          }
      }
  };
  }
  
  static bool EvalChecksigPreTapscript(const valtype& vchSig, const valtype& vchPubKey, CScript::const_iterator pbegincodehash, CScript::const_iterator pend, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptError* serror, bool& fSuccess)
  {
      assert(sigversion == SigVersion::BASE || sigversion == SigVersion::WITNESS_V0);
  
      // Subset of script starting at the most recent codeseparator
      CScript scriptCode(pbegincodehash, pend);
  
      // Drop the signature in pre-segwit scripts but not segwit scripts
      if (sigversion == SigVersion::BASE) {
          int found = FindAndDelete(scriptCode, CScript() << vchSig);
          if (found > 0 && (flags & SCRIPT_VERIFY_CONST_SCRIPTCODE))
              return set_error(serror, SCRIPT_ERR_SIG_FINDANDDELETE);
      }
  
      if (!CheckSignatureEncoding(vchSig, flags, serror) || !CheckPubKeyEncoding(vchPubKey, flags, sigversion, serror)) {
          //serror is set
          return false;
      }
  	
      bool no_forkid = ForkIdDisabled(flags);
      fSuccess = checker.CheckECDSASignature(vchSig, vchPubKey, scriptCode, sigversion, no_forkid);
  
      if (!fSuccess && (flags & SCRIPT_VERIFY_NULLFAIL) && vchSig.size())
          return set_error(serror, SCRIPT_ERR_SIG_NULLFAIL);
  
      return true;
  }
  
  static bool EvalChecksigTapscript(const valtype& sig, const valtype& pubkey, ScriptExecutionData& execdata, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptError* serror, bool& success)
  {
      assert(sigversion == SigVersion::TAPSCRIPT);
  
      /*
       *  The following validation sequence is consensus critical. Please note how --
       *    upgradable public key versions precede other rules;
       *    the script execution fails when using empty signature with invalid public key;
       *    the script execution fails when using non-empty invalid signature.
       */
      success = !sig.empty();
      if (success) {
          // Implement the sigops/witnesssize ratio test.
          // Passing with an upgradable public key version is also counted.
          assert(execdata.m_validation_weight_left_init);
          execdata.m_validation_weight_left -= VALIDATION_WEIGHT_PER_SIGOP_PASSED;
          if (execdata.m_validation_weight_left < 0) {
              return set_error(serror, SCRIPT_ERR_TAPSCRIPT_VALIDATION_WEIGHT);
          }
      }
      if (pubkey.size() == 0) {
          return set_error(serror, SCRIPT_ERR_PUBKEYTYPE);
      } else if (pubkey.size() == 32) {
          if (success && !checker.CheckSchnorrSignature(sig, pubkey, sigversion, execdata, serror)) {
              return false; // serror is set
          }
      } else {
          /*
           *  New public key version softforks should be defined before this `else` block.
           *  Generally, the new code should not do anything but failing the script execution. To avoid
           *  consensus bugs, it should not modify any existing values (including `success`).
           */
          if ((flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_PUBKEYTYPE) != 0) {
              return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_PUBKEYTYPE);
          }
      }
  
      return true;
  }
  
  /** Helper for OP_CHECKSIG, OP_CHECKSIGVERIFY, and (in Tapscript) OP_CHECKSIGADD.
   *
   * A return value of false means the script fails entirely. When true is returned, the
   * success variable indicates whether the signature check itself succeeded.
   */
  static bool EvalChecksig(const valtype& sig, const valtype& pubkey, CScript::const_iterator pbegincodehash, CScript::const_iterator pend, ScriptExecutionData& execdata, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptError* serror, bool& success)
  {
      switch (sigversion) {
      case SigVersion::BASE:
      case SigVersion::WITNESS_V0:
          return EvalChecksigPreTapscript(sig, pubkey, pbegincodehash, pend, flags, checker, sigversion, serror, success);
      case SigVersion::TAPSCRIPT:
          return EvalChecksigTapscript(sig, pubkey, execdata, flags, checker, sigversion, serror, success);
      case SigVersion::TAPROOT:
          // Key path spending in Taproot has no script, so this is unreachable.
          break;
      }
      assert(false);
  }
  
  bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptExecutionData& execdata, ScriptError* serror)
  {
      static const CScriptNum bnZero(0);
      static const CScriptNum bnOne(1);
      // static const CScriptNum bnFalse(0);
      // static const CScriptNum bnTrue(1);
      static const valtype vchFalse(0);
      // static const valtype vchZero(0);
      static const valtype vchTrue(1, 1);
  
      // sigversion cannot be TAPROOT here, as it admits no script execution.
      assert(sigversion == SigVersion::BASE || sigversion == SigVersion::WITNESS_V0 || sigversion == SigVersion::TAPSCRIPT);
  
      CScript::const_iterator pc = script.begin();
      CScript::const_iterator pend = script.end();
      CScript::const_iterator pbegincodehash = script.begin();
      opcodetype opcode;
      valtype vchPushValue;
      ConditionStack vfExec;
      std::vector<valtype> altstack;
      set_error(serror, SCRIPT_ERR_UNKNOWN_ERROR);
      if ((sigversion == SigVersion::BASE || sigversion == SigVersion::WITNESS_V0) && script.size() > MAX_SCRIPT_SIZE) {
          return set_error(serror, SCRIPT_ERR_SCRIPT_SIZE);
      }
      int nOpCount = 0;
      bool fRequireMinimal = (flags & SCRIPT_VERIFY_MINIMALDATA) != 0;
      uint32_t opcode_pos = 0;
      execdata.m_codeseparator_pos = 0xFFFFFFFFUL;
      execdata.m_codeseparator_pos_init = true;
  
      try
      {
          for (; pc < pend; ++opcode_pos) {
              bool fExec = vfExec.all_true();
  
              //
              // Read instruction
              //
              if (!script.GetOp(pc, opcode, vchPushValue))
                  return set_error(serror, SCRIPT_ERR_BAD_OPCODE);
              if (vchPushValue.size() > MAX_SCRIPT_ELEMENT_SIZE)
                  return set_error(serror, SCRIPT_ERR_PUSH_SIZE);
  
              if (sigversion == SigVersion::BASE || sigversion == SigVersion::WITNESS_V0) {
                  // Note how OP_RESERVED does not count towards the opcode limit.
                  if (opcode > OP_16 && ++nOpCount > MAX_OPS_PER_SCRIPT) {
                      return set_error(serror, SCRIPT_ERR_OP_COUNT);
                  }
              }
  
              if (opcode == OP_CAT ||
                  opcode == OP_SUBSTR ||
                  opcode == OP_LEFT ||
                  opcode == OP_RIGHT ||
                  opcode == OP_INVERT ||
                  opcode == OP_AND ||
                  opcode == OP_OR ||
                  opcode == OP_XOR ||
                  opcode == OP_2MUL ||
                  opcode == OP_2DIV ||
                  opcode == OP_MUL ||
                  opcode == OP_DIV ||
                  opcode == OP_MOD ||
                  opcode == OP_LSHIFT ||
                  opcode == OP_RSHIFT)
                  return set_error(serror, SCRIPT_ERR_DISABLED_OPCODE); // Disabled opcodes (CVE-2010-5137).
  
              // With SCRIPT_VERIFY_CONST_SCRIPTCODE, OP_CODESEPARATOR in non-segwit script is rejected even in an unexecuted branch
              if (opcode == OP_CODESEPARATOR && sigversion == SigVersion::BASE && (flags & SCRIPT_VERIFY_CONST_SCRIPTCODE))
                  return set_error(serror, SCRIPT_ERR_OP_CODESEPARATOR);
  
              if (fExec && 0 <= opcode && opcode <= OP_PUSHDATA4) {
                  if (fRequireMinimal && !CheckMinimalPush(vchPushValue, opcode)) {
                      return set_error(serror, SCRIPT_ERR_MINIMALDATA);
                  }
                  stack.push_back(vchPushValue);
              } else if (fExec || (OP_IF <= opcode && opcode <= OP_ENDIF))
              switch (opcode)
              {
                  //
                  // Push value
                  //
                  case OP_1NEGATE:
                  case OP_1:
                  case OP_2:
                  case OP_3:
                  case OP_4:
                  case OP_5:
                  case OP_6:
                  case OP_7:
                  case OP_8:
                  case OP_9:
                  case OP_10:
                  case OP_11:
                  case OP_12:
                  case OP_13:
                  case OP_14:
                  case OP_15:
                  case OP_16:
                  {
                      // ( -- value)
                      CScriptNum bn((int)opcode - (int)(OP_1 - 1));
                      stack.push_back(bn.getvch());
                      // The result of these opcodes should always be the minimal way to push the data
                      // they push, so no need for a CheckMinimalPush here.
                  }
                  break;
  
  
                  //
                  // Control
                  //
                  case OP_NOP:
                      break;
  
                  case OP_CHECKLOCKTIMEVERIFY:
                  {
                      if (!(flags & SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY)) {
                          // not enabled; treat as a NOP2
                          break;
                      }
  
                      if (stack.size() < 1)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
  
                      // Note that elsewhere numeric opcodes are limited to
                      // operands in the range -2**31+1 to 2**31-1, however it is
                      // legal for opcodes to produce results exceeding that
                      // range. This limitation is implemented by CScriptNum's
                      // default 4-byte limit.
                      //
                      // If we kept to that limit we'd have a year 2038 problem,
                      // even though the nLockTime field in transactions
                      // themselves is uint32 which only becomes meaningless
                      // after the year 2106.
                      //
                      // Thus as a special case we tell CScriptNum to accept up
                      // to 5-byte bignums, which are good until 2**39-1, well
                      // beyond the 2**32-1 limit of the nLockTime field itself.
                      const CScriptNum nLockTime(stacktop(-1), fRequireMinimal, 5);
  
                      // In the rare event that the argument may be < 0 due to
                      // some arithmetic being done first, you can always use
                      // 0 MAX CHECKLOCKTIMEVERIFY.
                      if (nLockTime < 0)
                          return set_error(serror, SCRIPT_ERR_NEGATIVE_LOCKTIME);
  
                      // Actually compare the specified lock time with the transaction.
                      if (!checker.CheckLockTime(nLockTime))
                          return set_error(serror, SCRIPT_ERR_UNSATISFIED_LOCKTIME);
  
                      break;
                  }
  
                  case OP_CHECKSEQUENCEVERIFY:
                  {
                      if (!(flags & SCRIPT_VERIFY_CHECKSEQUENCEVERIFY)) {
                          // not enabled; treat as a NOP3
                          break;
                      }
  
                      if (stack.size() < 1)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
  
                      // nSequence, like nLockTime, is a 32-bit unsigned integer
                      // field. See the comment in CHECKLOCKTIMEVERIFY regarding
                      // 5-byte numeric operands.
                      const CScriptNum nSequence(stacktop(-1), fRequireMinimal, 5);
  
                      // In the rare event that the argument may be < 0 due to
                      // some arithmetic being done first, you can always use
                      // 0 MAX CHECKSEQUENCEVERIFY.
                      if (nSequence < 0)
                          return set_error(serror, SCRIPT_ERR_NEGATIVE_LOCKTIME);
  
                      // To provide for future soft-fork extensibility, if the
                      // operand has the disabled lock-time flag set,
                      // CHECKSEQUENCEVERIFY behaves as a NOP.
                      if ((nSequence & CTxIn::SEQUENCE_LOCKTIME_DISABLE_FLAG) != 0)
                          break;
  
                      // Compare the specified sequence number with the input.
                      if (!checker.CheckSequence(nSequence))
                          return set_error(serror, SCRIPT_ERR_UNSATISFIED_LOCKTIME);
  
                      break;
                  }
  
                  case OP_NOP1: case OP_NOP4: case OP_NOP5:
                  case OP_NOP6: case OP_NOP7: case OP_NOP8: case OP_NOP9: case OP_NOP10:
                  {
                      if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS)
                          return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_NOPS);
                  }
                  break;
  
                  case OP_IF:
                  case OP_NOTIF:
                  {
                      // <expression> if [statements] [else [statements]] endif
                      bool fValue = false;
                      if (fExec)
                      {
                          if (stack.size() < 1)
                              return set_error(serror, SCRIPT_ERR_UNBALANCED_CONDITIONAL);
                          valtype& vch = stacktop(-1);
                          // Tapscript requires minimal IF/NOTIF inputs as a consensus rule.
                          if (sigversion == SigVersion::TAPSCRIPT) {
                              // The input argument to the OP_IF and OP_NOTIF opcodes must be either
                              // exactly 0 (the empty vector) or exactly 1 (the one-byte vector with value 1).
                              if (vch.size() > 1 || (vch.size() == 1 && vch[0] != 1)) {
                                  return set_error(serror, SCRIPT_ERR_TAPSCRIPT_MINIMALIF);
                              }
                          }
                          // Under witness v0 rules it is only a policy rule, enabled through SCRIPT_VERIFY_MINIMALIF.
                          if (sigversion == SigVersion::WITNESS_V0 && (flags & SCRIPT_VERIFY_MINIMALIF)) {
                              if (vch.size() > 1)
                                  return set_error(serror, SCRIPT_ERR_MINIMALIF);
                              if (vch.size() == 1 && vch[0] != 1)
                                  return set_error(serror, SCRIPT_ERR_MINIMALIF);
                          }
                          fValue = CastToBool(vch);
                          if (opcode == OP_NOTIF)
                              fValue = !fValue;
                          popstack(stack);
                      }
                      vfExec.push_back(fValue);
                  }
                  break;
  
                  case OP_ELSE:
                  {
                      if (vfExec.empty())
                          return set_error(serror, SCRIPT_ERR_UNBALANCED_CONDITIONAL);
                      vfExec.toggle_top();
                  }
                  break;
  
                  case OP_ENDIF:
                  {
                      if (vfExec.empty())
                          return set_error(serror, SCRIPT_ERR_UNBALANCED_CONDITIONAL);
                      vfExec.pop_back();
                  }
                  break;
  
                  case OP_VERIFY:
                  {
                      // (true -- ) or
                      // (false -- false) and return
                      if (stack.size() < 1)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      bool fValue = CastToBool(stacktop(-1));
                      if (fValue)
                          popstack(stack);
                      else
                          return set_error(serror, SCRIPT_ERR_VERIFY);
                  }
                  break;
  
                  case OP_RETURN:
                  {
                      return set_error(serror, SCRIPT_ERR_OP_RETURN);
                  }
                  break;
  
  
                  //
                  // Stack ops
                  //
                  case OP_TOALTSTACK:
                  {
                      if (stack.size() < 1)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      altstack.push_back(stacktop(-1));
                      popstack(stack);
                  }
                  break;
  
                  case OP_FROMALTSTACK:
                  {
                      if (altstack.size() < 1)
                          return set_error(serror, SCRIPT_ERR_INVALID_ALTSTACK_OPERATION);
                      stack.push_back(altstacktop(-1));
                      popstack(altstack);
                  }
                  break;
  
                  case OP_2DROP:
                  {
                      // (x1 x2 -- )
                      if (stack.size() < 2)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      popstack(stack);
                      popstack(stack);
                  }
                  break;
  
                  case OP_2DUP:
                  {
                      // (x1 x2 -- x1 x2 x1 x2)
                      if (stack.size() < 2)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      valtype vch1 = stacktop(-2);
                      valtype vch2 = stacktop(-1);
                      stack.push_back(vch1);
                      stack.push_back(vch2);
                  }
                  break;
  
                  case OP_3DUP:
                  {
                      // (x1 x2 x3 -- x1 x2 x3 x1 x2 x3)
                      if (stack.size() < 3)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      valtype vch1 = stacktop(-3);
                      valtype vch2 = stacktop(-2);
                      valtype vch3 = stacktop(-1);
                      stack.push_back(vch1);
                      stack.push_back(vch2);
                      stack.push_back(vch3);
                  }
                  break;
  
                  case OP_2OVER:
                  {
                      // (x1 x2 x3 x4 -- x1 x2 x3 x4 x1 x2)
                      if (stack.size() < 4)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      valtype vch1 = stacktop(-4);
                      valtype vch2 = stacktop(-3);
                      stack.push_back(vch1);
                      stack.push_back(vch2);
                  }
                  break;
  
                  case OP_2ROT:
                  {
                      // (x1 x2 x3 x4 x5 x6 -- x3 x4 x5 x6 x1 x2)
                      if (stack.size() < 6)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      valtype vch1 = stacktop(-6);
                      valtype vch2 = stacktop(-5);
                      stack.erase(stack.end()-6, stack.end()-4);
                      stack.push_back(vch1);
                      stack.push_back(vch2);
                  }
                  break;
  
                  case OP_2SWAP:
                  {
                      // (x1 x2 x3 x4 -- x3 x4 x1 x2)
                      if (stack.size() < 4)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      swap(stacktop(-4), stacktop(-2));
                      swap(stacktop(-3), stacktop(-1));
                  }
                  break;
  
                  case OP_IFDUP:
                  {
                      // (x - 0 | x x)
                      if (stack.size() < 1)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      valtype vch = stacktop(-1);
                      if (CastToBool(vch))
                          stack.push_back(vch);
                  }
                  break;
  
                  case OP_DEPTH:
                  {
                      // -- stacksize
                      CScriptNum bn(stack.size());
                      stack.push_back(bn.getvch());
                  }
                  break;
  
                  case OP_DROP:
                  {
                      // (x -- )
                      if (stack.size() < 1)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      popstack(stack);
                  }
                  break;
  
                  case OP_DUP:
                  {
                      // (x -- x x)
                      if (stack.size() < 1)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      valtype vch = stacktop(-1);
                      stack.push_back(vch);
                  }
                  break;
  
                  case OP_NIP:
                  {
                      // (x1 x2 -- x2)
                      if (stack.size() < 2)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      stack.erase(stack.end() - 2);
                  }
                  break;
  
                  case OP_OVER:
                  {
                      // (x1 x2 -- x1 x2 x1)
                      if (stack.size() < 2)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      valtype vch = stacktop(-2);
                      stack.push_back(vch);
                  }
                  break;
  
                  case OP_PICK:
                  case OP_ROLL:
                  {
                      // (xn ... x2 x1 x0 n - xn ... x2 x1 x0 xn)
                      // (xn ... x2 x1 x0 n - ... x2 x1 x0 xn)
                      if (stack.size() < 2)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      int n = CScriptNum(stacktop(-1), fRequireMinimal).getint();
                      popstack(stack);
                      if (n < 0 || n >= (int)stack.size())
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      valtype vch = stacktop(-n-1);
                      if (opcode == OP_ROLL)
                          stack.erase(stack.end()-n-1);
                      stack.push_back(vch);
                  }
                  break;
  
                  case OP_ROT:
                  {
                      // (x1 x2 x3 -- x2 x3 x1)
                      //  x2 x1 x3  after first swap
                      //  x2 x3 x1  after second swap
                      if (stack.size() < 3)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      swap(stacktop(-3), stacktop(-2));
                      swap(stacktop(-2), stacktop(-1));
                  }
                  break;
  
                  case OP_SWAP:
                  {
                      // (x1 x2 -- x2 x1)
                      if (stack.size() < 2)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      swap(stacktop(-2), stacktop(-1));
                  }
                  break;
  
                  case OP_TUCK:
                  {
                      // (x1 x2 -- x2 x1 x2)
                      if (stack.size() < 2)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      valtype vch = stacktop(-1);
                      stack.insert(stack.end()-2, vch);
                  }
                  break;
  
  
                  case OP_SIZE:
                  {
                      // (in -- in size)
                      if (stack.size() < 1)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      CScriptNum bn(stacktop(-1).size());
                      stack.push_back(bn.getvch());
                  }
                  break;
  
  
                  //
                  // Bitwise logic
                  //
                  case OP_EQUAL:
                  case OP_EQUALVERIFY:
                  //case OP_NOTEQUAL: // use OP_NUMNOTEQUAL
                  {
                      // (x1 x2 - bool)
                      if (stack.size() < 2)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      valtype& vch1 = stacktop(-2);
                      valtype& vch2 = stacktop(-1);
                      bool fEqual = (vch1 == vch2);
                      // OP_NOTEQUAL is disabled because it would be too easy to say
                      // something like n != 1 and have some wiseguy pass in 1 with extra
                      // zero bytes after it (numerically, 0x01 == 0x0001 == 0x000001)
                      //if (opcode == OP_NOTEQUAL)
                      //    fEqual = !fEqual;
                      popstack(stack);
                      popstack(stack);
                      stack.push_back(fEqual ? vchTrue : vchFalse);
                      if (opcode == OP_EQUALVERIFY)
                      {
                          if (fEqual)
                              popstack(stack);
                          else
                              return set_error(serror, SCRIPT_ERR_EQUALVERIFY);
                      }
                  }
                  break;
  
  
                  //
                  // Numeric
                  //
                  case OP_1ADD:
                  case OP_1SUB:
                  case OP_NEGATE:
                  case OP_ABS:
                  case OP_NOT:
                  case OP_0NOTEQUAL:
                  {
                      // (in -- out)
                      if (stack.size() < 1)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      CScriptNum bn(stacktop(-1), fRequireMinimal);
                      switch (opcode)
                      {
                      case OP_1ADD:       bn += bnOne; break;
                      case OP_1SUB:       bn -= bnOne; break;
                      case OP_NEGATE:     bn = -bn; break;
                      case OP_ABS:        if (bn < bnZero) bn = -bn; break;
                      case OP_NOT:        bn = (bn == bnZero); break;
                      case OP_0NOTEQUAL:  bn = (bn != bnZero); break;
                      default:            assert(!"invalid opcode"); break;
                      }
                      popstack(stack);
                      stack.push_back(bn.getvch());
                  }
                  break;
  
                  case OP_ADD:
                  case OP_SUB:
                  case OP_BOOLAND:
                  case OP_BOOLOR:
                  case OP_NUMEQUAL:
                  case OP_NUMEQUALVERIFY:
                  case OP_NUMNOTEQUAL:
                  case OP_LESSTHAN:
                  case OP_GREATERTHAN:
                  case OP_LESSTHANOREQUAL:
                  case OP_GREATERTHANOREQUAL:
                  case OP_MIN:
                  case OP_MAX:
                  {
                      // (x1 x2 -- out)
                      if (stack.size() < 2)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      CScriptNum bn1(stacktop(-2), fRequireMinimal);
                      CScriptNum bn2(stacktop(-1), fRequireMinimal);
                      CScriptNum bn(0);
                      switch (opcode)
                      {
                      case OP_ADD:
                          bn = bn1 + bn2;
                          break;
  
                      case OP_SUB:
                          bn = bn1 - bn2;
                          break;
  
                      case OP_BOOLAND:             bn = (bn1 != bnZero && bn2 != bnZero); break;
                      case OP_BOOLOR:              bn = (bn1 != bnZero || bn2 != bnZero); break;
                      case OP_NUMEQUAL:            bn = (bn1 == bn2); break;
                      case OP_NUMEQUALVERIFY:      bn = (bn1 == bn2); break;
                      case OP_NUMNOTEQUAL:         bn = (bn1 != bn2); break;
                      case OP_LESSTHAN:            bn = (bn1 < bn2); break;
                      case OP_GREATERTHAN:         bn = (bn1 > bn2); break;
                      case OP_LESSTHANOREQUAL:     bn = (bn1 <= bn2); break;
                      case OP_GREATERTHANOREQUAL:  bn = (bn1 >= bn2); break;
                      case OP_MIN:                 bn = (bn1 < bn2 ? bn1 : bn2); break;
                      case OP_MAX:                 bn = (bn1 > bn2 ? bn1 : bn2); break;
                      default:                     assert(!"invalid opcode"); break;
                      }
                      popstack(stack);
                      popstack(stack);
                      stack.push_back(bn.getvch());
  
                      if (opcode == OP_NUMEQUALVERIFY)
                      {
                          if (CastToBool(stacktop(-1)))
                              popstack(stack);
                          else
                              return set_error(serror, SCRIPT_ERR_NUMEQUALVERIFY);
                      }
                  }
                  break;
  
                  case OP_WITHIN:
                  {
                      // (x min max -- out)
                      if (stack.size() < 3)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      CScriptNum bn1(stacktop(-3), fRequireMinimal);
                      CScriptNum bn2(stacktop(-2), fRequireMinimal);
                      CScriptNum bn3(stacktop(-1), fRequireMinimal);
                      bool fValue = (bn2 <= bn1 && bn1 < bn3);
                      popstack(stack);
                      popstack(stack);
                      popstack(stack);
                      stack.push_back(fValue ? vchTrue : vchFalse);
                  }
                  break;
  
  
                  //
                  // Crypto
                  //
                  case OP_RIPEMD160:
                  case OP_SHA1:
                  case OP_SHA256:
                  case OP_HASH160:
                  case OP_HASH256:
                  {
                      // (in -- hash)
                      if (stack.size() < 1)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      valtype& vch = stacktop(-1);
                      valtype vchHash((opcode == OP_RIPEMD160 || opcode == OP_SHA1 || opcode == OP_HASH160) ? 20 : 32);
                      if (opcode == OP_RIPEMD160)
                          CRIPEMD160().Write(vch.data(), vch.size()).Finalize(vchHash.data());
                      else if (opcode == OP_SHA1)
                          CSHA1().Write(vch.data(), vch.size()).Finalize(vchHash.data());
                      else if (opcode == OP_SHA256)
                          CSHA256().Write(vch.data(), vch.size()).Finalize(vchHash.data());
                      else if (opcode == OP_HASH160)
                          CHash160().Write(vch).Finalize(vchHash);
                      else if (opcode == OP_HASH256)
                          CHash256().Write(vch).Finalize(vchHash);
                      popstack(stack);
                      stack.push_back(vchHash);
                  }
                  break;
  
                  case OP_CODESEPARATOR:
                  {
                      // If SCRIPT_VERIFY_CONST_SCRIPTCODE flag is set, use of OP_CODESEPARATOR is rejected in pre-segwit
                      // script, even in an unexecuted branch (this is checked above the opcode case statement).
  
                      // Hash starts after the code separator
                      pbegincodehash = pc;
                      execdata.m_codeseparator_pos = opcode_pos;
                  }
                  break;
  
                  case OP_CHECKSIG:
                  case OP_CHECKSIGVERIFY:
                  {
                      // (sig pubkey -- bool)
                      if (stack.size() < 2)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
  
                      valtype& vchSig    = stacktop(-2);
                      valtype& vchPubKey = stacktop(-1);
  
                      bool fSuccess = true;
                   
                      if (!EvalChecksig(vchSig, vchPubKey, pbegincodehash, pend, execdata, flags, checker, sigversion, serror, fSuccess)) return false;
                      popstack(stack);
                      popstack(stack);
                      stack.push_back(fSuccess ? vchTrue : vchFalse);
                      if (opcode == OP_CHECKSIGVERIFY)
                      {
                          if (fSuccess)
                              popstack(stack);
                          else
                              return set_error(serror, SCRIPT_ERR_CHECKSIGVERIFY);
                      }
                  }
                  break;
  
                  case OP_CHECKSIGADD:
                  {
                      // OP_CHECKSIGADD is only available in Tapscript
                      if (sigversion == SigVersion::BASE || sigversion == SigVersion::WITNESS_V0) return set_error(serror, SCRIPT_ERR_BAD_OPCODE);
  
                      // (sig num pubkey -- num)
                      if (stack.size() < 3) return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
  
                      const valtype& sig = stacktop(-3);
                      const CScriptNum num(stacktop(-2), fRequireMinimal);
                      const valtype& pubkey = stacktop(-1);
  
                      bool success = true;
                      if (!EvalChecksig(sig, pubkey, pbegincodehash, pend, execdata, flags, checker, sigversion, serror, success)) return false;
                      popstack(stack);
                      popstack(stack);
                      popstack(stack);
                      stack.push_back((num + (success ? 1 : 0)).getvch());
                  }
                  break;
  
                  case OP_CHECKMULTISIG:
                  case OP_CHECKMULTISIGVERIFY:
                  {
                      if (sigversion == SigVersion::TAPSCRIPT) return set_error(serror, SCRIPT_ERR_TAPSCRIPT_CHECKMULTISIG);
  
                      // ([sig ...] num_of_signatures [pubkey ...] num_of_pubkeys -- bool)
  
                      int i = 1;
                      if ((int)stack.size() < i)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
  
                      int nKeysCount = CScriptNum(stacktop(-i), fRequireMinimal).getint();
                      if (nKeysCount < 0 || nKeysCount > MAX_PUBKEYS_PER_MULTISIG)
                          return set_error(serror, SCRIPT_ERR_PUBKEY_COUNT);
                      nOpCount += nKeysCount;
                      if (nOpCount > MAX_OPS_PER_SCRIPT)
                          return set_error(serror, SCRIPT_ERR_OP_COUNT);
                      int ikey = ++i;
                      // ikey2 is the position of last non-signature item in the stack. Top stack item = 1.
                      // With SCRIPT_VERIFY_NULLFAIL, this is used for cleanup if operation fails.
                      int ikey2 = nKeysCount + 2;
                      i += nKeysCount;
                      if ((int)stack.size() < i)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
  
                      int nSigsCount = CScriptNum(stacktop(-i), fRequireMinimal).getint();
                      if (nSigsCount < 0 || nSigsCount > nKeysCount)
                          return set_error(serror, SCRIPT_ERR_SIG_COUNT);
                      int isig = ++i;
                      i += nSigsCount;
                      if ((int)stack.size() < i)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
  
                      // Subset of script starting at the most recent codeseparator
                      CScript scriptCode(pbegincodehash, pend);
  
                      // Drop the signature in pre-segwit scripts but not segwit scripts
                      for (int k = 0; k < nSigsCount; k++)
                      {
                          valtype& vchSig = stacktop(-isig-k);
                          if (sigversion == SigVersion::BASE) {
                              int found = FindAndDelete(scriptCode, CScript() << vchSig);
                              if (found > 0 && (flags & SCRIPT_VERIFY_CONST_SCRIPTCODE))
                                  return set_error(serror, SCRIPT_ERR_SIG_FINDANDDELETE);
                          }
                      }
  
                      bool fSuccess = true;
                      while (fSuccess && nSigsCount > 0)
                      {
                          valtype& vchSig    = stacktop(-isig);
                          valtype& vchPubKey = stacktop(-ikey);
  
                          // Note how this makes the exact order of pubkey/signature evaluation
                          // distinguishable by CHECKMULTISIG NOT if the STRICTENC flag is set.
                          // See the script_(in)valid tests for details.
                          if (!CheckSignatureEncoding(vchSig, flags, serror) || !CheckPubKeyEncoding(vchPubKey, flags, sigversion, serror)) {
                              // serror is set
                              return false;
                          }
  
                          // Check signature
                          bool no_forkid = ForkIdDisabled(flags);
                          bool fOk = checker.CheckECDSASignature(vchSig, vchPubKey, scriptCode, sigversion, no_forkid);
  
                          if (fOk) {
                              isig++;
                              nSigsCount--;
                          }
                          ikey++;
                          nKeysCount--;
  
                          // If there are more signatures left than keys left,
                          // then too many signatures have failed. Exit early,
                          // without checking any further signatures.
                          if (nSigsCount > nKeysCount)
                              fSuccess = false;
                      }
  
                      // Clean up stack of actual arguments
                      while (i-- > 1) {
                          // If the operation failed, we require that all signatures must be empty vector
                          if (!fSuccess && (flags & SCRIPT_VERIFY_NULLFAIL) && !ikey2 && stacktop(-1).size())
                              return set_error(serror, SCRIPT_ERR_SIG_NULLFAIL);
                          if (ikey2 > 0)
                              ikey2--;
                          popstack(stack);
                      }
  
                      // A bug causes CHECKMULTISIG to consume one extra argument
                      // whose contents were not checked in any way.
                      //
                      // Unfortunately this is a potential source of mutability,
                      // so optionally verify it is exactly equal to zero prior
                      // to removing it from the stack.
                      if (stack.size() < 1)
                          return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
                      if ((flags & SCRIPT_VERIFY_NULLDUMMY) && stacktop(-1).size())
                          return set_error(serror, SCRIPT_ERR_SIG_NULLDUMMY);
                      popstack(stack);
  
                      stack.push_back(fSuccess ? vchTrue : vchFalse);
  
                      if (opcode == OP_CHECKMULTISIGVERIFY)
                      {
                          if (fSuccess)
                              popstack(stack);
                          else
                              return set_error(serror, SCRIPT_ERR_CHECKMULTISIGVERIFY);
                      }
                  }
                  break;
  
                  default:
                      return set_error(serror, SCRIPT_ERR_BAD_OPCODE);
              }
  
              // Size limits
              if (stack.size() + altstack.size() > MAX_STACK_SIZE)
                  return set_error(serror, SCRIPT_ERR_STACK_SIZE);
          }
      }
      catch (...)
      {
          return set_error(serror, SCRIPT_ERR_UNKNOWN_ERROR);
      }
  
      if (!vfExec.empty())
          return set_error(serror, SCRIPT_ERR_UNBALANCED_CONDITIONAL);
  
      return set_success(serror);
  }
  
  bool EvalScript(std::vector<std::vector<unsigned char> >& stack, const CScript& script, unsigned int flags, const BaseSignatureChecker& checker, SigVersion sigversion, ScriptError* serror)
  {
      ScriptExecutionData execdata;
      return EvalScript(stack, script, flags, checker, sigversion, execdata, serror);
  }
  
  namespace {
  
  /**
   * Wrapper that serializes like CTransaction, but with the modifications
   *  required for the signature hash done in-place
   */
  template <class T>
  class CTransactionSignatureSerializer
  {
  private:
      const T& txTo;             //!< reference to the spending transaction (the one being serialized)
      const CScript& scriptCode; //!< output script being consumed
      const unsigned int nIn;    //!< input index of txTo being signed
      const bool fAnyoneCanPay;  //!< whether the hashtype has the SIGHASH_ANYONECANPAY flag set
      const bool fForkID;        //!< whether the hashtype has the SIGHASH_FORKID flag set
      const bool fHashSingle;    //!< whether the hashtype is SIGHASH_SINGLE
      const bool fHashNone;      //!< whether the hashtype is SIGHASH_NONE
  
  public:
      CTransactionSignatureSerializer(const T& txToIn, const CScript& scriptCodeIn, unsigned int nInIn, int nHashTypeIn) :
          txTo(txToIn), scriptCode(scriptCodeIn), nIn(nInIn),
          fAnyoneCanPay(!!(nHashTypeIn & SIGHASH_ANYONECANPAY)),
          fForkID(!!(nHashTypeIn & SIGHASH_FORKID)),
          fHashSingle((nHashTypeIn & 0x1f) == SIGHASH_SINGLE),
          fHashNone((nHashTypeIn & 0x1f) == SIGHASH_NONE) {}
  
      /** Serialize the passed scriptCode, skipping OP_CODESEPARATORs */
      template<typename S>
      void SerializeScriptCode(S &s) const {
          CScript::const_iterator it = scriptCode.begin();
          CScript::const_iterator itBegin = it;
          opcodetype opcode;
          unsigned int nCodeSeparators = 0;
          while (scriptCode.GetOp(it, opcode)) {
              if (opcode == OP_CODESEPARATOR)
                  nCodeSeparators++;
          }
          ::WriteCompactSize(s, scriptCode.size() - nCodeSeparators);
          it = itBegin;
          while (scriptCode.GetOp(it, opcode)) {
              if (opcode == OP_CODESEPARATOR) {
                  s.write((char*)&itBegin[0], it-itBegin-1);
                  itBegin = it;
              }
          }
          if (itBegin != scriptCode.end())
              s.write((char*)&itBegin[0], it-itBegin);
      }
  
      /** Serialize an input of txTo */
      template<typename S>
      void SerializeInput(S &s, unsigned int nInput) const {
          // In case of SIGHASH_ANYONECANPAY, only the input being signed is serialized
          if (fAnyoneCanPay)
              nInput = nIn;
          // Serialize the prevout
          ::Serialize(s, txTo.vin[nInput].prevout);
          // Serialize the script
          if (nInput != nIn)
              // Blank out other inputs' signatures
              ::Serialize(s, CScript());
          else
              SerializeScriptCode(s);
          // Serialize the nSequence
          if (nInput != nIn && (fHashSingle || fHashNone))
              // let the others update at will
              ::Serialize(s, (int)0);
          else
              ::Serialize(s, txTo.vin[nInput].nSequence);
      }
  
      /** Serialize an output of txTo */
      template<typename S>
      void SerializeOutput(S &s, unsigned int nOutput) const {
          if (fHashSingle && nOutput != nIn)
              // Do not lock-in the txout payee at other indices as txin
              ::Serialize(s, CTxOut());
          else
              ::Serialize(s, txTo.vout[nOutput]);
      }
  
      /** Serialize txTo */
      template<typename S>
      void Serialize(S &s) const {
          // Serialize nVersion
          ::Serialize(s, txTo.nVersion);
          // Serialize vin
          unsigned int nInputs = fAnyoneCanPay ? 1 : txTo.vin.size();
          ::WriteCompactSize(s, nInputs);
          for (unsigned int nInput = 0; nInput < nInputs; nInput++)
               SerializeInput(s, nInput);
          // Serialize vout
          unsigned int nOutputs = fHashNone ? 0 : (fHashSingle ? nIn+1 : txTo.vout.size());
          ::WriteCompactSize(s, nOutputs);
          for (unsigned int nOutput = 0; nOutput < nOutputs; nOutput++)
               SerializeOutput(s, nOutput);
          // Serialize nLockTime
          ::Serialize(s, txTo.nLockTime);
      }
  };
  
  /** Compute the (single) SHA256 of the concatenation of all prevouts of a tx. */
  template <class T>
  uint256 GetPrevoutsSHA256(const T& txTo)
  {
      CHashWriter ss(SER_GETHASH, 0);
      for (const auto& txin : txTo.vin) {
          ss << txin.prevout;
      }
      return ss.GetSHA256();
  }
  
  /** Compute the (single) SHA256 of the concatenation of all nSequences of a tx. */
  template <class T>
  uint256 GetSequencesSHA256(const T& txTo)
  {
      CHashWriter ss(SER_GETHASH, 0);
      for (const auto& txin : txTo.vin) {
          ss << txin.nSequence;
      }
      return ss.GetSHA256();
  }
  
  /** Compute the (single) SHA256 of the concatenation of all txouts of a tx. */
  template <class T>
  uint256 GetOutputsSHA256(const T& txTo)
  {
      CHashWriter ss(SER_GETHASH, 0);
      for (const auto& txout : txTo.vout) {
          ss << txout;
      }
      return ss.GetSHA256();
  }
  
  /** Compute the (single) SHA256 of the concatenation of all amounts spent by a tx. */
  uint256 GetSpentAmountsSHA256(const std::vector<CTxOut>& outputs_spent)
  {
      CHashWriter ss(SER_GETHASH, 0);
      for (const auto& txout : outputs_spent) {
          ss << txout.nValue;
      }
      return ss.GetSHA256();
  }
  
  /** Compute the (single) SHA256 of the concatenation of all scriptPubKeys spent by a tx. */
  uint256 GetSpentScriptsSHA256(const std::vector<CTxOut>& outputs_spent)
  {
      CHashWriter ss(SER_GETHASH, 0);
      for (const auto& txout : outputs_spent) {
          ss << txout.scriptPubKey;
      }
      return ss.GetSHA256();
  }
  
  
  } // namespace
  
  template <class T>
  void PrecomputedTransactionData::Init(const T& txTo, std::vector<CTxOut>&& spent_outputs)
  {
      assert(!m_spent_outputs_ready);
  
      m_spent_outputs = std::move(spent_outputs);
      if (!m_spent_outputs.empty()) {
          assert(m_spent_outputs.size() == txTo.vin.size());
          m_spent_outputs_ready = true;
      }
  
      // Determine which precomputation-impacting features this transaction uses.
      bool uses_bip143_segwit = false;
      bool uses_bip341_taproot = false;
      for (size_t inpos = 0; inpos < txTo.vin.size(); ++inpos) {
          if (!txTo.vin[inpos].scriptWitness.IsNull()) {
              if (m_spent_outputs_ready && m_spent_outputs[inpos].scriptPubKey.size() == 2 + WITNESS_V1_TAPROOT_SIZE &&
                  m_spent_outputs[inpos].scriptPubKey[0] == OP_1) {
                  // Treat every witness-bearing spend with 34-byte scriptPubKey that starts with OP_1 as a Taproot
                  // spend. This only works if spent_outputs was provided as well, but if it wasn't, actual validation
                  // will fail anyway. Note that this branch may trigger for scriptPubKeys that aren't actually segwit
                  // but in that case validation will fail as SCRIPT_ERR_WITNESS_UNEXPECTED anyway.
                  uses_bip341_taproot = true;
              } else {
                  // Treat every spend that's not known to native witness v1 as a Witness v0 spend. This branch may
                  // also be taken for unknown witness versions, but it is harmless, and being precise would require
                  // P2SH evaluation to find the redeemScript.
                  uses_bip143_segwit = true;
              }
          }
          if (uses_bip341_taproot && uses_bip143_segwit) break; // No need to scan further if we already need all.
      }
  
      if (uses_bip143_segwit || uses_bip341_taproot) {
          // Computations shared between both sighash schemes.
          m_prevouts_single_hash = GetPrevoutsSHA256(txTo);
          m_sequences_single_hash = GetSequencesSHA256(txTo);
          m_outputs_single_hash = GetOutputsSHA256(txTo);
      }
      if (uses_bip143_segwit) {
          hashPrevouts = SHA256Uint256(m_prevouts_single_hash);
          hashSequence = SHA256Uint256(m_sequences_single_hash);
          hashOutputs = SHA256Uint256(m_outputs_single_hash);
          m_bip143_segwit_ready = true;
      }
      if (uses_bip341_taproot) {
          m_spent_amounts_single_hash = GetSpentAmountsSHA256(m_spent_outputs);
          m_spent_scripts_single_hash = GetSpentScriptsSHA256(m_spent_outputs);
          m_bip341_taproot_ready = true;
      }
  }
  
  template <class T>
  PrecomputedTransactionData::PrecomputedTransactionData(const T& txTo)
  {
      Init(txTo, {});
  }
  
  // explicit instantiation
  template void PrecomputedTransactionData::Init(const CTransaction& txTo, std::vector<CTxOut>&& spent_outputs);
  template void PrecomputedTransactionData::Init(const CMutableTransaction& txTo, std::vector<CTxOut>&& spent_outputs);
  template PrecomputedTransactionData::PrecomputedTransactionData(const CTransaction& txTo);
  template PrecomputedTransactionData::PrecomputedTransactionData(const CMutableTransaction& txTo);
  
  static const CHashWriter HASHER_TAPSIGHASH = TaggedHash("TapSighash");
  static const CHashWriter HASHER_TAPLEAF = TaggedHash("TapLeaf");
  static const CHashWriter HASHER_TAPBRANCH = TaggedHash("TapBranch");
  static const CHashWriter HASHER_TAPTWEAK = TaggedHash("TapTweak");
  
  template<typename T>
  bool SignatureHashSchnorr(uint256& hash_out, const ScriptExecutionData& execdata, const T& tx_to, uint32_t in_pos, uint8_t hash_type, SigVersion sigversion, const PrecomputedTransactionData& cache)
  {
      uint8_t ext_flag, key_version;
      switch (sigversion) {
      case SigVersion::TAPROOT:
          ext_flag = 0;
          // key_version is not used and left uninitialized.
          break;
      case SigVersion::TAPSCRIPT:
          ext_flag = 1;
          // key_version must be 0 for now, representing the current version of
          // 32-byte public keys in the tapscript signature opcode execution.
          // An upgradable public key version (with a size not 32-byte) may
          // request a different key_version with a new sigversion.
          key_version = 0;
          break;
      default:
          assert(false);
      }
      assert(in_pos < tx_to.vin.size());
      assert(cache.m_bip341_taproot_ready && cache.m_spent_outputs_ready);
  
      CHashWriter ss = HASHER_TAPSIGHASH;
  
      // Epoch
      static constexpr uint8_t EPOCH = 0;
      ss << EPOCH;
  
      // Hash type
      const uint8_t output_type = (hash_type == SIGHASH_DEFAULT) ? SIGHASH_ALL : (hash_type & SIGHASH_OUTPUT_MASK); // Default (no sighash byte) is equivalent to SIGHASH_ALL
      const uint8_t input_type = hash_type & SIGHASH_INPUT_MASK;
      if (!(hash_type <= 0x03 || (hash_type >= 0x81 && hash_type <= 0x83))) return false;
      ss << hash_type;
  
      // Transaction level data
      ss << tx_to.nVersion;
      ss << tx_to.nLockTime;
      if (input_type != SIGHASH_ANYONECANPAY) {
          ss << cache.m_prevouts_single_hash;
          ss << cache.m_spent_amounts_single_hash;
          ss << cache.m_spent_scripts_single_hash;
          ss << cache.m_sequences_single_hash;
      }
      if (output_type == SIGHASH_ALL) {
          ss << cache.m_outputs_single_hash;
      }
  
      // Data about the input/prevout being spent
      assert(execdata.m_annex_init);
      const bool have_annex = execdata.m_annex_present;
      const uint8_t spend_type = (ext_flag << 1) + (have_annex ? 1 : 0); // The low bit indicates whether an annex is present.
      ss << spend_type;
      if (input_type == SIGHASH_ANYONECANPAY) {
          ss << tx_to.vin[in_pos].prevout;
          ss << cache.m_spent_outputs[in_pos];
          ss << tx_to.vin[in_pos].nSequence;
      } else {
          ss << in_pos;
      }
      if (have_annex) {
          ss << execdata.m_annex_hash;
      }
  
      // Data about the output (if only one).
      if (output_type == SIGHASH_SINGLE) {
          if (in_pos >= tx_to.vout.size()) return false;
          CHashWriter sha_single_output(SER_GETHASH, 0);
          sha_single_output << tx_to.vout[in_pos];
          ss << sha_single_output.GetSHA256();
      }
  
      // Additional data for BIP 342 signatures
      if (sigversion == SigVersion::TAPSCRIPT) {
          assert(execdata.m_tapleaf_hash_init);
          ss << execdata.m_tapleaf_hash;
          ss << key_version;
          assert(execdata.m_codeseparator_pos_init);
          ss << execdata.m_codeseparator_pos;
      }
  
      hash_out = ss.GetSHA256();
      return true;
  }
  
  template <class T>
  uint256 SignatureHash(const CScript& scriptCode, const T& txTo, unsigned int nIn, int nHashType, const CAmount& amount, SigVersion sigversion, bool no_forkid, const PrecomputedTransactionData* cache, int forkid)
  {
      assert(nIn < txTo.vin.size());
  
      bool use_forkid = false;
      int nForkHashType = nHashType;
      if (!no_forkid) {
          use_forkid = UsesForkId(nHashType);
          if (use_forkid) {
              nForkHashType |= forkid << 8;
          }
      }
  
      // force new tx with FORKID to use bip143 transaction digest algorithm
      // see https://github.com/bitcoin/bips/blob/master/bip-0143.mediawiki
      if (sigversion == SigVersion::WITNESS_V0 || use_forkid) {
          uint256 hashPrevouts;
          uint256 hashSequence;
          uint256 hashOutputs;
          const bool cacheready = cache && cache->m_bip143_segwit_ready;
  
          if (!(nHashType & SIGHASH_ANYONECANPAY)) {
              hashPrevouts = cacheready ? cache->hashPrevouts : SHA256Uint256(GetPrevoutsSHA256(txTo));
          }
  
          if (!(nHashType & SIGHASH_ANYONECANPAY) && (nHashType & 0x1f) != SIGHASH_SINGLE && (nHashType & 0x1f) != SIGHASH_NONE) {
              hashSequence = cacheready ? cache->hashSequence : SHA256Uint256(GetSequencesSHA256(txTo));
          }
  
  
          if ((nHashType & 0x1f) != SIGHASH_SINGLE && (nHashType & 0x1f) != SIGHASH_NONE) {
              hashOutputs = cacheready ? cache->hashOutputs : SHA256Uint256(GetOutputsSHA256(txTo));
          } else if ((nHashType & 0x1f) == SIGHASH_SINGLE && nIn < txTo.vout.size()) {
              CHashWriter ss(SER_GETHASH, 0);
              ss << txTo.vout[nIn];
              hashOutputs = ss.GetHash();
          }
  
          CHashWriter ss(SER_GETHASH, 0);
          // Version
          ss << txTo.nVersion;
          // Input prevouts/nSequence (none/all, depending on flags)
          ss << hashPrevouts;
          ss << hashSequence;
          // The input being signed (replacing the scriptSig with scriptCode + amount)
          // The prevout may already be contained in hashPrevout, and the nSequence
          // may already be contain in hashSequence.
          ss << txTo.vin[nIn].prevout;
          ss << scriptCode;
          ss << amount;
          ss << txTo.vin[nIn].nSequence;
          // Outputs (none/one/all, depending on flags)
          ss << hashOutputs;
          // Locktime
          ss << txTo.nLockTime;
          // Sighash type
          ss << nForkHashType;
  
          return ss.GetHash();
      }
  
      // Check for invalid use of SIGHASH_SINGLE
      if ((nHashType & 0x1f) == SIGHASH_SINGLE) {
          if (nIn >= txTo.vout.size()) {
              //  nOut out of range
              return uint256::ONE;
          }
      }
  
      // Wrapper to serialize only the necessary parts of the transaction being signed
      CTransactionSignatureSerializer<T> txTmp(txTo, scriptCode, nIn, nHashType);
  
      // Serialize and hash
      CHashWriter ss(SER_GETHASH, 0);
      ss << txTmp << nForkHashType;
      return ss.GetHash();
  }
  
  template <class T>
  bool GenericTransactionSignatureChecker<T>::VerifyECDSASignature(const std::vector<unsigned char>& vchSig, const CPubKey& pubkey, const uint256& sighash) const
  {
      return pubkey.Verify(sighash, vchSig);
  }
  
  template <class T>
  bool GenericTransactionSignatureChecker<T>::VerifySchnorrSignature(Span<const unsigned char> sig, const XOnlyPubKey& pubkey, const uint256& sighash) const
  {
      return pubkey.VerifySchnorr(sighash, sig);
  }
  
  template <class T>
  bool GenericTransactionSignatureChecker<T>::CheckECDSASignature(const std::vector<unsigned char>& vchSigIn, const std::vector<unsigned char>& vchPubKey, const CScript& scriptCode, SigVersion sigversion, bool no_forkid) const
  {
      CPubKey pubkey(vchPubKey);
      if (!pubkey.IsValid())
          return false;
  
      // Hash type is one byte tacked on to the end of the signature
      std::vector<unsigned char> vchSig(vchSigIn);
      if (vchSig.empty())
          return false;
      int nHashType = GetHashType(vchSig);
      vchSig.pop_back();
  
      uint256 sighash = SignatureHash(scriptCode, *txTo, nIn, nHashType, amount, sigversion, no_forkid, this->txdata);
  
      if (!VerifyECDSASignature(vchSig, pubkey, sighash))
          return false;
  
      return true;
  }
  
  template <class T>
  bool GenericTransactionSignatureChecker<T>::CheckSchnorrSignature(Span<const unsigned char> sig, Span<const unsigned char> pubkey_in, SigVersion sigversion, const ScriptExecutionData& execdata, ScriptError* serror) const
  {
      assert(sigversion == SigVersion::TAPROOT || sigversion == SigVersion::TAPSCRIPT);
      // Schnorr signatures have 32-byte public keys. The caller is responsible for enforcing this.
      assert(pubkey_in.size() == 32);
      // Note that in Tapscript evaluation, empty signatures are treated specially (invalid signature that does not
      // abort script execution). This is implemented in EvalChecksigTapscript, which won't invoke
      // CheckSchnorrSignature in that case. In other contexts, they are invalid like every other signature with
      // size different from 64 or 65.
      if (sig.size() != 64 && sig.size() != 65) return set_error(serror, SCRIPT_ERR_SCHNORR_SIG_SIZE);
  
      XOnlyPubKey pubkey{pubkey_in};
  
      uint8_t hashtype = SIGHASH_DEFAULT;
      if (sig.size() == 65) {
          hashtype = SpanPopBack(sig);
          if (hashtype == SIGHASH_DEFAULT) return set_error(serror, SCRIPT_ERR_SCHNORR_SIG_HASHTYPE);
      }
      uint256 sighash;
      assert(this->txdata);
      if (!SignatureHashSchnorr(sighash, execdata, *txTo, nIn, hashtype, sigversion, *this->txdata)) {
          return set_error(serror, SCRIPT_ERR_SCHNORR_SIG_HASHTYPE);
      }
      if (!VerifySchnorrSignature(sig, pubkey, sighash)) return set_error(serror, SCRIPT_ERR_SCHNORR_SIG);
      return true;
  }
  
  template <class T>
  bool GenericTransactionSignatureChecker<T>::CheckLockTime(const CScriptNum& nLockTime) const
  {
      // There are two kinds of nLockTime: lock-by-blockheight
      // and lock-by-blocktime, distinguished by whether
      // nLockTime < LOCKTIME_THRESHOLD.
      //
      // We want to compare apples to apples, so fail the script
      // unless the type of nLockTime being tested is the same as
      // the nLockTime in the transaction.
      if (!(
          (txTo->nLockTime <  LOCKTIME_THRESHOLD && nLockTime <  LOCKTIME_THRESHOLD) ||
          (txTo->nLockTime >= LOCKTIME_THRESHOLD && nLockTime >= LOCKTIME_THRESHOLD)
      ))
          return false;
  
      // Now that we know we're comparing apples-to-apples, the
      // comparison is a simple numeric one.
      if (nLockTime > (int64_t)txTo->nLockTime)
          return false;
  
      // Finally the nLockTime feature can be disabled and thus
      // CHECKLOCKTIMEVERIFY bypassed if every txin has been
      // finalized by setting nSequence to maxint. The
      // transaction would be allowed into the blockchain, making
      // the opcode ineffective.
      //
      // Testing if this vin is not final is sufficient to
      // prevent this condition. Alternatively we could test all
      // inputs, but testing just this input minimizes the data
      // required to prove correct CHECKLOCKTIMEVERIFY execution.
      if (CTxIn::SEQUENCE_FINAL == txTo->vin[nIn].nSequence)
          return false;
  
      return true;
  }
  
  template <class T>
  bool GenericTransactionSignatureChecker<T>::CheckSequence(const CScriptNum& nSequence) const
  {
      // Relative lock times are supported by comparing the passed
      // in operand to the sequence number of the input.
      const int64_t txToSequence = (int64_t)txTo->vin[nIn].nSequence;
  
      // Fail if the transaction's version number is not set high
      // enough to trigger BIP 68 rules.
      if (static_cast<uint32_t>(txTo->nVersion) < 2)
          return false;
  
      // Sequence numbers with their most significant bit set are not
      // consensus constrained. Testing that the transaction's sequence
      // number do not have this bit set prevents using this property
      // to get around a CHECKSEQUENCEVERIFY check.
      if (txToSequence & CTxIn::SEQUENCE_LOCKTIME_DISABLE_FLAG)
          return false;
  
      // Mask off any bits that do not have consensus-enforced meaning
      // before doing the integer comparisons
      const uint32_t nLockTimeMask = CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG | CTxIn::SEQUENCE_LOCKTIME_MASK;
      const int64_t txToSequenceMasked = txToSequence & nLockTimeMask;
      const CScriptNum nSequenceMasked = nSequence & nLockTimeMask;
  
      // There are two kinds of nSequence: lock-by-blockheight
      // and lock-by-blocktime, distinguished by whether
      // nSequenceMasked < CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG.
      //
      // We want to compare apples to apples, so fail the script
      // unless the type of nSequenceMasked being tested is the same as
      // the nSequenceMasked in the transaction.
      if (!(
          (txToSequenceMasked <  CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG && nSequenceMasked <  CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG) ||
          (txToSequenceMasked >= CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG && nSequenceMasked >= CTxIn::SEQUENCE_LOCKTIME_TYPE_FLAG)
      )) {
          return false;
      }
  
      // Now that we know we're comparing apples-to-apples, the
      // comparison is a simple numeric one.
      if (nSequenceMasked > txToSequenceMasked)
          return false;
  
      return true;
  }
  
  // explicit instantiation
  template class GenericTransactionSignatureChecker<CTransaction>;
  template class GenericTransactionSignatureChecker<CMutableTransaction>;
  
  static bool ExecuteWitnessScript(const Span<const valtype>& stack_span, const CScript& scriptPubKey, unsigned int flags, SigVersion sigversion, const BaseSignatureChecker& checker, ScriptExecutionData& execdata, ScriptError* serror)
  {
      std::vector<valtype> stack{stack_span.begin(), stack_span.end()};
  
      if (sigversion == SigVersion::TAPSCRIPT) {
          // OP_SUCCESSx processing overrides everything, including stack element size limits
          CScript::const_iterator pc = scriptPubKey.begin();
          while (pc < scriptPubKey.end()) {
              opcodetype opcode;
              if (!scriptPubKey.GetOp(pc, opcode)) {
                  // Note how this condition would not be reached if an unknown OP_SUCCESSx was found
                  return set_error(serror, SCRIPT_ERR_BAD_OPCODE);
              }
              // New opcodes will be listed here. May use a different sigversion to modify existing opcodes.
              if (IsOpSuccess(opcode)) {
                  if (flags & SCRIPT_VERIFY_DISCOURAGE_OP_SUCCESS) {
                      return set_error(serror, SCRIPT_ERR_DISCOURAGE_OP_SUCCESS);
                  }
                  return set_success(serror);
              }
          }
  
          // Tapscript enforces initial stack size limits (altstack is empty here)
          if (stack.size() > MAX_STACK_SIZE) return set_error(serror, SCRIPT_ERR_STACK_SIZE);
      }
  
      // Disallow stack item size > MAX_SCRIPT_ELEMENT_SIZE in witness stack
      for (const valtype& elem : stack) {
          if (elem.size() > MAX_SCRIPT_ELEMENT_SIZE) return set_error(serror, SCRIPT_ERR_PUSH_SIZE);
      }
  
      // Run the script interpreter.
      if (!EvalScript(stack, scriptPubKey, flags, checker, sigversion, execdata, serror)) return false;
  
      // Scripts inside witness implicitly require cleanstack behaviour
      if (stack.size() != 1) return set_error(serror, SCRIPT_ERR_CLEANSTACK);
      if (!CastToBool(stack.back())) return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
      return true;
  }
  
  static bool VerifyTaprootCommitment(const std::vector<unsigned char>& control, const std::vector<unsigned char>& program, const CScript& script, uint256& tapleaf_hash)
  {
      const int path_len = (control.size() - TAPROOT_CONTROL_BASE_SIZE) / TAPROOT_CONTROL_NODE_SIZE;
      const XOnlyPubKey p{uint256(std::vector<unsigned char>(control.begin() + 1, control.begin() + TAPROOT_CONTROL_BASE_SIZE))};
      const XOnlyPubKey q{uint256(program)};
      tapleaf_hash = (CHashWriter(HASHER_TAPLEAF) << uint8_t(control[0] & TAPROOT_LEAF_MASK) << script).GetSHA256();
      uint256 k = tapleaf_hash;
      for (int i = 0; i < path_len; ++i) {
          CHashWriter ss_branch{HASHER_TAPBRANCH};
          Span<const unsigned char> node(control.data() + TAPROOT_CONTROL_BASE_SIZE + TAPROOT_CONTROL_NODE_SIZE * i, TAPROOT_CONTROL_NODE_SIZE);
          if (std::lexicographical_compare(k.begin(), k.end(), node.begin(), node.end())) {
              ss_branch << k << node;
          } else {
              ss_branch << node << k;
          }
          k = ss_branch.GetSHA256();
      }
      k = (CHashWriter(HASHER_TAPTWEAK) << MakeSpan(p) << k).GetSHA256();
      return q.CheckPayToContract(p, k, control[0] & 1);
  }
  
  static bool VerifyWitnessProgram(const CScriptWitness& witness, int witversion, const std::vector<unsigned char>& program, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror, bool is_p2sh)
  {
      CScript exec_script; //!< Actually executed script (last stack item in P2WSH; implied P2PKH script in P2WPKH; leaf script in P2TR)
      Span<const valtype> stack{witness.stack};
      ScriptExecutionData execdata;
  
      if (witversion == 0) {
          if (program.size() == WITNESS_V0_SCRIPTHASH_SIZE) {
              // BIP141 P2WSH: 32-byte witness v0 program (which encodes SHA256(script))
              if (stack.size() == 0) {
                  return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_WITNESS_EMPTY);
              }
              const valtype& script_bytes = SpanPopBack(stack);
              exec_script = CScript(script_bytes.begin(), script_bytes.end());
              uint256 hash_exec_script;
              CSHA256().Write(&exec_script[0], exec_script.size()).Finalize(hash_exec_script.begin());
              if (memcmp(hash_exec_script.begin(), program.data(), 32)) {
                  return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH);
              }
              return ExecuteWitnessScript(stack, exec_script, flags, SigVersion::WITNESS_V0, checker, execdata, serror);
          } else if (program.size() == WITNESS_V0_KEYHASH_SIZE) {
              // BIP141 P2WPKH: 20-byte witness v0 program (which encodes Hash160(pubkey))
              if (stack.size() != 2) {
                  return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH); // 2 items in witness
              }
              exec_script << OP_DUP << OP_HASH160 << program << OP_EQUALVERIFY << OP_CHECKSIG;
              return ExecuteWitnessScript(stack, exec_script, flags, SigVersion::WITNESS_V0, checker, execdata, serror);
          } else {
              return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_WRONG_LENGTH);
          }
      } else if (witversion == 1 && program.size() == WITNESS_V1_TAPROOT_SIZE && !is_p2sh) {
          // BIP341 Taproot: 32-byte non-P2SH witness v1 program (which encodes a P2C-tweaked pubkey)
          if (!(flags & SCRIPT_VERIFY_TAPROOT)) return set_success(serror);
          if (stack.size() == 0) return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_WITNESS_EMPTY);
          if (stack.size() >= 2 && !stack.back().empty() && stack.back()[0] == ANNEX_TAG) {
              // Drop annex (this is non-standard; see IsWitnessStandard)
              const valtype& annex = SpanPopBack(stack);
              execdata.m_annex_hash = (CHashWriter(SER_GETHASH, 0) << annex).GetSHA256();
              execdata.m_annex_present = true;
          } else {
              execdata.m_annex_present = false;
          }
          execdata.m_annex_init = true;
          if (stack.size() == 1) {
              // Key path spending (stack size is 1 after removing optional annex)
              if (!checker.CheckSchnorrSignature(stack.front(), program, SigVersion::TAPROOT, execdata, serror)) {
                  return false; // serror is set
              }
              return set_success(serror);
          } else {
              // Script path spending (stack size is >1 after removing optional annex)
              const valtype& control = SpanPopBack(stack);
              const valtype& script_bytes = SpanPopBack(stack);
              exec_script = CScript(script_bytes.begin(), script_bytes.end());
              if (control.size() < TAPROOT_CONTROL_BASE_SIZE || control.size() > TAPROOT_CONTROL_MAX_SIZE || ((control.size() - TAPROOT_CONTROL_BASE_SIZE) % TAPROOT_CONTROL_NODE_SIZE) != 0) {
                  return set_error(serror, SCRIPT_ERR_TAPROOT_WRONG_CONTROL_SIZE);
              }
              if (!VerifyTaprootCommitment(control, program, exec_script, execdata.m_tapleaf_hash)) {
                  return set_error(serror, SCRIPT_ERR_WITNESS_PROGRAM_MISMATCH);
              }
              execdata.m_tapleaf_hash_init = true;
              if ((control[0] & TAPROOT_LEAF_MASK) == TAPROOT_LEAF_TAPSCRIPT) {
                  // Tapscript (leaf version 0xc0)
                  execdata.m_validation_weight_left = ::GetSerializeSize(witness.stack, PROTOCOL_VERSION) + VALIDATION_WEIGHT_OFFSET;
                  execdata.m_validation_weight_left_init = true;
                  return ExecuteWitnessScript(stack, exec_script, flags, SigVersion::TAPSCRIPT, checker, execdata, serror);
              }
              if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_TAPROOT_VERSION) {
                  return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_TAPROOT_VERSION);
              }
              return set_success(serror);
          }
      } else {
          if (flags & SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM) {
              return set_error(serror, SCRIPT_ERR_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM);
          }
          // Other version/size/p2sh combinations return true for future softfork compatibility
          return true;
      }
      // There is intentionally no return statement here, to be able to use "control reaches end of non-void function" warnings to detect gaps in the logic above.
  }
  
  bool VerifyScript(const CScript& scriptSig, const CScript& scriptPubKey, const CScriptWitness* witness, unsigned int flags, const BaseSignatureChecker& checker, ScriptError* serror)
  {
      static const CScriptWitness emptyWitness;
      if (witness == nullptr) {
          witness = &emptyWitness;
      }
      bool hadWitness = false;
  
      set_error(serror, SCRIPT_ERR_UNKNOWN_ERROR);
  
      if ((flags & SCRIPT_VERIFY_SIGPUSHONLY) != 0 && !scriptSig.IsPushOnly()) {
          return set_error(serror, SCRIPT_ERR_SIG_PUSHONLY);
      }
  
      // scriptSig and scriptPubKey must be evaluated sequentially on the same stack
      // rather than being simply concatenated (see CVE-2010-5141)
      std::vector<std::vector<unsigned char> > stack, stackCopy;
      if (!EvalScript(stack, scriptSig, flags, checker, SigVersion::BASE, serror))
          // serror is set
          return false;
      if (flags & SCRIPT_VERIFY_P2SH)
          stackCopy = stack;
      if (!EvalScript(stack, scriptPubKey, flags, checker, SigVersion::BASE, serror))
          // serror is set
          return false;
      if (stack.empty())
          return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
      if (CastToBool(stack.back()) == false)
          return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
  
      // Bare witness programs
      int witnessversion;
      std::vector<unsigned char> witnessprogram;
      if (flags & SCRIPT_VERIFY_WITNESS) {
          if (scriptPubKey.IsWitnessProgram(witnessversion, witnessprogram)) {
              hadWitness = true;
              if (scriptSig.size() != 0) {
                  // The scriptSig must be _exactly_ CScript(), otherwise we reintroduce malleability.
                  return set_error(serror, SCRIPT_ERR_WITNESS_MALLEATED);
              }
              if (!VerifyWitnessProgram(*witness, witnessversion, witnessprogram, flags, checker, serror, /* is_p2sh */ false)) {
                  return false;
              }
              // Bypass the cleanstack check at the end. The actual stack is obviously not clean
              // for witness programs.
              stack.resize(1);
          }
      }
  
      // Additional validation for spend-to-script-hash transactions:
      if ((flags & SCRIPT_VERIFY_P2SH) && scriptPubKey.IsPayToScriptHash())
      {
          // scriptSig must be literals-only or validation fails
          if (!scriptSig.IsPushOnly())
              return set_error(serror, SCRIPT_ERR_SIG_PUSHONLY);
  
          // Restore stack.
          swap(stack, stackCopy);
  
          // stack cannot be empty here, because if it was the
          // P2SH  HASH <> EQUAL  scriptPubKey would be evaluated with
          // an empty stack and the EvalScript above would return false.
          assert(!stack.empty());
  
          const valtype& pubKeySerialized = stack.back();
          CScript pubKey2(pubKeySerialized.begin(), pubKeySerialized.end());
          popstack(stack);
  
          if (!EvalScript(stack, pubKey2, flags, checker, SigVersion::BASE, serror))
              // serror is set
              return false;
          if (stack.empty())
              return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
          if (!CastToBool(stack.back()))
              return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
  
          // P2SH witness program
          if (flags & SCRIPT_VERIFY_WITNESS) {
              if (pubKey2.IsWitnessProgram(witnessversion, witnessprogram)) {
                  hadWitness = true;
                  if (scriptSig != CScript() << std::vector<unsigned char>(pubKey2.begin(), pubKey2.end())) {
                      // The scriptSig must be _exactly_ a single push of the redeemScript. Otherwise we
                      // reintroduce malleability.
                      return set_error(serror, SCRIPT_ERR_WITNESS_MALLEATED_P2SH);
                  }
                  if (!VerifyWitnessProgram(*witness, witnessversion, witnessprogram, flags, checker, serror, /* is_p2sh */ true)) {
                      return false;
                  }
                  // Bypass the cleanstack check at the end. The actual stack is obviously not clean
                  // for witness programs.
                  stack.resize(1);
              }
          }
      }
  
      // The CLEANSTACK check is only performed after potential P2SH evaluation,
      // as the non-P2SH evaluation of a P2SH script will obviously not result in
      // a clean stack (the P2SH inputs remain). The same holds for witness evaluation.
      if ((flags & SCRIPT_VERIFY_CLEANSTACK) != 0) {
          // Disallow CLEANSTACK without P2SH, as otherwise a switch CLEANSTACK->P2SH+CLEANSTACK
          // would be possible, which is not a softfork (and P2SH should be one).
          assert((flags & SCRIPT_VERIFY_P2SH) != 0);
          assert((flags & SCRIPT_VERIFY_WITNESS) != 0);
          if (stack.size() != 1) {
              return set_error(serror, SCRIPT_ERR_CLEANSTACK);
          }
      }
  
      if (flags & SCRIPT_VERIFY_WITNESS) {
          // We can't check for correct unexpected witness data if P2SH was off, so require
          // that WITNESS implies P2SH. Otherwise, going from WITNESS->P2SH+WITNESS would be
          // possible, which is not a softfork.
          assert((flags & SCRIPT_VERIFY_P2SH) != 0);
          if (!hadWitness && !witness->IsNull()) {
              return set_error(serror, SCRIPT_ERR_WITNESS_UNEXPECTED);
          }
      }
  
      return set_success(serror);
  }
  
  size_t static WitnessSigOps(int witversion, const std::vector<unsigned char>& witprogram, const CScriptWitness& witness)
  {
      if (witversion == 0) {
          if (witprogram.size() == WITNESS_V0_KEYHASH_SIZE)
              return 1;
  
          if (witprogram.size() == WITNESS_V0_SCRIPTHASH_SIZE && witness.stack.size() > 0) {
              CScript subscript(witness.stack.back().begin(), witness.stack.back().end());
              return subscript.GetSigOpCount(true);
          }
      }
  
      // Future flags may be implemented here.
      return 0;
  }
  
  size_t CountWitnessSigOps(const CScript& scriptSig, const CScript& scriptPubKey, const CScriptWitness* witness, unsigned int flags)
  {
      static const CScriptWitness witnessEmpty;
  
      if ((flags & SCRIPT_VERIFY_WITNESS) == 0) {
          return 0;
      }
      assert((flags & SCRIPT_VERIFY_P2SH) != 0);
  
      int witnessversion;
      std::vector<unsigned char> witnessprogram;
      if (scriptPubKey.IsWitnessProgram(witnessversion, witnessprogram)) {
          return WitnessSigOps(witnessversion, witnessprogram, witness ? *witness : witnessEmpty);
      }
  
      if (scriptPubKey.IsPayToScriptHash() && scriptSig.IsPushOnly()) {
          CScript::const_iterator pc = scriptSig.begin();
          std::vector<unsigned char> data;
          while (pc < scriptSig.end()) {
              opcodetype opcode;
              scriptSig.GetOp(pc, opcode, data);
          }
          CScript subscript(data.begin(), data.end());
          if (subscript.IsWitnessProgram(witnessversion, witnessprogram)) {
              return WitnessSigOps(witnessversion, witnessprogram, witness ? *witness : witnessEmpty);
          }
      }
  
      return 0;
  }
  