text: // Copyright (c) 2009-2010 Satoshi Nakamoto
  // Copyright (c) 2009-2018 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/sigcache.h>
  
  #include <pubkey.h>
  #include <random.h>
  #include <uint256.h>
  #include <util/system.h>
  
  #include <cuckoocache.h>
  #include <boost/thread.hpp>
  
  namespace {
  /**
   * Valid signature cache, to avoid doing expensive ECDSA signature checking
   * twice for every transaction (once when accepted into memory pool, and
   * again when accepted into the block chain)
   */
  class CSignatureCache
  {
  private:
       //! Entries are SHA256(nonce || signature hash || public key || signature):
      uint256 nonce;
      typedef CuckooCache::cache<uint256, SignatureCacheHasher> map_type;
      map_type setValid;
      boost::shared_mutex cs_sigcache;
  
  public:
      CSignatureCache()
      {
          GetRandBytes(nonce.begin(), 32);
      }
  
      void
      ComputeEntry(uint256& entry, const uint256 &hash, const std::vector<unsigned char>& vchSig, const CPubKey& pubkey)
      {
          CSHA256().Write(nonce.begin(), 32).Write(hash.begin(), 32).Write(&pubkey[0], pubkey.size()).Write(&vchSig[0], vchSig.size()).Finalize(entry.begin());
      }
  
      bool
      Get(const uint256& entry, const bool erase)
      {
          boost::shared_lock<boost::shared_mutex> lock(cs_sigcache);
          return setValid.contains(entry, erase);
      }
  
      void Set(uint256& entry)
      {
          boost::unique_lock<boost::shared_mutex> lock(cs_sigcache);
          setValid.insert(entry);
      }
      uint32_t setup_bytes(size_t n)
      {
          return setValid.setup_bytes(n);
      }
  };
  
  /* In previous versions of this code, signatureCache was a local static variable
   * in CachingTransactionSignatureChecker::VerifySignature.  We initialize
   * signatureCache outside of VerifySignature to avoid the atomic operation per
   * call overhead associated with local static variables even though
   * signatureCache could be made local to VerifySignature.
  */
  static CSignatureCache signatureCache;
  } // namespace
  
  // To be called once in AppInitMain/BasicTestingSetup to initialize the
  // signatureCache.
  void InitSignatureCache()
  {
      // nMaxCacheSize is unsigned. If -maxsigcachesize is set to zero,
      // setup_bytes creates the minimum possible cache (2 elements).
      size_t nMaxCacheSize = std::min(std::max((int64_t)0, gArgs.GetArg("-maxsigcachesize", DEFAULT_MAX_SIG_CACHE_SIZE) / 2), MAX_MAX_SIG_CACHE_SIZE) * ((size_t) 1 << 20);
      size_t nElems = signatureCache.setup_bytes(nMaxCacheSize);
      LogPrintf("Using %zu MiB out of %zu/2 requested for signature cache, able to store %zu elements\n",
              (nElems*sizeof(uint256)) >>20, (nMaxCacheSize*2)>>20, nElems);
  }
  
  bool CachingTransactionSignatureChecker::VerifySignature(const std::vector<unsigned char>& vchSig, const CPubKey& pubkey, const uint256& sighash) const
  {
      uint256 entry;
      signatureCache.ComputeEntry(entry, sighash, vchSig, pubkey);
      if (signatureCache.Get(entry, !store))
          return true;
      if (!TransactionSignatureChecker::VerifySignature(vchSig, pubkey, sighash))
          return false;
      if (store)
          signatureCache.Set(entry);
      return true;
  }
  