text: // Copyright 2005 Google Inc. All Rights Reserved.
  //
  // Redistribution and use in source and binary forms, with or without
  // modification, are permitted provided that the following conditions are
  // met:
  //
  //     * Redistributions of source code must retain the above copyright
  // notice, this list of conditions and the following disclaimer.
  //     * Redistributions in binary form must reproduce the above
  // copyright notice, this list of conditions and the following disclaimer
  // in the documentation and/or other materials provided with the
  // distribution.
  //     * Neither the name of Google Inc. nor the names of its
  // contributors may be used to endorse or promote products derived from
  // this software without specific prior written permission.
  //
  // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  
  #include "snappy.h"
  #include "snappy-internal.h"
  #include "snappy-sinksource.h"
  
  #include <stdio.h>
  
  #include <algorithm>
  #include <string>
  #include <vector>
  
  
  namespace snappy {
  
  // Any hash function will produce a valid compressed bitstream, but a good
  // hash function reduces the number of collisions and thus yields better
  // compression for compressible input, and more speed for incompressible
  // input. Of course, it doesn't hurt if the hash function is reasonably fast
  // either, as it gets called a lot.
  static inline uint32 HashBytes(uint32 bytes, int shift) {
    uint32 kMul = 0x1e35a7bd;
    return (bytes * kMul) >> shift;
  }
  static inline uint32 Hash(const char* p, int shift) {
    return HashBytes(UNALIGNED_LOAD32(p), shift);
  }
  
  size_t MaxCompressedLength(size_t source_len) {
    // Compressed data can be defined as:
    //    compressed := item* literal*
    //    item       := literal* copy
    //
    // The trailing literal sequence has a space blowup of at most 62/60
    // since a literal of length 60 needs one tag byte + one extra byte
    // for length information.
    //
    // Item blowup is trickier to measure.  Suppose the "copy" op copies
    // 4 bytes of data.  Because of a special check in the encoding code,
    // we produce a 4-byte copy only if the offset is < 65536.  Therefore
    // the copy op takes 3 bytes to encode, and this type of item leads
    // to at most the 62/60 blowup for representing literals.
    //
    // Suppose the "copy" op copies 5 bytes of data.  If the offset is big
    // enough, it will take 5 bytes to encode the copy op.  Therefore the
    // worst case here is a one-byte literal followed by a five-byte copy.
    // I.e., 6 bytes of input turn into 7 bytes of "compressed" data.
    //
    // This last factor dominates the blowup, so the final estimate is:
    return 32 + source_len + source_len/6;
  }
  
  enum {
    LITERAL = 0,
    COPY_1_BYTE_OFFSET = 1,  // 3 bit length + 3 bits of offset in opcode
    COPY_2_BYTE_OFFSET = 2,
    COPY_4_BYTE_OFFSET = 3
  };
  
  // Copy "len" bytes from "src" to "op", one byte at a time.  Used for
  // handling COPY operations where the input and output regions may
  // overlap.  For example, suppose:
  //    src    == "ab"
  //    op     == src + 2
  //    len    == 20
  // After IncrementalCopy(src, op, len), the result will have
  // eleven copies of "ab"
  //    ababababababababababab
  // Note that this does not match the semantics of either memcpy()
  // or memmove().
  static inline void IncrementalCopy(const char* src, char* op, int len) {
    DCHECK_GT(len, 0);
    do {
      *op++ = *src++;
    } while (--len > 0);
  }
  
  // Equivalent to IncrementalCopy except that it can write up to ten extra
  // bytes after the end of the copy, and that it is faster.
  //
  // The main part of this loop is a simple copy of eight bytes at a time until
  // we've copied (at least) the requested amount of bytes.  However, if op and
  // src are less than eight bytes apart (indicating a repeating pattern of
  // length < 8), we first need to expand the pattern in order to get the correct
  // results. For instance, if the buffer looks like this, with the eight-byte
  // <src> and <op> patterns marked as intervals:
  //
  //    abxxxxxxxxxxxx
  //    [------]           src
  //      [------]         op
  //
  // a single eight-byte copy from <src> to <op> will repeat the pattern once,
  // after which we can move <op> two bytes without moving <src>:
  //
  //    ababxxxxxxxxxx
  //    [------]           src
  //        [------]       op
  //
  // and repeat the exercise until the two no longer overlap.
  //
  // This allows us to do very well in the special case of one single byte
  // repeated many times, without taking a big hit for more general cases.
  //
  // The worst case of extra writing past the end of the match occurs when
  // op - src == 1 and len == 1; the last copy will read from byte positions
  // [0..7] and write to [4..11], whereas it was only supposed to write to
  // position 1. Thus, ten excess bytes.
  
  namespace {
  
  const int kMaxIncrementCopyOverflow = 10;
  
  }  // namespace
  
  static inline void IncrementalCopyFastPath(const char* src, char* op, int len) {
    while (op - src < 8) {
      UNALIGNED_STORE64(op, UNALIGNED_LOAD64(src));
      len -= op - src;
      op += op - src;
    }
    while (len > 0) {
      UNALIGNED_STORE64(op, UNALIGNED_LOAD64(src));
      src += 8;
      op += 8;
      len -= 8;
    }
  }
  
  static inline char* EmitLiteral(char* op,
                                  const char* literal,
                                  int len,
                                  bool allow_fast_path) {
    int n = len - 1;      // Zero-length literals are disallowed
    if (n < 60) {
      // Fits in tag byte
      *op++ = LITERAL | (n << 2);
  
      // The vast majority of copies are below 16 bytes, for which a
      // call to memcpy is overkill. This fast path can sometimes
      // copy up to 15 bytes too much, but that is okay in the
      // main loop, since we have a bit to go on for both sides:
      //
      //   - The input will always have kInputMarginBytes = 15 extra
      //     available bytes, as long as we're in the main loop, and
      //     if not, allow_fast_path = false.
      //   - The output will always have 32 spare bytes (see
      //     MaxCompressedLength).
      if (allow_fast_path && len <= 16) {
        UNALIGNED_STORE64(op, UNALIGNED_LOAD64(literal));
        UNALIGNED_STORE64(op + 8, UNALIGNED_LOAD64(literal + 8));
        return op + len;
      }
    } else {
      // Encode in upcoming bytes
      char* base = op;
      int count = 0;
      op++;
      while (n > 0) {
        *op++ = n & 0xff;
        n >>= 8;
        count++;
      }
      assert(count >= 1);
      assert(count <= 4);
      *base = LITERAL | ((59+count) << 2);
    }
    memcpy(op, literal, len);
    return op + len;
  }
  
  static inline char* EmitCopyLessThan64(char* op, int offset, int len) {
    DCHECK_LE(len, 64);
    DCHECK_GE(len, 4);
    DCHECK_LT(offset, 65536);
  
    if ((len < 12) && (offset < 2048)) {
      int len_minus_4 = len - 4;
      assert(len_minus_4 < 8);            // Must fit in 3 bits
      *op++ = COPY_1_BYTE_OFFSET | ((len_minus_4) << 2) | ((offset >> 8) << 5);
      *op++ = offset & 0xff;
    } else {
      *op++ = COPY_2_BYTE_OFFSET | ((len-1) << 2);
      LittleEndian::Store16(op, offset);
      op += 2;
    }
    return op;
  }
  
  static inline char* EmitCopy(char* op, int offset, int len) {
    // Emit 64 byte copies but make sure to keep at least four bytes reserved
    while (len >= 68) {
      op = EmitCopyLessThan64(op, offset, 64);
      len -= 64;
    }
  
    // Emit an extra 60 byte copy if have too much data to fit in one copy
    if (len > 64) {
      op = EmitCopyLessThan64(op, offset, 60);
      len -= 60;
    }
  
    // Emit remainder
    op = EmitCopyLessThan64(op, offset, len);
    return op;
  }
  
  
  bool GetUncompressedLength(const char* start, size_t n, size_t* result) {
    uint32 v = 0;
    const char* limit = start + n;
    if (Varint::Parse32WithLimit(start, limit, &v) != NULL) {
      *result = v;
      return true;
    } else {
      return false;
    }
  }
  
  namespace internal {
  uint16* WorkingMemory::GetHashTable(size_t input_size, int* table_size) {
    // Use smaller hash table when input.size() is smaller, since we
    // fill the table, incurring O(hash table size) overhead for
    // compression, and if the input is short, we won't need that
    // many hash table entries anyway.
    assert(kMaxHashTableSize >= 256);
    int htsize = 256;
    while (htsize < kMaxHashTableSize && htsize < input_size) {
      htsize <<= 1;
    }
    CHECK_EQ(0, htsize & (htsize - 1)) << ": must be power of two";
    CHECK_LE(htsize, kMaxHashTableSize) << ": hash table too large";
  
    uint16* table;
    if (htsize <= ARRAYSIZE(small_table_)) {
      table = small_table_;
    } else {
      if (large_table_ == NULL) {
        large_table_ = new uint16[kMaxHashTableSize];
      }
      table = large_table_;
    }
  
    *table_size = htsize;
    memset(table, 0, htsize * sizeof(*table));
    return table;
  }
  }  // end namespace internal
  
  // For 0 <= offset <= 4, GetUint32AtOffset(UNALIGNED_LOAD64(p), offset) will
  // equal UNALIGNED_LOAD32(p + offset).  Motivation: On x86-64 hardware we have
  // empirically found that overlapping loads such as
  //  UNALIGNED_LOAD32(p) ... UNALIGNED_LOAD32(p+1) ... UNALIGNED_LOAD32(p+2)
  // are slower than UNALIGNED_LOAD64(p) followed by shifts and casts to uint32.
  static inline uint32 GetUint32AtOffset(uint64 v, int offset) {
    DCHECK(0 <= offset && offset <= 4) << offset;
    return v >> (LittleEndian::IsLittleEndian() ? 8 * offset : 32 - 8 * offset);
  }
  
  // Flat array compression that does not emit the "uncompressed length"
  // prefix. Compresses "input" string to the "*op" buffer.
  //
  // REQUIRES: "input" is at most "kBlockSize" bytes long.
  // REQUIRES: "op" points to an array of memory that is at least
  // "MaxCompressedLength(input.size())" in size.
  // REQUIRES: All elements in "table[0..table_size-1]" are initialized to zero.
  // REQUIRES: "table_size" is a power of two
  //
  // Returns an "end" pointer into "op" buffer.
  // "end - op" is the compressed size of "input".
  namespace internal {
  char* CompressFragment(const char* input,
                         size_t input_size,
                         char* op,
                         uint16* table,
                         const int table_size) {
    // "ip" is the input pointer, and "op" is the output pointer.
    const char* ip = input;
    CHECK_LE(input_size, kBlockSize);
    CHECK_EQ(table_size & (table_size - 1), 0) << ": table must be power of two";
    const int shift = 32 - Bits::Log2Floor(table_size);
    DCHECK_EQ(kuint32max >> shift, table_size - 1);
    const char* ip_end = input + input_size;
    const char* base_ip = ip;
    // Bytes in [next_emit, ip) will be emitted as literal bytes.  Or
    // [next_emit, ip_end) after the main loop.
    const char* next_emit = ip;
  
    const int kInputMarginBytes = 15;
    if (PREDICT_TRUE(input_size >= kInputMarginBytes)) {
      const char* ip_limit = input + input_size - kInputMarginBytes;
  
      for (uint32 next_hash = Hash(++ip, shift); ; ) {
        DCHECK_LT(next_emit, ip);
        // The body of this loop calls EmitLiteral once and then EmitCopy one or
        // more times.  (The exception is that when we're close to exhausting
        // the input we goto emit_remainder.)
        //
        // In the first iteration of this loop we're just starting, so
        // there's nothing to copy, so calling EmitLiteral once is
        // necessary.  And we only start a new iteration when the
        // current iteration has determined that a call to EmitLiteral will
        // precede the next call to EmitCopy (if any).
        //
        // Step 1: Scan forward in the input looking for a 4-byte-long match.
        // If we get close to exhausting the input then goto emit_remainder.
        //
        // Heuristic match skipping: If 32 bytes are scanned with no matches
        // found, start looking only at every other byte. If 32 more bytes are
        // scanned, look at every third byte, etc.. When a match is found,
        // immediately go back to looking at every byte. This is a small loss
        // (~5% performance, ~0.1% density) for compressible data due to more
        // bookkeeping, but for non-compressible data (such as JPEG) it's a huge
        // win since the compressor quickly "realizes" the data is incompressible
        // and doesn't bother looking for matches everywhere.
        //
        // The "skip" variable keeps track of how many bytes there are since the
        // last match; dividing it by 32 (ie. right-shifting by five) gives the
        // number of bytes to move ahead for each iteration.
        uint32 skip = 32;
  
        const char* next_ip = ip;
        const char* candidate;
        do {
          ip = next_ip;
          uint32 hash = next_hash;
          DCHECK_EQ(hash, Hash(ip, shift));
          uint32 bytes_between_hash_lookups = skip++ >> 5;
          next_ip = ip + bytes_between_hash_lookups;
          if (PREDICT_FALSE(next_ip > ip_limit)) {
            goto emit_remainder;
          }
          next_hash = Hash(next_ip, shift);
          candidate = base_ip + table[hash];
          DCHECK_GE(candidate, base_ip);
          DCHECK_LT(candidate, ip);
  
          table[hash] = ip - base_ip;
        } while (PREDICT_TRUE(UNALIGNED_LOAD32(ip) !=
                              UNALIGNED_LOAD32(candidate)));
  
        // Step 2: A 4-byte match has been found.  We'll later see if more
        // than 4 bytes match.  But, prior to the match, input
        // bytes [next_emit, ip) are unmatched.  Emit them as "literal bytes."
        DCHECK_LE(next_emit + 16, ip_end);
        op = EmitLiteral(op, next_emit, ip - next_emit, true);
  
        // Step 3: Call EmitCopy, and then see if another EmitCopy could
        // be our next move.  Repeat until we find no match for the
        // input immediately after what was consumed by the last EmitCopy call.
        //
        // If we exit this loop normally then we need to call EmitLiteral next,
        // though we don't yet know how big the literal will be.  We handle that
        // by proceeding to the next iteration of the main loop.  We also can exit
        // this loop via goto if we get close to exhausting the input.
        uint64 input_bytes = 0;
        uint32 candidate_bytes = 0;
  
        do {
          // We have a 4-byte match at ip, and no need to emit any
          // "literal bytes" prior to ip.
          const char* base = ip;
          int matched = 4 + FindMatchLength(candidate + 4, ip + 4, ip_end);
          ip += matched;
          int offset = base - candidate;
          DCHECK_EQ(0, memcmp(base, candidate, matched));
          op = EmitCopy(op, offset, matched);
          // We could immediately start working at ip now, but to improve
          // compression we first update table[Hash(ip - 1, ...)].
          const char* insert_tail = ip - 1;
          next_emit = ip;
          if (PREDICT_FALSE(ip >= ip_limit)) {
            goto emit_remainder;
          }
          input_bytes = UNALIGNED_LOAD64(insert_tail);
          uint32 prev_hash = HashBytes(GetUint32AtOffset(input_bytes, 0), shift);
          table[prev_hash] = ip - base_ip - 1;
          uint32 cur_hash = HashBytes(GetUint32AtOffset(input_bytes, 1), shift);
          candidate = base_ip + table[cur_hash];
          candidate_bytes = UNALIGNED_LOAD32(candidate);
          table[cur_hash] = ip - base_ip;
        } while (GetUint32AtOffset(input_bytes, 1) == candidate_bytes);
  
        next_hash = HashBytes(GetUint32AtOffset(input_bytes, 2), shift);
        ++ip;
      }
    }
  
   emit_remainder:
    // Emit the remaining bytes as a literal
    if (next_emit < ip_end) {
      op = EmitLiteral(op, next_emit, ip_end - next_emit, false);
    }
  
    return op;
  }
  }  // end namespace internal
  
  // Signature of output types needed by decompression code.
  // The decompression code is templatized on a type that obeys this
  // signature so that we do not pay virtual function call overhead in
  // the middle of a tight decompression loop.
  //
  // class DecompressionWriter {
  //  public:
  //   // Called before decompression
  //   void SetExpectedLength(size_t length);
  //
  //   // Called after decompression
  //   bool CheckLength() const;
  //
  //   // Called repeatedly during decompression
  //   bool Append(const char* ip, uint32 length, bool allow_fast_path);
  //   bool AppendFromSelf(uint32 offset, uint32 length);
  // };
  //
  // "allow_fast_path" is a parameter that says if there is at least 16
  // readable bytes in "ip". It is currently only used by SnappyArrayWriter.
  
  // -----------------------------------------------------------------------
  // Lookup table for decompression code.  Generated by ComputeTable() below.
  // -----------------------------------------------------------------------
  
  // Mapping from i in range [0,4] to a mask to extract the bottom 8*i bits
  static const uint32 wordmask[] = {
    0u, 0xffu, 0xffffu, 0xffffffu, 0xffffffffu
  };
  
  // Data stored per entry in lookup table:
  //      Range   Bits-used       Description
  //      ------------------------------------
  //      1..64   0..7            Literal/copy length encoded in opcode byte
  //      0..7    8..10           Copy offset encoded in opcode byte / 256
  //      0..4    11..13          Extra bytes after opcode
  //
  // We use eight bits for the length even though 7 would have sufficed
  // because of efficiency reasons:
  //      (1) Extracting a byte is faster than a bit-field
  //      (2) It properly aligns copy offset so we do not need a <<8
  static const uint16 char_table[256] = {
    0x0001, 0x0804, 0x1001, 0x2001, 0x0002, 0x0805, 0x1002, 0x2002,
    0x0003, 0x0806, 0x1003, 0x2003, 0x0004, 0x0807, 0x1004, 0x2004,
    0x0005, 0x0808, 0x1005, 0x2005, 0x0006, 0x0809, 0x1006, 0x2006,
    0x0007, 0x080a, 0x1007, 0x2007, 0x0008, 0x080b, 0x1008, 0x2008,
    0x0009, 0x0904, 0x1009, 0x2009, 0x000a, 0x0905, 0x100a, 0x200a,
    0x000b, 0x0906, 0x100b, 0x200b, 0x000c, 0x0907, 0x100c, 0x200c,
    0x000d, 0x0908, 0x100d, 0x200d, 0x000e, 0x0909, 0x100e, 0x200e,
    0x000f, 0x090a, 0x100f, 0x200f, 0x0010, 0x090b, 0x1010, 0x2010,
    0x0011, 0x0a04, 0x1011, 0x2011, 0x0012, 0x0a05, 0x1012, 0x2012,
    0x0013, 0x0a06, 0x1013, 0x2013, 0x0014, 0x0a07, 0x1014, 0x2014,
    0x0015, 0x0a08, 0x1015, 0x2015, 0x0016, 0x0a09, 0x1016, 0x2016,
    0x0017, 0x0a0a, 0x1017, 0x2017, 0x0018, 0x0a0b, 0x1018, 0x2018,
    0x0019, 0x0b04, 0x1019, 0x2019, 0x001a, 0x0b05, 0x101a, 0x201a,
    0x001b, 0x0b06, 0x101b, 0x201b, 0x001c, 0x0b07, 0x101c, 0x201c,
    0x001d, 0x0b08, 0x101d, 0x201d, 0x001e, 0x0b09, 0x101e, 0x201e,
    0x001f, 0x0b0a, 0x101f, 0x201f, 0x0020, 0x0b0b, 0x1020, 0x2020,
    0x0021, 0x0c04, 0x1021, 0x2021, 0x0022, 0x0c05, 0x1022, 0x2022,
    0x0023, 0x0c06, 0x1023, 0x2023, 0x0024, 0x0c07, 0x1024, 0x2024,
    0x0025, 0x0c08, 0x1025, 0x2025, 0x0026, 0x0c09, 0x1026, 0x2026,
    0x0027, 0x0c0a, 0x1027, 0x2027, 0x0028, 0x0c0b, 0x1028, 0x2028,
    0x0029, 0x0d04, 0x1029, 0x2029, 0x002a, 0x0d05, 0x102a, 0x202a,
    0x002b, 0x0d06, 0x102b, 0x202b, 0x002c, 0x0d07, 0x102c, 0x202c,
    0x002d, 0x0d08, 0x102d, 0x202d, 0x002e, 0x0d09, 0x102e, 0x202e,
    0x002f, 0x0d0a, 0x102f, 0x202f, 0x0030, 0x0d0b, 0x1030, 0x2030,
    0x0031, 0x0e04, 0x1031, 0x2031, 0x0032, 0x0e05, 0x1032, 0x2032,
    0x0033, 0x0e06, 0x1033, 0x2033, 0x0034, 0x0e07, 0x1034, 0x2034,
    0x0035, 0x0e08, 0x1035, 0x2035, 0x0036, 0x0e09, 0x1036, 0x2036,
    0x0037, 0x0e0a, 0x1037, 0x2037, 0x0038, 0x0e0b, 0x1038, 0x2038,
    0x0039, 0x0f04, 0x1039, 0x2039, 0x003a, 0x0f05, 0x103a, 0x203a,
    0x003b, 0x0f06, 0x103b, 0x203b, 0x003c, 0x0f07, 0x103c, 0x203c,
    0x0801, 0x0f08, 0x103d, 0x203d, 0x1001, 0x0f09, 0x103e, 0x203e,
    0x1801, 0x0f0a, 0x103f, 0x203f, 0x2001, 0x0f0b, 0x1040, 0x2040
  };
  
  /*// In debug mode, allow optional computation of the table at startup.
  // Also, check that the decompression table is correct.
  #ifndef NDEBUG
  DEFINE_bool(snappy_dump_decompression_table, false, "If true, we print the decompression table at startup.");
  
  static uint16 MakeEntry(unsigned int extra,
                          unsigned int len,
                          unsigned int copy_offset) {
    // Check that all of the fields fit within the allocated space
    DCHECK_EQ(extra,       extra & 0x7);          // At most 3 bits
    DCHECK_EQ(copy_offset, copy_offset & 0x7);    // At most 3 bits
    DCHECK_EQ(len,         len & 0x7f);           // At most 7 bits
    return len | (copy_offset << 8) | (extra << 11);
  }
  
  static void ComputeTable() {
    uint16 dst[256];
  
    // Place invalid entries in all places to detect missing initialization
    int assigned = 0;
    for (int i = 0; i < 256; i++) {
      dst[i] = 0xffff;
    }
  
    // Small LITERAL entries.  We store (len-1) in the top 6 bits.
    for (unsigned int len = 1; len <= 60; len++) {
      dst[LITERAL | ((len-1) << 2)] = MakeEntry(0, len, 0);
      assigned++;
    }
  
    // Large LITERAL entries.  We use 60..63 in the high 6 bits to
    // encode the number of bytes of length info that follow the opcode.
    for (unsigned int extra_bytes = 1; extra_bytes <= 4; extra_bytes++) {
      // We set the length field in the lookup table to 1 because extra
      // bytes encode len-1.
      dst[LITERAL | ((extra_bytes+59) << 2)] = MakeEntry(extra_bytes, 1, 0);
      assigned++;
    }
  
    // COPY_1_BYTE_OFFSET.
    //
    // The tag byte in the compressed data stores len-4 in 3 bits, and
    // offset/256 in 5 bits.  offset%256 is stored in the next byte.
    //
    // This format is used for length in range [4..11] and offset in
    // range [0..2047]
    for (unsigned int len = 4; len < 12; len++) {
      for (unsigned int offset = 0; offset < 2048; offset += 256) {
        dst[COPY_1_BYTE_OFFSET | ((len-4)<<2) | ((offset>>8)<<5)] =
          MakeEntry(1, len, offset>>8);
        assigned++;
      }
    }
  
    // COPY_2_BYTE_OFFSET.
    // Tag contains len-1 in top 6 bits, and offset in next two bytes.
    for (unsigned int len = 1; len <= 64; len++) {
      dst[COPY_2_BYTE_OFFSET | ((len-1)<<2)] = MakeEntry(2, len, 0);
      assigned++;
    }
  
    // COPY_4_BYTE_OFFSET.
    // Tag contents len-1 in top 6 bits, and offset in next four bytes.
    for (unsigned int len = 1; len <= 64; len++) {
      dst[COPY_4_BYTE_OFFSET | ((len-1)<<2)] = MakeEntry(4, len, 0);
      assigned++;
    }
  
    // Check that each entry was initialized exactly once.
    CHECK_EQ(assigned, 256);
    for (int i = 0; i < 256; i++) {
      CHECK_NE(dst[i], 0xffff);
    }
  
    if (FLAGS_snappy_dump_decompression_table) {
      printf("static const uint16 char_table[256] = {\n  ");
      for (int i = 0; i < 256; i++) {
        printf("0x%04x%s",
               dst[i],
               ((i == 255) ? "\n" : (((i%8) == 7) ? ",\n  " : ", ")));
      }
      printf("};\n");
    }
  
    // Check that computed table matched recorded table
    for (int i = 0; i < 256; i++) {
      CHECK_EQ(dst[i], char_table[i]);
    }
  }
  REGISTER_MODULE_INITIALIZER(snappy, ComputeTable());
  #endif  !NDEBUG */
  
  // Helper class for decompression
  class SnappyDecompressor {
   private:
    Source*       reader_;         // Underlying source of bytes to decompress
    const char*   ip_;             // Points to next buffered byte
    const char*   ip_limit_;       // Points just past buffered bytes
    uint32        peeked_;         // Bytes peeked from reader (need to skip)
    bool          eof_;            // Hit end of input without an error?
    char          scratch_[5];     // Temporary buffer for PeekFast() boundaries
  
    // Ensure that all of the tag metadata for the next tag is available
    // in [ip_..ip_limit_-1].  Also ensures that [ip,ip+4] is readable even
    // if (ip_limit_ - ip_ < 5).
    //
    // Returns true on success, false on error or end of input.
    bool RefillTag();
  
   public:
    explicit SnappyDecompressor(Source* reader)
        : reader_(reader),
          ip_(NULL),
          ip_limit_(NULL),
          peeked_(0),
          eof_(false) {
    }
  
    ~SnappyDecompressor() {
      // Advance past any bytes we peeked at from the reader
      reader_->Skip(peeked_);
    }
  
    // Returns true iff we have hit the end of the input without an error.
    bool eof() const {
      return eof_;
    }
  
    // Read the uncompressed length stored at the start of the compressed data.
    // On succcess, stores the length in *result and returns true.
    // On failure, returns false.
    bool ReadUncompressedLength(uint32* result) {
      DCHECK(ip_ == NULL);       // Must not have read anything yet
      // Length is encoded in 1..5 bytes
      *result = 0;
      uint32 shift = 0;
      while (true) {
        if (shift >= 32) return false;
        size_t n;
        const char* ip = reader_->Peek(&n);
        if (n == 0) return false;
        const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip));
        reader_->Skip(1);
        *result |= static_cast<uint32>(c & 0x7f) << shift;
        if (c < 128) {
          break;
        }
        shift += 7;
      }
      return true;
    }
  
    // Process the next item found in the input.
    // Returns true if successful, false on error or end of input.
    template <class Writer>
    void DecompressAllTags(Writer* writer) {
      const char* ip = ip_;
      for ( ;; ) {
        if (ip_limit_ - ip < 5) {
          ip_ = ip;
          if (!RefillTag()) return;
          ip = ip_;
        }
  
        const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip++));
  
        if ((c & 0x3) == LITERAL) {
          uint32 literal_length = c >> 2;
          if (PREDICT_FALSE(literal_length >= 60)) {
            // Long literal.
            const uint32 literal_length_length = literal_length - 59;
            literal_length =
                LittleEndian::Load32(ip) & wordmask[literal_length_length];
            ip += literal_length_length;
          }
          ++literal_length;
  
          uint32 avail = ip_limit_ - ip;
          while (avail < literal_length) {
            if (!writer->Append(ip, avail, false)) return;
            literal_length -= avail;
            reader_->Skip(peeked_);
            size_t n;
            ip = reader_->Peek(&n);
            avail = n;
            peeked_ = avail;
            if (avail == 0) return;  // Premature end of input
            ip_limit_ = ip + avail;
          }
          bool allow_fast_path = (avail >= 16);
          if (!writer->Append(ip, literal_length, allow_fast_path)) {
            return;
          }
          ip += literal_length;
        } else {
          const uint32 entry = char_table[c];
          const uint32 trailer = LittleEndian::Load32(ip) & wordmask[entry >> 11];
          const uint32 length = entry & 0xff;
          ip += entry >> 11;
  
          // copy_offset/256 is encoded in bits 8..10.  By just fetching
          // those bits, we get copy_offset (since the bit-field starts at
          // bit 8).
          const uint32 copy_offset = entry & 0x700;
          if (!writer->AppendFromSelf(copy_offset + trailer, length)) {
            return;
          }
        }
      }
    }
  };
  
  bool SnappyDecompressor::RefillTag() {
    const char* ip = ip_;
    if (ip == ip_limit_) {
      // Fetch a new fragment from the reader
      reader_->Skip(peeked_);   // All peeked bytes are used up
      size_t n;
      ip = reader_->Peek(&n);
      peeked_ = n;
      if (n == 0) {
        eof_ = true;
        return false;
      }
      ip_limit_ = ip + n;
    }
  
    // Read the tag character
    DCHECK_LT(ip, ip_limit_);
    const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip));
    const uint32 entry = char_table[c];
    const uint32 needed = (entry >> 11) + 1;  // +1 byte for 'c'
    DCHECK_LE(needed, sizeof(scratch_));
  
    // Read more bytes from reader if needed
    uint32 nbuf = ip_limit_ - ip;
    if (nbuf < needed) {
      // Stitch together bytes from ip and reader to form the word
      // contents.  We store the needed bytes in "scratch_".  They
      // will be consumed immediately by the caller since we do not
      // read more than we need.
      memmove(scratch_, ip, nbuf);
      reader_->Skip(peeked_);  // All peeked bytes are used up
      peeked_ = 0;
      while (nbuf < needed) {
        size_t length;
        const char* src = reader_->Peek(&length);
        if (length == 0) return false;
        uint32 to_add = min<uint32>(needed - nbuf, length);
        memcpy(scratch_ + nbuf, src, to_add);
        nbuf += to_add;
        reader_->Skip(to_add);
      }
      DCHECK_EQ(nbuf, needed);
      ip_ = scratch_;
      ip_limit_ = scratch_ + needed;
    } else if (nbuf < 5) {
      // Have enough bytes, but move into scratch_ so that we do not
      // read past end of input
      memmove(scratch_, ip, nbuf);
      reader_->Skip(peeked_);  // All peeked bytes are used up
      peeked_ = 0;
      ip_ = scratch_;
      ip_limit_ = scratch_ + nbuf;
    } else {
      // Pass pointer to buffer returned by reader_.
      ip_ = ip;
    }
    return true;
  }
  
  template <typename Writer>
  static bool InternalUncompress(Source* r,
                                 Writer* writer,
                                 uint32 max_len) {
    // Read the uncompressed length from the front of the compressed input
    SnappyDecompressor decompressor(r);
    uint32 uncompressed_len = 0;
    if (!decompressor.ReadUncompressedLength(&uncompressed_len)) return false;
    // Protect against possible DoS attack
    if (static_cast<uint64>(uncompressed_len) > max_len) {
      return false;
    }
  
    writer->SetExpectedLength(uncompressed_len);
  
    // Process the entire input
    decompressor.DecompressAllTags(writer);
    return (decompressor.eof() && writer->CheckLength());
  }
  
  bool GetUncompressedLength(Source* source, uint32* result) {
    SnappyDecompressor decompressor(source);
    return decompressor.ReadUncompressedLength(result);
  }
  
  size_t Compress(Source* reader, Sink* writer) {
    size_t written = 0;
    int N = reader->Available();
    char ulength[Varint::kMax32];
    char* p = Varint::Encode32(ulength, N);
    writer->Append(ulength, p-ulength);
    written += (p - ulength);
  
    internal::WorkingMemory wmem;
    char* scratch = NULL;
    char* scratch_output = NULL;
  
    while (N > 0) {
      // Get next block to compress (without copying if possible)
      size_t fragment_size;
      const char* fragment = reader->Peek(&fragment_size);
      DCHECK_NE(fragment_size, 0) << ": premature end of input";
      const int num_to_read = min(N, kBlockSize);
      size_t bytes_read = fragment_size;
  
      int pending_advance = 0;
      if (bytes_read >= num_to_read) {
        // Buffer returned by reader is large enough
        pending_advance = num_to_read;
        fragment_size = num_to_read;
      } else {
        // Read into scratch buffer
        if (scratch == NULL) {
          // If this is the last iteration, we want to allocate N bytes
          // of space, otherwise the max possible kBlockSize space.
          // num_to_read contains exactly the correct value
          scratch = new char[num_to_read];
        }
        memcpy(scratch, fragment, bytes_read);
        reader->Skip(bytes_read);
  
        while (bytes_read < num_to_read) {
          fragment = reader->Peek(&fragment_size);
          size_t n = min<size_t>(fragment_size, num_to_read - bytes_read);
          memcpy(scratch + bytes_read, fragment, n);
          bytes_read += n;
          reader->Skip(n);
        }
        DCHECK_EQ(bytes_read, num_to_read);
        fragment = scratch;
        fragment_size = num_to_read;
      }
      DCHECK_EQ(fragment_size, num_to_read);
  
      // Get encoding table for compression
      int table_size;
      uint16* table = wmem.GetHashTable(num_to_read, &table_size);
  
      // Compress input_fragment and append to dest
      const int max_output = MaxCompressedLength(num_to_read);
  
      // Need a scratch buffer for the output, in case the byte sink doesn't
      // have room for us directly.
      if (scratch_output == NULL) {
        scratch_output = new char[max_output];
      } else {
        // Since we encode kBlockSize regions followed by a region
        // which is <= kBlockSize in length, a previously allocated
        // scratch_output[] region is big enough for this iteration.
      }
      char* dest = writer->GetAppendBuffer(max_output, scratch_output);
      char* end = internal::CompressFragment(fragment, fragment_size,
                                             dest, table, table_size);
      writer->Append(dest, end - dest);
      written += (end - dest);
  
      N -= num_to_read;
      reader->Skip(pending_advance);
    }
  
    delete[] scratch;
    delete[] scratch_output;
  
    return written;
  }
  
  // -----------------------------------------------------------------------
  // Flat array interfaces
  // -----------------------------------------------------------------------
  
  // A type that writes to a flat array.
  // Note that this is not a "ByteSink", but a type that matches the
  // Writer template argument to SnappyDecompressor::DecompressAllTags().
  class SnappyArrayWriter {
   private:
    char* base_;
    char* op_;
    char* op_limit_;
  
   public:
    inline explicit SnappyArrayWriter(char* dst)
        : base_(dst),
          op_(dst) {
    }
  
    inline void SetExpectedLength(size_t len) {
      op_limit_ = op_ + len;
    }
  
    inline bool CheckLength() const {
      return op_ == op_limit_;
    }
  
    inline bool Append(const char* ip, uint32 len, bool allow_fast_path) {
      char* op = op_;
      const int space_left = op_limit_ - op;
      if (allow_fast_path && len <= 16 && space_left >= 16) {
        // Fast path, used for the majority (about 90%) of dynamic invocations.
        UNALIGNED_STORE64(op, UNALIGNED_LOAD64(ip));
        UNALIGNED_STORE64(op + 8, UNALIGNED_LOAD64(ip + 8));
      } else {
        if (space_left < len) {
          return false;
        }
        memcpy(op, ip, len);
      }
      op_ = op + len;
      return true;
    }
  
    inline bool AppendFromSelf(uint32 offset, uint32 len) {
      char* op = op_;
      const int space_left = op_limit_ - op;
  
      if (op - base_ <= offset - 1u) {  // -1u catches offset==0
        return false;
      }
      if (len <= 16 && offset >= 8 && space_left >= 16) {
        // Fast path, used for the majority (70-80%) of dynamic invocations.
        UNALIGNED_STORE64(op, UNALIGNED_LOAD64(op - offset));
        UNALIGNED_STORE64(op + 8, UNALIGNED_LOAD64(op - offset + 8));
      } else {
        if (space_left >= len + kMaxIncrementCopyOverflow) {
          IncrementalCopyFastPath(op - offset, op, len);
        } else {
          if (space_left < len) {
            return false;
          }
          IncrementalCopy(op - offset, op, len);
        }
      }
  
      op_ = op + len;
      return true;
    }
  };
  
  bool RawUncompress(const char* compressed, size_t n, char* uncompressed) {
    ByteArraySource reader(compressed, n);
    return RawUncompress(&reader, uncompressed);
  }
  
  bool RawUncompress(Source* compressed, char* uncompressed) {
    SnappyArrayWriter output(uncompressed);
    return InternalUncompress(compressed, &output, kuint32max);
  }
  
  bool Uncompress(const char* compressed, size_t n, string* uncompressed) {
    size_t ulength;
    if (!GetUncompressedLength(compressed, n, &ulength)) {
      return false;
    }
    // Protect against possible DoS attack
    if ((static_cast<uint64>(ulength) + uncompressed->size()) >
        uncompressed->max_size()) {
      return false;
    }
    STLStringResizeUninitialized(uncompressed, ulength);
    return RawUncompress(compressed, n, string_as_array(uncompressed));
  }
  
  
  // A Writer that drops everything on the floor and just does validation
  class SnappyDecompressionValidator {
   private:
    size_t expected_;
    size_t produced_;
  
   public:
    inline SnappyDecompressionValidator() : produced_(0) { }
    inline void SetExpectedLength(size_t len) {
      expected_ = len;
    }
    inline bool CheckLength() const {
      return expected_ == produced_;
    }
    inline bool Append(const char* ip, uint32 len, bool allow_fast_path) {
      produced_ += len;
      return produced_ <= expected_;
    }
    inline bool AppendFromSelf(uint32 offset, uint32 len) {
      if (produced_ <= offset - 1u) return false;  // -1u catches offset==0
      produced_ += len;
      return produced_ <= expected_;
    }
  };
  
  bool IsValidCompressedBuffer(const char* compressed, size_t n) {
    ByteArraySource reader(compressed, n);
    SnappyDecompressionValidator writer;
    return InternalUncompress(&reader, &writer, kuint32max);
  }
  
  void RawCompress(const char* input,
                   size_t input_length,
                   char* compressed,
                   size_t* compressed_length) {
    ByteArraySource reader(input, input_length);
    UncheckedByteArraySink writer(compressed);
    Compress(&reader, &writer);
  
    // Compute how many bytes were added
    *compressed_length = (writer.CurrentDestination() - compressed);
  }
  
  size_t Compress(const char* input, size_t input_length, string* compressed) {
    // Pre-grow the buffer to the max length of the compressed output
    compressed->resize(MaxCompressedLength(input_length));
  
    size_t compressed_length;
    RawCompress(input, input_length, string_as_array(compressed),
                &compressed_length);
    compressed->resize(compressed_length);
    return compressed_length;
  }
  
  
  } // end namespace snappy
  
  