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"
  
  #if !defined(SNAPPY_HAVE_SSSE3)
  // __SSSE3__ is defined by GCC and Clang. Visual Studio doesn't target SIMD
  // support between SSE2 and AVX (so SSSE3 instructions require AVX support), and
  // defines __AVX__ when AVX support is available.
  #if defined(__SSSE3__) || defined(__AVX__)
  #define SNAPPY_HAVE_SSSE3 1
  #else
  #define SNAPPY_HAVE_SSSE3 0
  #endif
  #endif  // !defined(SNAPPY_HAVE_SSSE3)
  
  #if !defined(SNAPPY_HAVE_BMI2)
  // __BMI2__ is defined by GCC and Clang. Visual Studio doesn't target BMI2
  // specifically, but it does define __AVX2__ when AVX2 support is available.
  // Fortunately, AVX2 was introduced in Haswell, just like BMI2.
  //
  // BMI2 is not defined as a subset of AVX2 (unlike SSSE3 and AVX above). So,
  // GCC and Clang can build code with AVX2 enabled but BMI2 disabled, in which
  // case issuing BMI2 instructions results in a compiler error.
  #if defined(__BMI2__) || (defined(_MSC_VER) && defined(__AVX2__))
  #define SNAPPY_HAVE_BMI2 1
  #else
  #define SNAPPY_HAVE_BMI2 0
  #endif
  #endif  // !defined(SNAPPY_HAVE_BMI2)
  
  #if SNAPPY_HAVE_SSSE3
  // Please do not replace with <x86intrin.h>. or with headers that assume more
  // advanced SSE versions without checking with all the OWNERS.
  #include <tmmintrin.h>
  #endif
  
  #if SNAPPY_HAVE_BMI2
  // Please do not replace with <x86intrin.h>. or with headers that assume more
  // advanced SSE versions without checking with all the OWNERS.
  #include <immintrin.h>
  #endif
  
  #include <stdio.h>
  
  #include <algorithm>
  #include <string>
  #include <vector>
  #include <util/generic/string.h>
  
  namespace snappy {
  
  using internal::COPY_1_BYTE_OFFSET;
  using internal::COPY_2_BYTE_OFFSET;
  using internal::LITERAL;
  using internal::char_table;
  using internal::kMaximumTagLength;
  
  // 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;
  }
  
  namespace {
  
  void UnalignedCopy64(const void* src, void* dst) {
    char tmp[8];
    memcpy(tmp, src, 8);
    memcpy(dst, tmp, 8);
  }
  
  void UnalignedCopy128(const void* src, void* dst) {
    // memcpy gets vectorized when the appropriate compiler options are used.
    // For example, x86 compilers targeting SSE2+ will optimize to an SSE2 load
    // and store.
    char tmp[16];
    memcpy(tmp, src, 16);
    memcpy(dst, tmp, 16);
  }
  
  // Copy [src, src+(op_limit-op)) to [op, (op_limit-op)) a 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
  //    op_limit  == op + 20
  // After IncrementalCopySlow(src, op, op_limit), the result will have eleven
  // copies of "ab"
  //    ababababababababababab
  // Note that this does not match the semantics of either memcpy() or memmove().
  inline char* IncrementalCopySlow(const char* src, char* op,
                                   char* const op_limit) {
    // TODO: Remove pragma when LLVM is aware this
    // function is only called in cold regions and when cold regions don't get
    // vectorized or unrolled.
  #ifdef __clang__
  #pragma clang loop unroll(disable)
  #endif
    while (op < op_limit) {
      *op++ = *src++;
    }
    return op_limit;
  }
  
  #if SNAPPY_HAVE_SSSE3
  
  // This is a table of shuffle control masks that can be used as the source
  // operand for PSHUFB to permute the contents of the destination XMM register
  // into a repeating byte pattern.
  alignas(16) const char pshufb_fill_patterns[7][16] = {
    {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0},
    {0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1},
    {0, 1, 2, 0, 1, 2, 0, 1, 2, 0, 1, 2, 0, 1, 2, 0},
    {0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3},
    {0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0},
    {0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3},
    {0, 1, 2, 3, 4, 5, 6, 0, 1, 2, 3, 4, 5, 6, 0, 1},
  };
  
  #endif  // SNAPPY_HAVE_SSSE3
  
  // Copy [src, src+(op_limit-op)) to [op, (op_limit-op)) but faster than
  // IncrementalCopySlow. buf_limit is the address past the end of the writable
  // region of the buffer.
  inline char* IncrementalCopy(const char* src, char* op, char* const op_limit,
                               char* const buf_limit) {
    // Terminology:
    //
    // slop = buf_limit - op
    // pat  = op - src
    // len  = limit - op
    assert(src < op);
    assert(op <= op_limit);
    assert(op_limit <= buf_limit);
    // NOTE: The compressor always emits 4 <= len <= 64. It is ok to assume that
    // to optimize this function but we have to also handle other cases in case
    // the input does not satisfy these conditions.
  
    size_t pattern_size = op - src;
    // The cases are split into different branches to allow the branch predictor,
    // FDO, and static prediction hints to work better. For each input we list the
    // ratio of invocations that match each condition.
    //
    // input        slop < 16   pat < 8  len > 16
    // ------------------------------------------
    // html|html4|cp   0%         1.01%    27.73%
    // urls            0%         0.88%    14.79%
    // jpg             0%        64.29%     7.14%
    // pdf             0%         2.56%    58.06%
    // txt[1-4]        0%         0.23%     0.97%
    // pb              0%         0.96%    13.88%
    // bin             0.01%     22.27%    41.17%
    //
    // It is very rare that we don't have enough slop for doing block copies. It
    // is also rare that we need to expand a pattern. Small patterns are common
    // for incompressible formats and for those we are plenty fast already.
    // Lengths are normally not greater than 16 but they vary depending on the
    // input. In general if we always predict len <= 16 it would be an ok
    // prediction.
    //
    // In order to be fast we want a pattern >= 8 bytes and an unrolled loop
    // copying 2x 8 bytes at a time.
  
    // Handle the uncommon case where pattern is less than 8 bytes.
    if (SNAPPY_PREDICT_FALSE(pattern_size < 8)) {
  #if SNAPPY_HAVE_SSSE3
      // Load the first eight bytes into an 128-bit XMM register, then use PSHUFB
      // to permute the register's contents in-place into a repeating sequence of
      // the first "pattern_size" bytes.
      // For example, suppose:
      //    src       == "abc"
      //    op        == op + 3
      // After _mm_shuffle_epi8(), "pattern" will have five copies of "abc"
      // followed by one byte of slop: abcabcabcabcabca.
      //
      // The non-SSE fallback implementation suffers from store-forwarding stalls
      // because its loads and stores partly overlap. By expanding the pattern
      // in-place, we avoid the penalty.
      if (SNAPPY_PREDICT_TRUE(op <= buf_limit - 16)) {
        const __m128i shuffle_mask = _mm_load_si128(
            reinterpret_cast<const __m128i*>(pshufb_fill_patterns)
            + pattern_size - 1);
        const __m128i pattern = _mm_shuffle_epi8(
            _mm_loadl_epi64(reinterpret_cast<const __m128i*>(src)), shuffle_mask);
        // Uninitialized bytes are masked out by the shuffle mask.
        // TODO: remove annotation and macro defs once MSan is fixed.
        SNAPPY_ANNOTATE_MEMORY_IS_INITIALIZED(&pattern, sizeof(pattern));
        pattern_size *= 16 / pattern_size;
        char* op_end = std::min(op_limit, buf_limit - 15);
        while (op < op_end) {
          _mm_storeu_si128(reinterpret_cast<__m128i*>(op), pattern);
          op += pattern_size;
        }
        if (SNAPPY_PREDICT_TRUE(op >= op_limit)) return op_limit;
      }
      return IncrementalCopySlow(src, op, op_limit);
  #else  // !SNAPPY_HAVE_SSSE3
      // If plenty of buffer space remains, expand the pattern to at least 8
      // bytes. The way the following loop is written, we need 8 bytes of buffer
      // space if pattern_size >= 4, 11 bytes if pattern_size is 1 or 3, and 10
      // bytes if pattern_size is 2.  Precisely encoding that is probably not
      // worthwhile; instead, invoke the slow path if we cannot write 11 bytes
      // (because 11 are required in the worst case).
      if (SNAPPY_PREDICT_TRUE(op <= buf_limit - 11)) {
        while (pattern_size < 8) {
          UnalignedCopy64(src, op);
          op += pattern_size;
          pattern_size *= 2;
        }
        if (SNAPPY_PREDICT_TRUE(op >= op_limit)) return op_limit;
      } else {
        return IncrementalCopySlow(src, op, op_limit);
      }
  #endif  // SNAPPY_HAVE_SSSE3
    }
    assert(pattern_size >= 8);
  
    // Copy 2x 8 bytes at a time. Because op - src can be < 16, a single
    // UnalignedCopy128 might overwrite data in op. UnalignedCopy64 is safe
    // because expanding the pattern to at least 8 bytes guarantees that
    // op - src >= 8.
    //
    // Typically, the op_limit is the gating factor so try to simplify the loop
    // based on that.
    if (SNAPPY_PREDICT_TRUE(op_limit <= buf_limit - 16)) {
      // There is at least one, and at most four 16-byte blocks. Writing four
      // conditionals instead of a loop allows FDO to layout the code with respect
      // to the actual probabilities of each length.
      // TODO: Replace with loop with trip count hint.
      UnalignedCopy64(src, op);
      UnalignedCopy64(src + 8, op + 8);
  
      if (op + 16 < op_limit) {
        UnalignedCopy64(src + 16, op + 16);
        UnalignedCopy64(src + 24, op + 24);
      }
      if (op + 32 < op_limit) {
        UnalignedCopy64(src + 32, op + 32);
        UnalignedCopy64(src + 40, op + 40);
      }
      if (op + 48 < op_limit) {
        UnalignedCopy64(src + 48, op + 48);
        UnalignedCopy64(src + 56, op + 56);
      }
      return op_limit;
    }
  
    // Fall back to doing as much as we can with the available slop in the
    // buffer. This code path is relatively cold however so we save code size by
    // avoiding unrolling and vectorizing.
    //
    // TODO: Remove pragma when when cold regions don't get vectorized
    // or unrolled.
  #ifdef __clang__
  #pragma clang loop unroll(disable)
  #endif
    for (char *op_end = buf_limit - 16; op < op_end; op += 16, src += 16) {
      UnalignedCopy64(src, op);
      UnalignedCopy64(src + 8, op + 8);
    }
    if (op >= op_limit)
      return op_limit;
  
    // We only take this branch if we didn't have enough slop and we can do a
    // single 8 byte copy.
    if (SNAPPY_PREDICT_FALSE(op <= buf_limit - 8)) {
      UnalignedCopy64(src, op);
      src += 8;
      op += 8;
    }
    return IncrementalCopySlow(src, op, op_limit);
  }
  
  }  // namespace
  
  template <bool allow_fast_path>
  static inline char* EmitLiteral(char* op,
                                  const char* literal,
                                  int len) {
    // 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).
    assert(len > 0);      // Zero-length literals are disallowed
    int n = len - 1;
    if (allow_fast_path && len <= 16) {
      // Fits in tag byte
      *op++ = LITERAL | (n << 2);
  
      UnalignedCopy128(literal, op);
      return op + len;
    }
  
    if (n < 60) {
      // Fits in tag byte
      *op++ = LITERAL | (n << 2);
    } else {
      int count = (Bits::Log2Floor(n) >> 3) + 1;
      assert(count >= 1);
      assert(count <= 4);
      *op++ = LITERAL | ((59 + count) << 2);
      // Encode in upcoming bytes.
      // Write 4 bytes, though we may care about only 1 of them. The output buffer
      // is guaranteed to have at least 3 more spaces left as 'len >= 61' holds
      // here and there is a memcpy of size 'len' below.
      LittleEndian::Store32(op, n);
      op += count;
    }
    memcpy(op, literal, len);
    return op + len;
  }
  
  template <bool len_less_than_12>
  static inline char* EmitCopyAtMost64(char* op, size_t offset, size_t len) {
    assert(len <= 64);
    assert(len >= 4);
    assert(offset < 65536);
    assert(len_less_than_12 == (len < 12));
  
    if (len_less_than_12 && SNAPPY_PREDICT_TRUE(offset < 2048)) {
      // offset fits in 11 bits.  The 3 highest go in the top of the first byte,
      // and the rest go in the second byte.
      *op++ = COPY_1_BYTE_OFFSET + ((len - 4) << 2) + ((offset >> 3) & 0xe0);
      *op++ = offset & 0xff;
    } else {
      // Write 4 bytes, though we only care about 3 of them.  The output buffer
      // is required to have some slack, so the extra byte won't overrun it.
      uint32 u = COPY_2_BYTE_OFFSET + ((len - 1) << 2) + (offset << 8);
      LittleEndian::Store32(op, u);
      op += 3;
    }
    return op;
  }
  
  template <bool len_less_than_12>
  static inline char* EmitCopy(char* op, size_t offset, size_t len) {
    assert(len_less_than_12 == (len < 12));
    if (len_less_than_12) {
      return EmitCopyAtMost64</*len_less_than_12=*/true>(op, offset, len);
    } else {
      // A special case for len <= 64 might help, but so far measurements suggest
      // it's in the noise.
  
      // Emit 64 byte copies but make sure to keep at least four bytes reserved.
      while (SNAPPY_PREDICT_FALSE(len >= 68)) {
        op = EmitCopyAtMost64</*len_less_than_12=*/false>(op, offset, 64);
        len -= 64;
      }
  
      // One or two copies will now finish the job.
      if (len > 64) {
        op = EmitCopyAtMost64</*len_less_than_12=*/false>(op, offset, 60);
        len -= 60;
      }
  
      // Emit remainder.
      if (len < 12) {
        op = EmitCopyAtMost64</*len_less_than_12=*/true>(op, offset, len);
      } else {
        op = EmitCopyAtMost64</*len_less_than_12=*/false>(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 {
  uint32 CalculateTableSize(uint32 input_size) {
    static_assert(
        kMaxHashTableSize >= kMinHashTableSize,
        "kMaxHashTableSize should be greater or equal to kMinHashTableSize.");
    if (input_size > kMaxHashTableSize) {
      return kMaxHashTableSize;
    }
    if (input_size < kMinHashTableSize) {
      return kMinHashTableSize;
    }
    // This is equivalent to Log2Ceiling(input_size), assuming input_size > 1.
    // 2 << Log2Floor(x - 1) is equivalent to 1 << (1 + Log2Floor(x - 1)).
    return 2u << Bits::Log2Floor(input_size - 1);
  }
  }  // namespace
  
  namespace internal {
  WorkingMemory::WorkingMemory(size_t input_size) {
    const size_t max_fragment_size = std::min(input_size, kBlockSize);
    const size_t table_size = CalculateTableSize(max_fragment_size);
    size_ = table_size * sizeof(*table_) + max_fragment_size +
            MaxCompressedLength(max_fragment_size);
    mem_ = std::allocator<char>().allocate(size_);
    table_ = reinterpret_cast<uint16*>(mem_);
    input_ = mem_ + table_size * sizeof(*table_);
    output_ = input_ + max_fragment_size;
  }
  
  WorkingMemory::~WorkingMemory() {
    std::allocator<char>().deallocate(mem_, size_);
  }
  
  uint16* WorkingMemory::GetHashTable(size_t fragment_size,
                                      int* table_size) const {
    const size_t htsize = CalculateTableSize(fragment_size);
    memset(table_, 0, htsize * sizeof(*table_));
    *table_size = htsize;
    return table_;
  }
  }  // end namespace internal
  
  // For 0 <= offset <= 4, GetUint32AtOffset(GetEightBytesAt(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.
  //
  // We have different versions for 64- and 32-bit; ideally we would avoid the
  // two functions and just inline the UNALIGNED_LOAD64 call into
  // GetUint32AtOffset, but GCC (at least not as of 4.6) is seemingly not clever
  // enough to avoid loading the value multiple times then. For 64-bit, the load
  // is done when GetEightBytesAt() is called, whereas for 32-bit, the load is
  // done at GetUint32AtOffset() time.
  
  #ifdef ARCH_K8
  
  typedef uint64 EightBytesReference;
  
  static inline EightBytesReference GetEightBytesAt(const char* ptr) {
    return UNALIGNED_LOAD64(ptr);
  }
  
  static inline uint32 GetUint32AtOffset(uint64 v, int offset) {
    assert(offset >= 0);
    assert(offset <= 4);
    return v >> (LittleEndian::IsLittleEndian() ? 8 * offset : 32 - 8 * offset);
  }
  
  #else
  
  typedef const char* EightBytesReference;
  
  static inline EightBytesReference GetEightBytesAt(const char* ptr) {
    return ptr;
  }
  
  static inline uint32 GetUint32AtOffset(const char* v, int offset) {
    assert(offset >= 0);
    assert(offset <= 4);
    return UNALIGNED_LOAD32(v + offset);
  }
  
  #endif
  
  // 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;
    assert(input_size <= kBlockSize);
    assert((table_size & (table_size - 1)) == 0);  // table must be power of two
    const int shift = 32 - Bits::Log2Floor(table_size);
    assert(static_cast<int>(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 size_t kInputMarginBytes = 15;
    if (SNAPPY_PREDICT_TRUE(input_size >= kInputMarginBytes)) {
      const char* ip_limit = input + input_size - kInputMarginBytes;
  
      for (uint32 next_hash = Hash(++ip, shift); ; ) {
        assert(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 (or skipped), 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;
          assert(hash == Hash(ip, shift));
          uint32 bytes_between_hash_lookups = skip >> 5;
          skip += bytes_between_hash_lookups;
          next_ip = ip + bytes_between_hash_lookups;
          if (SNAPPY_PREDICT_FALSE(next_ip > ip_limit)) {
            goto emit_remainder;
          }
          next_hash = Hash(next_ip, shift);
          candidate = base_ip + table[hash];
          assert(candidate >= base_ip);
          assert(candidate < ip);
  
          table[hash] = ip - base_ip;
        } while (SNAPPY_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."
        assert(next_emit + 16 <= ip_end);
        op = EmitLiteral</*allow_fast_path=*/true>(op, next_emit, ip - next_emit);
  
        // 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.
        EightBytesReference input_bytes;
        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;
          std::pair<size_t, bool> p =
              FindMatchLength(candidate + 4, ip + 4, ip_end);
          size_t matched = 4 + p.first;
          ip += matched;
          size_t offset = base - candidate;
          assert(0 == memcmp(base, candidate, matched));
          if (p.second) {
            op = EmitCopy</*len_less_than_12=*/true>(op, offset, matched);
          } else {
            op = EmitCopy</*len_less_than_12=*/false>(op, offset, matched);
          }
          next_emit = ip;
          if (SNAPPY_PREDICT_FALSE(ip >= ip_limit)) {
            goto emit_remainder;
          }
          // We are now looking for a 4-byte match again.  We read
          // table[Hash(ip, shift)] for that.  To improve compression,
          // we also update table[Hash(ip - 1, shift)] and table[Hash(ip, shift)].
          input_bytes = GetEightBytesAt(ip - 1);
          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</*allow_fast_path=*/false>(op, next_emit,
                                                  ip_end - next_emit);
    }
  
    return op;
  }
  }  // end namespace internal
  
  // Called back at avery compression call to trace parameters and sizes.
  static inline void Report(const char *algorithm, size_t compressed_size,
                            size_t uncompressed_size) {}
  
  // 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, size_t length);
  //   bool AppendFromSelf(uint32 offset, size_t length);
  //
  //   // The rules for how TryFastAppend differs from Append are somewhat
  //   // convoluted:
  //   //
  //   //  - TryFastAppend is allowed to decline (return false) at any
  //   //    time, for any reason -- just "return false" would be
  //   //    a perfectly legal implementation of TryFastAppend.
  //   //    The intention is for TryFastAppend to allow a fast path
  //   //    in the common case of a small append.
  //   //  - TryFastAppend is allowed to read up to <available> bytes
  //   //    from the input buffer, whereas Append is allowed to read
  //   //    <length>. However, if it returns true, it must leave
  //   //    at least five (kMaximumTagLength) bytes in the input buffer
  //   //    afterwards, so that there is always enough space to read the
  //   //    next tag without checking for a refill.
  //   //  - TryFastAppend must always return decline (return false)
  //   //    if <length> is 61 or more, as in this case the literal length is not
  //   //    decoded fully. In practice, this should not be a big problem,
  //   //    as it is unlikely that one would implement a fast path accepting
  //   //    this much data.
  //   //
  //   bool TryFastAppend(const char* ip, size_t available, size_t length);
  // };
  
  static inline uint32 ExtractLowBytes(uint32 v, int n) {
    assert(n >= 0);
    assert(n <= 4);
  #if SNAPPY_HAVE_BMI2
    return _bzhi_u32(v, 8 * n);
  #else
    // This needs to be wider than uint32 otherwise `mask << 32` will be
    // undefined.
    uint64 mask = 0xffffffff;
    return v & ~(mask << (8 * n));
  #endif
  }
  
  static inline bool LeftShiftOverflows(uint8 value, uint32 shift) {
    assert(shift < 32);
    static const uint8 masks[] = {
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  //
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  //
        0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,  //
        0x00, 0x80, 0xc0, 0xe0, 0xf0, 0xf8, 0xfc, 0xfe};
    return (value & masks[shift]) != 0;
  }
  
  // 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_[kMaximumTagLength];  // See RefillTag().
  
    // 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 success, stores the length in *result and returns true.
    // On failure, returns false.
    bool ReadUncompressedLength(uint32* result) {
      assert(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);
        uint32 val = c & 0x7f;
        if (LeftShiftOverflows(static_cast<uint8>(val), shift)) return false;
        *result |= val << 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>
  #if defined(__GNUC__) && defined(__x86_64__)
    __attribute__((aligned(32)))
  #endif
    void DecompressAllTags(Writer* writer) {
      // In x86, pad the function body to start 16 bytes later. This function has
      // a couple of hotspots that are highly sensitive to alignment: we have
      // observed regressions by more than 20% in some metrics just by moving the
      // exact same code to a different position in the benchmark binary.
      //
      // Putting this code on a 32-byte-aligned boundary + 16 bytes makes us hit
      // the "lucky" case consistently. Unfortunately, this is a very brittle
      // workaround, and future differences in code generation may reintroduce
      // this regression. If you experience a big, difficult to explain, benchmark
      // performance regression here, first try removing this hack.
  #if defined(__GNUC__) && defined(__x86_64__)
      // Two 8-byte "NOP DWORD ptr [EAX + EAX*1 + 00000000H]" instructions.
      asm(".byte 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00");
      asm(".byte 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00");
  #endif
  
      const char* ip = ip_;
      // We could have put this refill fragment only at the beginning of the loop.
      // However, duplicating it at the end of each branch gives the compiler more
      // scope to optimize the <ip_limit_ - ip> expression based on the local
      // context, which overall increases speed.
      #define MAYBE_REFILL() \
          if (ip_limit_ - ip < kMaximumTagLength) { \
            ip_ = ip; \
            if (!RefillTag()) return; \
            ip = ip_; \
          }
  
      MAYBE_REFILL();
      for ( ;; ) {
        const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip++));
  
        // Ratio of iterations that have LITERAL vs non-LITERAL for different
        // inputs.
        //
        // input          LITERAL  NON_LITERAL
        // -----------------------------------
        // html|html4|cp   23%        77%
        // urls            36%        64%
        // jpg             47%        53%
        // pdf             19%        81%
        // txt[1-4]        25%        75%
        // pb              24%        76%
        // bin             24%        76%
        if (SNAPPY_PREDICT_FALSE((c & 0x3) == LITERAL)) {
          size_t literal_length = (c >> 2) + 1u;
          if (writer->TryFastAppend(ip, ip_limit_ - ip, literal_length)) {
            assert(literal_length < 61);
            ip += literal_length;
            // NOTE: There is no MAYBE_REFILL() here, as TryFastAppend()
            // will not return true unless there's already at least five spare
            // bytes in addition to the literal.
            continue;
          }
          if (SNAPPY_PREDICT_FALSE(literal_length >= 61)) {
            // Long literal.
            const size_t literal_length_length = literal_length - 60;
            literal_length =
                ExtractLowBytes(LittleEndian::Load32(ip), literal_length_length) +
                1;
            ip += literal_length_length;
          }
  
          size_t avail = ip_limit_ - ip;
          while (avail < literal_length) {
            if (!writer->Append(ip, avail)) 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;
          }
          if (!writer->Append(ip, literal_length)) {
            return;
          }
          ip += literal_length;
          MAYBE_REFILL();
        } else {
          const size_t entry = char_table[c];
          const size_t trailer =
              ExtractLowBytes(LittleEndian::Load32(ip), entry >> 11);
          const size_t 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 size_t copy_offset = entry & 0x700;
          if (!writer->AppendFromSelf(copy_offset + trailer, length)) {
            return;
          }
          MAYBE_REFILL();
        }
      }
  
  #undef MAYBE_REFILL
    }
  };
  
  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;
      eof_ = (n == 0);
      if (eof_) return false;
      ip_limit_ = ip + n;
    }
  
    // Read the tag character
    assert(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'
    assert(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 = std::min<uint32>(needed - nbuf, length);
        memcpy(scratch_ + nbuf, src, to_add);
        nbuf += to_add;
        reader_->Skip(to_add);
      }
      assert(nbuf == needed);
      ip_ = scratch_;
      ip_limit_ = scratch_ + needed;
    } else if (nbuf < kMaximumTagLength) {
      // 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) {
    // 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;
  
    return InternalUncompressAllTags(&decompressor, writer, r->Available(),
                                     uncompressed_len);
  }
  
  template <typename Writer>
  static bool InternalUncompressAllTags(SnappyDecompressor* decompressor,
                                        Writer* writer,
                                        uint32 compressed_len,
                                        uint32 uncompressed_len) {
    Report("snappy_uncompress", compressed_len, uncompressed_len);
  
    writer->SetExpectedLength(uncompressed_len);
  
    // Process the entire input
    decompressor->DecompressAllTags(writer);
    writer->Flush();
    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;
    size_t N = reader->Available();
    const size_t uncompressed_size = N;
    char ulength[Varint::kMax32];
    char* p = Varint::Encode32(ulength, N);
    writer->Append(ulength, p-ulength);
    written += (p - ulength);
  
    internal::WorkingMemory wmem(N);
  
    while (N > 0) {
      // Get next block to compress (without copying if possible)
      size_t fragment_size;
      const char* fragment = reader->Peek(&fragment_size);
      assert(fragment_size != 0);  // premature end of input
      const size_t num_to_read = std::min(N, kBlockSize);
      size_t bytes_read = fragment_size;
  
      size_t 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 {
        char* scratch = wmem.GetScratchInput();
        memcpy(scratch, fragment, bytes_read);
        reader->Skip(bytes_read);
  
        while (bytes_read < num_to_read) {
          fragment = reader->Peek(&fragment_size);
          size_t n = std::min<size_t>(fragment_size, num_to_read - bytes_read);
          memcpy(scratch + bytes_read, fragment, n);
          bytes_read += n;
          reader->Skip(n);
        }
        assert(bytes_read == num_to_read);
        fragment = scratch;
        fragment_size = num_to_read;
      }
      assert(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.
  
      // 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, wmem.GetScratchOutput());
      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);
    }
  
    Report("snappy_compress", written, uncompressed_size);
  
    return written;
  }
  
  // -----------------------------------------------------------------------
  // IOVec interfaces
  // -----------------------------------------------------------------------
  
  // A type that writes to an iovec.
  // Note that this is not a "ByteSink", but a type that matches the
  // Writer template argument to SnappyDecompressor::DecompressAllTags().
  class SnappyIOVecWriter {
   private:
    // output_iov_end_ is set to iov + count and used to determine when
    // the end of the iovs is reached.
    const struct iovec* output_iov_end_;
  
  #if !defined(NDEBUG)
    const struct iovec* output_iov_;
  #endif  // !defined(NDEBUG)
  
    // Current iov that is being written into.
    const struct iovec* curr_iov_;
  
    // Pointer to current iov's write location.
    char* curr_iov_output_;
  
    // Remaining bytes to write into curr_iov_output.
    size_t curr_iov_remaining_;
  
    // Total bytes decompressed into output_iov_ so far.
    size_t total_written_;
  
    // Maximum number of bytes that will be decompressed into output_iov_.
    size_t output_limit_;
  
    static inline char* GetIOVecPointer(const struct iovec* iov, size_t offset) {
      return reinterpret_cast<char*>(iov->iov_base) + offset;
    }
  
   public:
    // Does not take ownership of iov. iov must be valid during the
    // entire lifetime of the SnappyIOVecWriter.
    inline SnappyIOVecWriter(const struct iovec* iov, size_t iov_count)
        : output_iov_end_(iov + iov_count),
  #if !defined(NDEBUG)
          output_iov_(iov),
  #endif  // !defined(NDEBUG)
          curr_iov_(iov),
          curr_iov_output_(iov_count ? reinterpret_cast<char*>(iov->iov_base)
                                     : nullptr),
          curr_iov_remaining_(iov_count ? iov->iov_len : 0),
          total_written_(0),
          output_limit_(-1) {}
  
    inline void SetExpectedLength(size_t len) {
      output_limit_ = len;
    }
  
    inline bool CheckLength() const {
      return total_written_ == output_limit_;
    }
  
    inline bool Append(const char* ip, size_t len) {
      if (total_written_ + len > output_limit_) {
        return false;
      }
  
      return AppendNoCheck(ip, len);
    }
  
    inline bool AppendNoCheck(const char* ip, size_t len) {
      while (len > 0) {
        if (curr_iov_remaining_ == 0) {
          // This iovec is full. Go to the next one.
          if (curr_iov_ + 1 >= output_iov_end_) {
            return false;
          }
          ++curr_iov_;
          curr_iov_output_ = reinterpret_cast<char*>(curr_iov_->iov_base);
          curr_iov_remaining_ = curr_iov_->iov_len;
        }
  
        const size_t to_write = std::min(len, curr_iov_remaining_);
        memcpy(curr_iov_output_, ip, to_write);
        curr_iov_output_ += to_write;
        curr_iov_remaining_ -= to_write;
        total_written_ += to_write;
        ip += to_write;
        len -= to_write;
      }
  
      return true;
    }
  
    inline bool TryFastAppend(const char* ip, size_t available, size_t len) {
      const size_t space_left = output_limit_ - total_written_;
      if (len <= 16 && available >= 16 + kMaximumTagLength && space_left >= 16 &&
          curr_iov_remaining_ >= 16) {
        // Fast path, used for the majority (about 95%) of invocations.
        UnalignedCopy128(ip, curr_iov_output_);
        curr_iov_output_ += len;
        curr_iov_remaining_ -= len;
        total_written_ += len;
        return true;
      }
  
      return false;
    }
  
    inline bool AppendFromSelf(size_t offset, size_t len) {
      // See SnappyArrayWriter::AppendFromSelf for an explanation of
      // the "offset - 1u" trick.
      if (offset - 1u >= total_written_) {
        return false;
      }
      const size_t space_left = output_limit_ - total_written_;
      if (len > space_left) {
        return false;
      }
  
      // Locate the iovec from which we need to start the copy.
      const iovec* from_iov = curr_iov_;
      size_t from_iov_offset = curr_iov_->iov_len - curr_iov_remaining_;
      while (offset > 0) {
        if (from_iov_offset >= offset) {
          from_iov_offset -= offset;
          break;
        }
  
        offset -= from_iov_offset;
        --from_iov;
  #if !defined(NDEBUG)
        assert(from_iov >= output_iov_);
  #endif  // !defined(NDEBUG)
        from_iov_offset = from_iov->iov_len;
      }
  
      // Copy <len> bytes starting from the iovec pointed to by from_iov_index to
      // the current iovec.
      while (len > 0) {
        assert(from_iov <= curr_iov_);
        if (from_iov != curr_iov_) {
          const size_t to_copy =
              std::min(from_iov->iov_len - from_iov_offset, len);
          AppendNoCheck(GetIOVecPointer(from_iov, from_iov_offset), to_copy);
          len -= to_copy;
          if (len > 0) {
            ++from_iov;
            from_iov_offset = 0;
          }
        } else {
          size_t to_copy = curr_iov_remaining_;
          if (to_copy == 0) {
            // This iovec is full. Go to the next one.
            if (curr_iov_ + 1 >= output_iov_end_) {
              return false;
            }
            ++curr_iov_;
            curr_iov_output_ = reinterpret_cast<char*>(curr_iov_->iov_base);
            curr_iov_remaining_ = curr_iov_->iov_len;
            continue;
          }
          if (to_copy > len) {
            to_copy = len;
          }
  
          IncrementalCopy(GetIOVecPointer(from_iov, from_iov_offset),
                          curr_iov_output_, curr_iov_output_ + to_copy,
                          curr_iov_output_ + curr_iov_remaining_);
          curr_iov_output_ += to_copy;
          curr_iov_remaining_ -= to_copy;
          from_iov_offset += to_copy;
          total_written_ += to_copy;
          len -= to_copy;
        }
      }
  
      return true;
    }
  
    inline void Flush() {}
  };
  
  bool RawUncompressToIOVec(const char* compressed, size_t compressed_length,
                            const struct iovec* iov, size_t iov_cnt) {
    ByteArraySource reader(compressed, compressed_length);
    return RawUncompressToIOVec(&reader, iov, iov_cnt);
  }
  
  bool RawUncompressToIOVec(Source* compressed, const struct iovec* iov,
                            size_t iov_cnt) {
    SnappyIOVecWriter output(iov, iov_cnt);
    return InternalUncompress(compressed, &output);
  }
  
  // -----------------------------------------------------------------------
  // 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),
          op_limit_(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, size_t len) {
      char* op = op_;
      const size_t space_left = op_limit_ - op;
      if (space_left < len) {
        return false;
      }
      memcpy(op, ip, len);
      op_ = op + len;
      return true;
    }
  
    inline bool TryFastAppend(const char* ip, size_t available, size_t len) {
      char* op = op_;
      const size_t space_left = op_limit_ - op;
      if (len <= 16 && available >= 16 + kMaximumTagLength && space_left >= 16) {
        // Fast path, used for the majority (about 95%) of invocations.
        UnalignedCopy128(ip, op);
        op_ = op + len;
        return true;
      } else {
        return false;
      }
    }
  
    inline bool AppendFromSelf(size_t offset, size_t len) {
      char* const op_end = op_ + len;
  
      // Check if we try to append from before the start of the buffer.
      // Normally this would just be a check for "produced < offset",
      // but "produced <= offset - 1u" is equivalent for every case
      // except the one where offset==0, where the right side will wrap around
      // to a very big number. This is convenient, as offset==0 is another
      // invalid case that we also want to catch, so that we do not go
      // into an infinite loop.
      if (Produced() <= offset - 1u || op_end > op_limit_) return false;
      op_ = IncrementalCopy(op_ - offset, op_, op_end, op_limit_);
  
      return true;
    }
    inline size_t Produced() const {
      assert(op_ >= base_);
      return op_ - base_;
    }
    inline void Flush() {}
  };
  
  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);
  }
  
  bool Uncompress(const char* compressed, size_t n, std::string* uncompressed) {
    size_t ulength;
    if (!GetUncompressedLength(compressed, n, &ulength)) {
      return false;
    }
    // On 32-bit builds: max_size() < kuint32max.  Check for that instead
    // of crashing (e.g., consider externally specified compressed data).
    if (ulength > uncompressed->max_size()) {
      return false;
    }
    STLStringResizeUninitialized(uncompressed, ulength);
    return RawUncompress(compressed, n, string_as_array(uncompressed));
  }
  
  bool Uncompress(const char* compressed, size_t n, TString* uncompressed) {
    size_t ulength;
    if (!GetUncompressedLength(compressed, n, &ulength)) {
      return false;
    }
    // On 32-bit builds: max_size() < kuint32max.  Check for that instead
    // of crashing (e.g., consider externally specified compressed data).
    if (ulength > uncompressed->max_size()) {
      return false;
    }
    uncompressed->ReserveAndResize(ulength);
    return RawUncompress(compressed, n, uncompressed->begin());
  }
  
  // A Writer that drops everything on the floor and just does validation
  class SnappyDecompressionValidator {
   private:
    size_t expected_;
    size_t produced_;
  
   public:
    inline SnappyDecompressionValidator() : expected_(0), produced_(0) { }
    inline void SetExpectedLength(size_t len) {
      expected_ = len;
    }
    inline bool CheckLength() const {
      return expected_ == produced_;
    }
    inline bool Append(const char* ip, size_t len) {
      produced_ += len;
      return produced_ <= expected_;
    }
    inline bool TryFastAppend(const char* ip, size_t available, size_t length) {
      return false;
    }
    inline bool AppendFromSelf(size_t offset, size_t len) {
      // See SnappyArrayWriter::AppendFromSelf for an explanation of
      // the "offset - 1u" trick.
      if (produced_ <= offset - 1u) return false;
      produced_ += len;
      return produced_ <= expected_;
    }
    inline void Flush() {}
  };
  
  bool IsValidCompressedBuffer(const char* compressed, size_t n) {
    ByteArraySource reader(compressed, n);
    SnappyDecompressionValidator writer;
    return InternalUncompress(&reader, &writer);
  }
  
  bool IsValidCompressed(Source* compressed) {
    SnappyDecompressionValidator writer;
    return InternalUncompress(compressed, &writer);
  }
  
  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,
                  std::string* compressed) {
    // Pre-grow the buffer to the max length of the compressed output
    STLStringResizeUninitialized(compressed, MaxCompressedLength(input_length));
  
    size_t compressed_length;
    RawCompress(input, input_length, string_as_array(compressed),
                &compressed_length);
    compressed->resize(compressed_length);
    return compressed_length;
  }
  
  size_t Compress(const char* input, size_t input_length,
                  TString* compressed) {
    // Pre-grow the buffer to the max length of the compressed output
    compressed->ReserveAndResize(MaxCompressedLength(input_length));
  
    size_t compressed_length;
    RawCompress(input, input_length, compressed->begin(),
                &compressed_length);
    compressed->resize(compressed_length);
    return compressed_length;
  }
  
  // -----------------------------------------------------------------------
  // Sink interface
  // -----------------------------------------------------------------------
  
  // A type that decompresses into a Sink. The template parameter
  // Allocator must export one method "char* Allocate(int size);", which
  // allocates a buffer of "size" and appends that to the destination.
  template <typename Allocator>
  class SnappyScatteredWriter {
    Allocator allocator_;
  
    // We need random access into the data generated so far.  Therefore
    // we keep track of all of the generated data as an array of blocks.
    // All of the blocks except the last have length kBlockSize.
    std::vector<char*> blocks_;
    size_t expected_;
  
    // Total size of all fully generated blocks so far
    size_t full_size_;
  
    // Pointer into current output block
    char* op_base_;       // Base of output block
    char* op_ptr_;        // Pointer to next unfilled byte in block
    char* op_limit_;      // Pointer just past block
  
    inline size_t Size() const {
      return full_size_ + (op_ptr_ - op_base_);
    }
  
    bool SlowAppend(const char* ip, size_t len);
    bool SlowAppendFromSelf(size_t offset, size_t len);
  
   public:
    inline explicit SnappyScatteredWriter(const Allocator& allocator)
        : allocator_(allocator),
          full_size_(0),
          op_base_(NULL),
          op_ptr_(NULL),
          op_limit_(NULL) {
    }
  
    inline void SetExpectedLength(size_t len) {
      assert(blocks_.empty());
      expected_ = len;
    }
  
    inline bool CheckLength() const {
      return Size() == expected_;
    }
  
    // Return the number of bytes actually uncompressed so far
    inline size_t Produced() const {
      return Size();
    }
  
    inline bool Append(const char* ip, size_t len) {
      size_t avail = op_limit_ - op_ptr_;
      if (len <= avail) {
        // Fast path
        memcpy(op_ptr_, ip, len);
        op_ptr_ += len;
        return true;
      } else {
        return SlowAppend(ip, len);
      }
    }
  
    inline bool TryFastAppend(const char* ip, size_t available, size_t length) {
      char* op = op_ptr_;
      const int space_left = op_limit_ - op;
      if (length <= 16 && available >= 16 + kMaximumTagLength &&
          space_left >= 16) {
        // Fast path, used for the majority (about 95%) of invocations.
        UnalignedCopy128(ip, op);
        op_ptr_ = op + length;
        return true;
      } else {
        return false;
      }
    }
  
    inline bool AppendFromSelf(size_t offset, size_t len) {
      char* const op_end = op_ptr_ + len;
      // See SnappyArrayWriter::AppendFromSelf for an explanation of
      // the "offset - 1u" trick.
      if (SNAPPY_PREDICT_TRUE(offset - 1u < op_ptr_ - op_base_ &&
                            op_end <= op_limit_)) {
        // Fast path: src and dst in current block.
        op_ptr_ = IncrementalCopy(op_ptr_ - offset, op_ptr_, op_end, op_limit_);
        return true;
      }
      return SlowAppendFromSelf(offset, len);
    }
  
    // Called at the end of the decompress. We ask the allocator
    // write all blocks to the sink.
    inline void Flush() { allocator_.Flush(Produced()); }
  };
  
  template<typename Allocator>
  bool SnappyScatteredWriter<Allocator>::SlowAppend(const char* ip, size_t len) {
    size_t avail = op_limit_ - op_ptr_;
    while (len > avail) {
      // Completely fill this block
      memcpy(op_ptr_, ip, avail);
      op_ptr_ += avail;
      assert(op_limit_ - op_ptr_ == 0);
      full_size_ += (op_ptr_ - op_base_);
      len -= avail;
      ip += avail;
  
      // Bounds check
      if (full_size_ + len > expected_) {
        return false;
      }
  
      // Make new block
      size_t bsize = std::min<size_t>(kBlockSize, expected_ - full_size_);
      op_base_ = allocator_.Allocate(bsize);
      op_ptr_ = op_base_;
      op_limit_ = op_base_ + bsize;
      blocks_.push_back(op_base_);
      avail = bsize;
    }
  
    memcpy(op_ptr_, ip, len);
    op_ptr_ += len;
    return true;
  }
  
  template<typename Allocator>
  bool SnappyScatteredWriter<Allocator>::SlowAppendFromSelf(size_t offset,
                                                           size_t len) {
    // Overflow check
    // See SnappyArrayWriter::AppendFromSelf for an explanation of
    // the "offset - 1u" trick.
    const size_t cur = Size();
    if (offset - 1u >= cur) return false;
    if (expected_ - cur < len) return false;
  
    // Currently we shouldn't ever hit this path because Compress() chops the
    // input into blocks and does not create cross-block copies. However, it is
    // nice if we do not rely on that, since we can get better compression if we
    // allow cross-block copies and thus might want to change the compressor in
    // the future.
    size_t src = cur - offset;
    while (len-- > 0) {
      char c = blocks_[src >> kBlockLog][src & (kBlockSize-1)];
      Append(&c, 1);
      src++;
    }
    return true;
  }
  
  class SnappySinkAllocator {
   public:
    explicit SnappySinkAllocator(Sink* dest): dest_(dest) {}
    ~SnappySinkAllocator() {}
  
    char* Allocate(int size) {
      Datablock block(new char[size], size);
      blocks_.push_back(block);
      return block.data;
    }
  
    // We flush only at the end, because the writer wants
    // random access to the blocks and once we hand the
    // block over to the sink, we can't access it anymore.
    // Also we don't write more than has been actually written
    // to the blocks.
    void Flush(size_t size) {
      size_t size_written = 0;
      size_t block_size;
      for (int i = 0; i < blocks_.size(); ++i) {
        block_size = std::min<size_t>(blocks_[i].size, size - size_written);
        dest_->AppendAndTakeOwnership(blocks_[i].data, block_size,
                                      &SnappySinkAllocator::Deleter, NULL);
        size_written += block_size;
      }
      blocks_.clear();
    }
  
   private:
    struct Datablock {
      char* data;
      size_t size;
      Datablock(char* p, size_t s) : data(p), size(s) {}
    };
  
    static void Deleter(void* arg, const char* bytes, size_t size) {
      delete[] bytes;
    }
  
    Sink* dest_;
    std::vector<Datablock> blocks_;
  
    // Note: copying this object is allowed
  };
  
  size_t UncompressAsMuchAsPossible(Source* compressed, Sink* uncompressed) {
    SnappySinkAllocator allocator(uncompressed);
    SnappyScatteredWriter<SnappySinkAllocator> writer(allocator);
    InternalUncompress(compressed, &writer);
    return writer.Produced();
  }
  
  bool Uncompress(Source* compressed, Sink* uncompressed) {
    // Read the uncompressed length from the front of the compressed input
    SnappyDecompressor decompressor(compressed);
    uint32 uncompressed_len = 0;
    if (!decompressor.ReadUncompressedLength(&uncompressed_len)) {
      return false;
    }
  
    char c;
    size_t allocated_size;
    char* buf = uncompressed->GetAppendBufferVariable(
        1, uncompressed_len, &c, 1, &allocated_size);
  
    const size_t compressed_len = compressed->Available();
    // If we can get a flat buffer, then use it, otherwise do block by block
    // uncompression
    if (allocated_size >= uncompressed_len) {
      SnappyArrayWriter writer(buf);
      bool result = InternalUncompressAllTags(&decompressor, &writer,
                                              compressed_len, uncompressed_len);
      uncompressed->Append(buf, writer.Produced());
      return result;
    } else {
      SnappySinkAllocator allocator(uncompressed);
      SnappyScatteredWriter<SnappySinkAllocator> writer(allocator);
      return InternalUncompressAllTags(&decompressor, &writer, compressed_len,
                                       uncompressed_len);
    }
  }
  
  }  // namespace snappy
  