// 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 "butil/third_party/snappy/snappy.h"
#include "butil/third_party/snappy/snappy-internal.h"
#include "butil/third_party/snappy/snappy-sinksource.h"

#include <stdio.h>

#include <algorithm>
#include <string>
#include <vector>

namespace butil {
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_t HashBytes(uint32_t bytes, int shift) {
    uint32_t kMul = 0x1e35a7bd;
    return (bytes * kMul) >> shift;
}
static inline uint32_t 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
};
static const uint32_t kMaximumTagLength = 5;  // COPY_4_BYTE_OFFSET plus the actual offset.

// 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, ssize_t len) {
    assert(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 uint32_t kMaxIncrementCopyOverflow = 10;

inline void IncrementalCopyFastPath(const char* src, char* op, ssize_t len) {
    while (BAIDU_UNLIKELY(op - src < 8)) {
        UnalignedCopy64(src, op);
        len -= op - src;
        op += op - src;
    }
    while (len > 0) {
        UnalignedCopy64(src, op);
        src += 8;
        op += 8;
        len -= 8;
    }
}

}  // namespace

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) {
            UnalignedCopy64(literal, op);
            UnalignedCopy64(literal + 8, op + 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, size_t offset, int len) {
    assert(len <= 64);
    assert(len >= 4);
    assert(offset < 65536);

    if ((len < 12) && (offset < 2048)) {
        size_t 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, size_t offset, int len) {
    // Emit 64 byte copies but make sure to keep at least four bytes reserved
    while (BAIDU_UNLIKELY(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_t v = 0;
    const char* limit = start + n;
    if (Varint::Parse32WithLimit(start, limit, &v) != NULL) {
        *result = v;
        return true;
    } else {
        return false;
    }
}

namespace internal {
uint16_t* 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);
    size_t htsize = 256;
    while (htsize < kMaxHashTableSize && htsize < input_size) {
        htsize <<= 1;
    }

    uint16_t* table;
    if (htsize <= arraysize(small_table_)) {
        table = small_table_;
    } else {
        if (large_table_ == NULL) {
            large_table_ = new uint16_t[kMaxHashTableSize];
        }
        table = large_table_;
    }

    *table_size = htsize;
    memset(table, 0, htsize * sizeof(*table));
    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_t EightBytesReference;

static inline EightBytesReference GetEightBytesAt(const char* ptr) {
    return UNALIGNED_LOAD64(ptr);
}

static inline uint32_t GetUint32AtOffset(uint64_t 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_t 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_t* 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 (BAIDU_LIKELY(input_size >= kInputMarginBytes)) {
        const char* ip_limit = input + input_size - kInputMarginBytes;

        for (uint32_t 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, 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_t skip = 32;

            const char* next_ip = ip;
            const char* candidate;
            do {
                ip = next_ip;
                uint32_t hash = next_hash;
                assert(hash == Hash(ip, shift));
                uint32_t bytes_between_hash_lookups = skip++ >> 5;
                next_ip = ip + bytes_between_hash_lookups;
                if (BAIDU_UNLIKELY(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 (BAIDU_LIKELY(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(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.
            EightBytesReference input_bytes;
            uint32_t 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;
                size_t offset = base - candidate;
                assert(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 (BAIDU_UNLIKELY(ip >= ip_limit)) {
                    goto emit_remainder;
                }
                input_bytes = GetEightBytesAt(insert_tail);
                uint32_t prev_hash = HashBytes(GetUint32AtOffset(input_bytes, 0), shift);
                table[prev_hash] = ip - base_ip - 1;
                uint32_t 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, size_t length);
//   bool AppendFromSelf(uint32_t 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);
// };

// -----------------------------------------------------------------------
// 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_t 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_t 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.
#if 0
DEFINE_bool(snappy_dump_decompression_table, false,
            "If true, we print the decompression table at startup.");

static uint16_t MakeEntry(unsigned int extra,
                          unsigned int len,
                          unsigned int copy_offset) {
    // Check that all of the fields fit within the allocated space
    assert(extra       == (extra & 0x7));          // At most 3 bits
    assert(copy_offset == (copy_offset & 0x7));    // At most 3 bits
    assert(len         == (len & 0x7f));           // At most 7 bits
    return len | (copy_offset << 8) | (extra << 11);
}

static void ALLOW_UNUSED ComputeTable() {
    uint16_t 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.
    if (assigned != 256) {
        fprintf(stderr, "ComputeTable: assigned only %d of 256\n", assigned);
        abort();
    }
    for (int i = 0; i < 256; i++) {
        if (dst[i] == 0xffff) {
            fprintf(stderr, "ComputeTable: did not assign byte %d\n", i);
            abort();
        }
    }

    if (FLAGS_snappy_dump_decompression_table) {
        printf("static const uint16_t 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++) {
        if (dst[i] != char_table[i]) {
            fprintf(stderr, "ComputeTable: byte %d: computed (%x), expect (%x)\n",
                    i, static_cast<int>(dst[i]), static_cast<int>(char_table[i]));
            abort();
        }
    }
}
#endif //disabled

// 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_t        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 succcess, stores the length in *result and returns true.
    // On failure, returns false.
    bool ReadUncompressedLength(uint32_t* result) {
        assert(ip_ == NULL);       // Must not have read anything yet
        // Length is encoded in 1..5 bytes
        *result = 0;
        uint32_t 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_t>(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_;

        // 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++));

            if ((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(user): 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 (BAIDU_UNLIKELY(literal_length >= 61)) {
                    // Long literal.
                    const size_t literal_length_length = literal_length - 60;
                    literal_length =
                        (LittleEndian::Load32(ip) & wordmask[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 uint32_t entry = char_table[c];
                const uint32_t trailer = LittleEndian::Load32(ip) & wordmask[entry >> 11];
                const uint32_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 uint32_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;
        if (n == 0) {
            eof_ = true;
            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_t entry = char_table[c];
    const uint32_t needed = (entry >> 11) + 1;  // +1 byte for 'c'
    assert(needed <= sizeof(scratch_));

    // Read more bytes from reader if needed
    uint32_t 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_t to_add = std::min<uint32_t>(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_t uncompressed_len = 0;
    if (!decompressor.ReadUncompressedLength(&uncompressed_len)) return false;
    return InternalUncompressAllTags(&decompressor, writer, uncompressed_len);
}

template <typename Writer>
static bool InternalUncompressAllTags(SnappyDecompressor* decompressor,
                                      Writer* writer,
                                      uint32_t 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_t* result) {
    SnappyDecompressor decompressor(source);
    return decompressor.ReadUncompressedLength(result);
}

size_t Compress(Source* reader, Sink* writer) {
    size_t written = 0;
    size_t 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);
        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 {
            // 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 = 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_t* 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;
}

// -----------------------------------------------------------------------
// 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:
    const struct iovec* output_iov_;
    const size_t output_iov_count_;

    // We are currently writing into output_iov_[curr_iov_index_].
    int curr_iov_index_;

    // Bytes written to output_iov_[curr_iov_index_] so far.
    size_t curr_iov_written_;

    // 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_;

    inline char* GetIOVecPointer(int index, size_t offset) {
        return reinterpret_cast<char*>(output_iov_[index].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_(iov),
          output_iov_count_(iov_count),
          curr_iov_index_(0),
          curr_iov_written_(0),
          total_written_(0),
          output_limit_((size_t)-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;
        }

        while (len > 0) {
            assert(curr_iov_written_ <= output_iov_[curr_iov_index_].iov_len);
            if (curr_iov_written_ >= output_iov_[curr_iov_index_].iov_len) {
                // This iovec is full. Go to the next one.
                if ((size_t)(curr_iov_index_ + 1) >= output_iov_count_) {
                    return false;
                }
                curr_iov_written_ = 0;
                ++curr_iov_index_;
            }

            const size_t to_write = std::min(
                len, output_iov_[curr_iov_index_].iov_len - curr_iov_written_);
            memcpy(GetIOVecPointer(curr_iov_index_, curr_iov_written_),
                   ip,
                   to_write);
            curr_iov_written_ += 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 &&
            output_iov_[curr_iov_index_].iov_len - curr_iov_written_ >= 16) {
            // Fast path, used for the majority (about 95%) of invocations.
            char* ptr = GetIOVecPointer(curr_iov_index_, curr_iov_written_);
            UnalignedCopy64(ip, ptr);
            UnalignedCopy64(ip + 8, ptr + 8);
            curr_iov_written_ += len;
            total_written_ += len;
            return true;
        }

        return false;
    }

    inline bool AppendFromSelf(size_t offset, size_t len) {
        if (offset > total_written_ || offset == 0) {
            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.
        int from_iov_index = curr_iov_index_;
        size_t from_iov_offset = curr_iov_written_;
        while (offset > 0) {
            if (from_iov_offset >= offset) {
                from_iov_offset -= offset;
                break;
            }

            offset -= from_iov_offset;
            --from_iov_index;
            assert(from_iov_index >= 0);
            from_iov_offset = output_iov_[from_iov_index].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_index <= curr_iov_index_);
            if (from_iov_index != curr_iov_index_) {
                const size_t to_copy = std::min(
                    output_iov_[from_iov_index].iov_len - from_iov_offset,
                    len);
                Append(GetIOVecPointer(from_iov_index, from_iov_offset), to_copy);
                len -= to_copy;
                if (len > 0) {
                    ++from_iov_index;
                    from_iov_offset = 0;
                }
            } else {
                assert(curr_iov_written_ <= output_iov_[curr_iov_index_].iov_len);
                size_t to_copy = std::min(output_iov_[curr_iov_index_].iov_len -
                                          curr_iov_written_,
                                          len);
                if (to_copy == 0) {
                    // This iovec is full. Go to the next one.
                    if ((size_t)(curr_iov_index_ + 1) >= output_iov_count_) {
                        return false;
                    }
                    ++curr_iov_index_;
                    curr_iov_written_ = 0;
                    continue;
                }
                if (to_copy > len) {
                    to_copy = len;
                }
                IncrementalCopy(GetIOVecPointer(from_iov_index, from_iov_offset),
                                GetIOVecPointer(curr_iov_index_, curr_iov_written_),
                                to_copy);
                curr_iov_written_ += 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.
            UnalignedCopy64(ip, op);
            UnalignedCopy64(ip + 8, op + 8);
            op_ = op + len;
            return true;
        } else {
            return false;
        }
    }

    inline bool AppendFromSelf(size_t offset, size_t len) {
        char* op = op_;
        const size_t space_left = op_limit_ - op;

        // 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.
        assert(op >= base_);
        size_t produced = op - base_;
        if (produced <= offset - 1u) {
            return false;
        }
        if (len <= 16 && offset >= 8 && space_left >= 16) {
            // Fast path, used for the majority (70-80%) of dynamic invocations.
            UnalignedCopy64(op - offset, op);
            UnalignedCopy64(op - offset + 8, op + 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;
    }
    inline size_t Produced() const {
        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));
}

// 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
    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;
}

// -----------------------------------------------------------------------
// 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.
            UNALIGNED_STORE64(op, UNALIGNED_LOAD64(ip));
            UNALIGNED_STORE64(op + 8, UNALIGNED_LOAD64(ip + 8));
            op_ptr_ = op + length;
            return true;
        } else {
            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 < (size_t)(op_ptr_ - op_base_)) {
            //               ^ Fix warning in gcc 3.4
            const size_t space_left = op_limit_ - op_ptr_;
            if (space_left >= len + kMaxIncrementCopyOverflow) {
                // Fast path: src and dst in current block.
                IncrementalCopyFastPath(op_ptr_ - offset, op_ptr_, len);
                op_ptr_ += len;
                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 (size_t 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_t 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);

    // 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, uncompressed_len);
        uncompressed->Append(buf, writer.Produced());
        return result;
    } else {
        SnappySinkAllocator allocator(uncompressed);
        SnappyScatteredWriter<SnappySinkAllocator> writer(allocator);
        return InternalUncompressAllTags(&decompressor, &writer, uncompressed_len);
    }
}

} // end namespace snappy
} // end namespace butil
