The Tor Browser: other-licenses/snappy/src/snappy.cc@97036ab72558 (annotated)

other-licenses/snappy/src/snappy.cc@97036ab72558 (annotated)

other-licenses/snappy/src/snappy.cc

Tue, 06 Jan 2015 21:39:09 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Tue, 06 Jan 2015 21:39:09 +0100
branch: TOR_BUG_9701
changeset 8: 97036ab72558
permissions: -rw-r--r--

Conditionally force memory storage according to privacy.thirdparty.isolate;
This solves Tor bug #9701, complying with disk avoidance documented in
https://www.torproject.org/projects/torbrowser/design/#disk-avoidance.

 // Copyright 2005 Google Inc. All Rights Reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 // copyright notice, this list of conditions and the following disclaimer
 // in the documentation and/or other materials provided with the
 // distribution.
 //     * Neither the name of Google Inc. nor the names of its
 // contributors may be used to endorse or promote products derived from
 // this software without specific prior written permission.
 //
 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include "snappy.h"
 #include "snappy-internal.h"
 #include "snappy-sinksource.h"
 #include <stdio.h>
 #include <algorithm>
 #include <string>
 #include <vector>
 namespace snappy {
 // Any hash function will produce a valid compressed bitstream, but a good
 // hash function reduces the number of collisions and thus yields better
 // compression for compressible input, and more speed for incompressible
 // input. Of course, it doesn't hurt if the hash function is reasonably fast
 // either, as it gets called a lot.
 static inline uint32 HashBytes(uint32 bytes, int shift) {
   uint32 kMul = 0x1e35a7bd;
   return (bytes * kMul) >> shift;
 }
 static inline uint32 Hash(const char* p, int shift) {
   return HashBytes(UNALIGNED_LOAD32(p), shift);
 }
 size_t MaxCompressedLength(size_t source_len) {
   // Compressed data can be defined as:
   //    compressed := item* literal*
   //    item       := literal* copy
   //
   // The trailing literal sequence has a space blowup of at most 62/60
   // since a literal of length 60 needs one tag byte + one extra byte
   // for length information.
   //
   // Item blowup is trickier to measure.  Suppose the "copy" op copies
   // 4 bytes of data.  Because of a special check in the encoding code,
   // we produce a 4-byte copy only if the offset is < 65536.  Therefore
   // the copy op takes 3 bytes to encode, and this type of item leads
   // to at most the 62/60 blowup for representing literals.
   //
   // Suppose the "copy" op copies 5 bytes of data.  If the offset is big
   // enough, it will take 5 bytes to encode the copy op.  Therefore the
   // worst case here is a one-byte literal followed by a five-byte copy.
   // I.e., 6 bytes of input turn into 7 bytes of "compressed" data.
   //
   // This last factor dominates the blowup, so the final estimate is:
   return 32 + source_len + source_len/6;
 }
 enum {
   LITERAL = 0,
   COPY_1_BYTE_OFFSET = 1,  // 3 bit length + 3 bits of offset in opcode
   COPY_2_BYTE_OFFSET = 2,
   COPY_4_BYTE_OFFSET = 3
 };
 // Copy "len" bytes from "src" to "op", one byte at a time.  Used for
 // handling COPY operations where the input and output regions may
 // overlap.  For example, suppose:
 //    src    == "ab"
 //    op     == src + 2
 //    len    == 20
 // After IncrementalCopy(src, op, len), the result will have
 // eleven copies of "ab"
 //    ababababababababababab
 // Note that this does not match the semantics of either memcpy()
 // or memmove().
 static inline void IncrementalCopy(const char* src, char* op, int len) {
   DCHECK_GT(len, 0);
   do {
     *op++ = *src++;
   } while (--len > 0);
 }
 // Equivalent to IncrementalCopy except that it can write up to ten extra
 // bytes after the end of the copy, and that it is faster.
 //
 // The main part of this loop is a simple copy of eight bytes at a time until
 // we've copied (at least) the requested amount of bytes.  However, if op and
 // src are less than eight bytes apart (indicating a repeating pattern of
 // length < 8), we first need to expand the pattern in order to get the correct
 // results. For instance, if the buffer looks like this, with the eight-byte
 // <src> and <op> patterns marked as intervals:
 //
 //    abxxxxxxxxxxxx
 //    [------]           src
 //      [------]         op
 //
 // a single eight-byte copy from <src> to <op> will repeat the pattern once,
 // after which we can move <op> two bytes without moving <src>:
 //
 //    ababxxxxxxxxxx
 //    [------]           src
 //        [------]       op
 //
 // and repeat the exercise until the two no longer overlap.
 //
 // This allows us to do very well in the special case of one single byte
 // repeated many times, without taking a big hit for more general cases.
 //
 // The worst case of extra writing past the end of the match occurs when
 // op - src == 1 and len == 1; the last copy will read from byte positions
 // [0..7] and write to [4..11], whereas it was only supposed to write to
 // position 1. Thus, ten excess bytes.
 namespace {
 const int kMaxIncrementCopyOverflow = 10;
 }  // namespace
 static inline void IncrementalCopyFastPath(const char* src, char* op, int len) {
   while (op - src < 8) {
     UNALIGNED_STORE64(op, UNALIGNED_LOAD64(src));
     len -= op - src;
     op += op - src;
   }
   while (len > 0) {
     UNALIGNED_STORE64(op, UNALIGNED_LOAD64(src));
     src += 8;
     op += 8;
     len -= 8;
   }
 }
 static inline char* EmitLiteral(char* op,
                                 const char* literal,
                                 int len,
                                 bool allow_fast_path) {
   int n = len - 1;      // Zero-length literals are disallowed
   if (n < 60) {
     // Fits in tag byte
     *op++ = LITERAL | (n << 2);
     // The vast majority of copies are below 16 bytes, for which a
     // call to memcpy is overkill. This fast path can sometimes
     // copy up to 15 bytes too much, but that is okay in the
     // main loop, since we have a bit to go on for both sides:
     //
     //   - The input will always have kInputMarginBytes = 15 extra
     //     available bytes, as long as we're in the main loop, and
     //     if not, allow_fast_path = false.
     //   - The output will always have 32 spare bytes (see
     //     MaxCompressedLength).
     if (allow_fast_path && len <= 16) {
       UNALIGNED_STORE64(op, UNALIGNED_LOAD64(literal));
       UNALIGNED_STORE64(op + 8, UNALIGNED_LOAD64(literal + 8));
       return op + len;
     }
   } else {
     // Encode in upcoming bytes
     char* base = op;
     int count = 0;
     op++;
     while (n > 0) {
       *op++ = n & 0xff;
       n >>= 8;
       count++;
     }
     assert(count >= 1);
     assert(count <= 4);
     *base = LITERAL | ((59+count) << 2);
   }
   memcpy(op, literal, len);
   return op + len;
 }
 static inline char* EmitCopyLessThan64(char* op, size_t offset, int len) {
   DCHECK_LE(len, 64);
   DCHECK_GE(len, 4);
   DCHECK_LT(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 (len >= 68) {
     op = EmitCopyLessThan64(op, offset, 64);
     len -= 64;
   }
   // Emit an extra 60 byte copy if have too much data to fit in one copy
   if (len > 64) {
     op = EmitCopyLessThan64(op, offset, 60);
     len -= 60;
   }
   // Emit remainder
   op = EmitCopyLessThan64(op, offset, len);
   return op;
 }
 bool GetUncompressedLength(const char* start, size_t n, size_t* result) {
   uint32 v = 0;
   const char* limit = start + n;
   if (Varint::Parse32WithLimit(start, limit, &v) != NULL) {
     *result = v;
     return true;
   } else {
     return false;
   }
 }
 namespace internal {
 uint16* WorkingMemory::GetHashTable(size_t input_size, int* table_size) {
   // Use smaller hash table when input.size() is smaller, since we
   // fill the table, incurring O(hash table size) overhead for
   // compression, and if the input is short, we won't need that
   // many hash table entries anyway.
   assert(kMaxHashTableSize >= 256);
   size_t htsize = 256;
   while (htsize < kMaxHashTableSize && htsize < input_size) {
     htsize <<= 1;
   }
   CHECK_EQ(0, htsize & (htsize - 1)) << ": must be power of two";
   CHECK_LE(htsize, kMaxHashTableSize) << ": hash table too large";
   uint16* table;
   if (htsize <= ARRAYSIZE(small_table_)) {
     table = small_table_;
   } else {
     if (large_table_ == NULL) {
       large_table_ = new uint16[kMaxHashTableSize];
     }
     table = large_table_;
   }
   *table_size = htsize;
   memset(table, 0, htsize * sizeof(*table));
   return table;
 }
 }  // end namespace internal
 // For 0 <= offset <= 4, GetUint32AtOffset(UNALIGNED_LOAD64(p), offset) will
 // equal UNALIGNED_LOAD32(p + offset).  Motivation: On x86-64 hardware we have
 // empirically found that overlapping loads such as
 //  UNALIGNED_LOAD32(p) ... UNALIGNED_LOAD32(p+1) ... UNALIGNED_LOAD32(p+2)
 // are slower than UNALIGNED_LOAD64(p) followed by shifts and casts to uint32.
 static inline uint32 GetUint32AtOffset(uint64 v, int offset) {
   DCHECK(0 <= offset && offset <= 4) << offset;
   return v >> (LittleEndian::IsLittleEndian() ? 8 * offset : 32 - 8 * offset);
 }
 // Flat array compression that does not emit the "uncompressed length"
 // prefix. Compresses "input" string to the "*op" buffer.
 //
 // REQUIRES: "input" is at most "kBlockSize" bytes long.
 // REQUIRES: "op" points to an array of memory that is at least
 // "MaxCompressedLength(input.size())" in size.
 // REQUIRES: All elements in "table[0..table_size-1]" are initialized to zero.
 // REQUIRES: "table_size" is a power of two
 //
 // Returns an "end" pointer into "op" buffer.
 // "end - op" is the compressed size of "input".
 namespace internal {
 char* CompressFragment(const char* input,
                        size_t input_size,
                        char* op,
                        uint16* table,
                        const int table_size) {
   // "ip" is the input pointer, and "op" is the output pointer.
   const char* ip = input;
   CHECK_LE(input_size, kBlockSize);
   CHECK_EQ(table_size & (table_size - 1), 0) << ": table must be power of two";
   const int shift = 32 - Bits::Log2Floor(table_size);
   DCHECK_EQ(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 (PREDICT_TRUE(input_size >= kInputMarginBytes)) {
     const char* ip_limit = input + input_size - kInputMarginBytes;
     for (uint32 next_hash = Hash(++ip, shift); ; ) {
       DCHECK_LT(next_emit, ip);
       // The body of this loop calls EmitLiteral once and then EmitCopy one or
       // more times.  (The exception is that when we're close to exhausting
       // the input we goto emit_remainder.)
       //
       // In the first iteration of this loop we're just starting, so
       // there's nothing to copy, so calling EmitLiteral once is
       // necessary.  And we only start a new iteration when the
       // current iteration has determined that a call to EmitLiteral will
       // precede the next call to EmitCopy (if any).
       //
       // Step 1: Scan forward in the input looking for a 4-byte-long match.
       // If we get close to exhausting the input then goto emit_remainder.
       //
       // Heuristic match skipping: If 32 bytes are scanned with no matches
       // found, start looking only at every other byte. If 32 more bytes are
       // scanned, look at every third byte, etc.. When a match is found,
       // immediately go back to looking at every byte. This is a small loss
       // (~5% performance, ~0.1% density) for compressible data due to more
       // bookkeeping, but for non-compressible data (such as JPEG) it's a huge
       // win since the compressor quickly "realizes" the data is incompressible
       // and doesn't bother looking for matches everywhere.
       //
       // The "skip" variable keeps track of how many bytes there are since the
       // last match; dividing it by 32 (ie. right-shifting by five) gives the
       // number of bytes to move ahead for each iteration.
       uint32 skip = 32;
       const char* next_ip = ip;
       const char* candidate;
       do {
         ip = next_ip;
         uint32 hash = next_hash;
         DCHECK_EQ(hash, Hash(ip, shift));
         uint32 bytes_between_hash_lookups = skip++ >> 5;
         next_ip = ip + bytes_between_hash_lookups;
         if (PREDICT_FALSE(next_ip > ip_limit)) {
           goto emit_remainder;
         }
         next_hash = Hash(next_ip, shift);
         candidate = base_ip + table[hash];
         DCHECK_GE(candidate, base_ip);
         DCHECK_LT(candidate, ip);
         table[hash] = ip - base_ip;
       } while (PREDICT_TRUE(UNALIGNED_LOAD32(ip) !=
                             UNALIGNED_LOAD32(candidate)));
       // Step 2: A 4-byte match has been found.  We'll later see if more
       // than 4 bytes match.  But, prior to the match, input
       // bytes [next_emit, ip) are unmatched.  Emit them as "literal bytes."
       DCHECK_LE(next_emit + 16, ip_end);
       op = EmitLiteral(op, next_emit, ip - next_emit, true);
       // Step 3: Call EmitCopy, and then see if another EmitCopy could
       // be our next move.  Repeat until we find no match for the
       // input immediately after what was consumed by the last EmitCopy call.
       //
       // If we exit this loop normally then we need to call EmitLiteral next,
       // though we don't yet know how big the literal will be.  We handle that
       // by proceeding to the next iteration of the main loop.  We also can exit
       // this loop via goto if we get close to exhausting the input.
       uint64 input_bytes = 0;
       uint32 candidate_bytes = 0;
       do {
         // We have a 4-byte match at ip, and no need to emit any
         // "literal bytes" prior to ip.
         const char* base = ip;
         int matched = 4 + FindMatchLength(candidate + 4, ip + 4, ip_end);
         ip += matched;
         size_t offset = base - candidate;
         DCHECK_EQ(0, memcmp(base, candidate, matched));
         op = EmitCopy(op, offset, matched);
         // We could immediately start working at ip now, but to improve
         // compression we first update table[Hash(ip - 1, ...)].
         const char* insert_tail = ip - 1;
         next_emit = ip;
         if (PREDICT_FALSE(ip >= ip_limit)) {
           goto emit_remainder;
         }
         input_bytes = UNALIGNED_LOAD64(insert_tail);
         uint32 prev_hash = HashBytes(GetUint32AtOffset(input_bytes, 0), shift);
         table[prev_hash] = ip - base_ip - 1;
         uint32 cur_hash = HashBytes(GetUint32AtOffset(input_bytes, 1), shift);
         candidate = base_ip + table[cur_hash];
         candidate_bytes = UNALIGNED_LOAD32(candidate);
         table[cur_hash] = ip - base_ip;
       } while (GetUint32AtOffset(input_bytes, 1) == candidate_bytes);
       next_hash = HashBytes(GetUint32AtOffset(input_bytes, 2), shift);
       ++ip;
     }
   }
  emit_remainder:
   // Emit the remaining bytes as a literal
   if (next_emit < ip_end) {
     op = EmitLiteral(op, next_emit, ip_end - next_emit, false);
   }
   return op;
 }
 }  // end namespace internal
 // Signature of output types needed by decompression code.
 // The decompression code is templatized on a type that obeys this
 // signature so that we do not pay virtual function call overhead in
 // the middle of a tight decompression loop.
 //
 // class DecompressionWriter {
 //  public:
 //   // Called before decompression
 //   void SetExpectedLength(size_t length);
 //
 //   // Called after decompression
 //   bool CheckLength() const;
 //
 //   // Called repeatedly during decompression
 //   bool Append(const char* ip, size_t length);
 //   bool AppendFromSelf(uint32 offset, size_t length);
 //
 //   // The difference between TryFastAppend and Append is that TryFastAppend
 //   // is allowed to read up to <available> bytes from the input buffer,
 //   // whereas Append is allowed to read <length>.
 //   //
 //   // Also, TryFastAppend is allowed to return false, declining the append,
 //   // without it being a fatal error -- 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.
 //   //
 //   // NOTE(user): 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 wordmask[] = {
 u, 0xffu, 0xffffu, 0xffffffu, 0xffffffffu
 };
 // Data stored per entry in lookup table:
 //      Range   Bits-used       Description
 //      ------------------------------------
 //      1..64   0..7            Literal/copy length encoded in opcode byte
 //      0..7    8..10           Copy offset encoded in opcode byte / 256
 //      0..4    11..13          Extra bytes after opcode
 //
 // We use eight bits for the length even though 7 would have sufficed
 // because of efficiency reasons:
 //      (1) Extracting a byte is faster than a bit-field
 //      (2) It properly aligns copy offset so we do not need a <<8
 static const uint16 char_table[256] = {
 x0001, 0x0804, 0x1001, 0x2001, 0x0002, 0x0805, 0x1002, 0x2002,
 x0003, 0x0806, 0x1003, 0x2003, 0x0004, 0x0807, 0x1004, 0x2004,
 x0005, 0x0808, 0x1005, 0x2005, 0x0006, 0x0809, 0x1006, 0x2006,
 x0007, 0x080a, 0x1007, 0x2007, 0x0008, 0x080b, 0x1008, 0x2008,
 x0009, 0x0904, 0x1009, 0x2009, 0x000a, 0x0905, 0x100a, 0x200a,
 x000b, 0x0906, 0x100b, 0x200b, 0x000c, 0x0907, 0x100c, 0x200c,
 x000d, 0x0908, 0x100d, 0x200d, 0x000e, 0x0909, 0x100e, 0x200e,
 x000f, 0x090a, 0x100f, 0x200f, 0x0010, 0x090b, 0x1010, 0x2010,
 x0011, 0x0a04, 0x1011, 0x2011, 0x0012, 0x0a05, 0x1012, 0x2012,
 x0013, 0x0a06, 0x1013, 0x2013, 0x0014, 0x0a07, 0x1014, 0x2014,
 x0015, 0x0a08, 0x1015, 0x2015, 0x0016, 0x0a09, 0x1016, 0x2016,
 x0017, 0x0a0a, 0x1017, 0x2017, 0x0018, 0x0a0b, 0x1018, 0x2018,
 x0019, 0x0b04, 0x1019, 0x2019, 0x001a, 0x0b05, 0x101a, 0x201a,
 x001b, 0x0b06, 0x101b, 0x201b, 0x001c, 0x0b07, 0x101c, 0x201c,
 x001d, 0x0b08, 0x101d, 0x201d, 0x001e, 0x0b09, 0x101e, 0x201e,
 x001f, 0x0b0a, 0x101f, 0x201f, 0x0020, 0x0b0b, 0x1020, 0x2020,
 x0021, 0x0c04, 0x1021, 0x2021, 0x0022, 0x0c05, 0x1022, 0x2022,
 x0023, 0x0c06, 0x1023, 0x2023, 0x0024, 0x0c07, 0x1024, 0x2024,
 x0025, 0x0c08, 0x1025, 0x2025, 0x0026, 0x0c09, 0x1026, 0x2026,
 x0027, 0x0c0a, 0x1027, 0x2027, 0x0028, 0x0c0b, 0x1028, 0x2028,
 x0029, 0x0d04, 0x1029, 0x2029, 0x002a, 0x0d05, 0x102a, 0x202a,
 x002b, 0x0d06, 0x102b, 0x202b, 0x002c, 0x0d07, 0x102c, 0x202c,
 x002d, 0x0d08, 0x102d, 0x202d, 0x002e, 0x0d09, 0x102e, 0x202e,
 x002f, 0x0d0a, 0x102f, 0x202f, 0x0030, 0x0d0b, 0x1030, 0x2030,
 x0031, 0x0e04, 0x1031, 0x2031, 0x0032, 0x0e05, 0x1032, 0x2032,
 x0033, 0x0e06, 0x1033, 0x2033, 0x0034, 0x0e07, 0x1034, 0x2034,
 x0035, 0x0e08, 0x1035, 0x2035, 0x0036, 0x0e09, 0x1036, 0x2036,
 x0037, 0x0e0a, 0x1037, 0x2037, 0x0038, 0x0e0b, 0x1038, 0x2038,
 x0039, 0x0f04, 0x1039, 0x2039, 0x003a, 0x0f05, 0x103a, 0x203a,
 x003b, 0x0f06, 0x103b, 0x203b, 0x003c, 0x0f07, 0x103c, 0x203c,
 x0801, 0x0f08, 0x103d, 0x203d, 0x1001, 0x0f09, 0x103e, 0x203e,
 x1801, 0x0f0a, 0x103f, 0x203f, 0x2001, 0x0f0b, 0x1040, 0x2040
 };
 // In debug mode, allow optional computation of the table at startup.
 // Also, check that the decompression table is correct.
 #ifndef NDEBUG
 DEFINE_bool(snappy_dump_decompression_table, false,
             "If true, we print the decompression table at startup.");
 static uint16 MakeEntry(unsigned int extra,
                         unsigned int len,
                         unsigned int copy_offset) {
   // Check that all of the fields fit within the allocated space
   DCHECK_EQ(extra,       extra & 0x7);          // At most 3 bits
   DCHECK_EQ(copy_offset, copy_offset & 0x7);    // At most 3 bits
   DCHECK_EQ(len,         len & 0x7f);           // At most 7 bits
   return len | (copy_offset << 8) | (extra << 11);
 }
 static void ComputeTable() {
   uint16 dst[256];
   // Place invalid entries in all places to detect missing initialization
   int assigned = 0;
   for (int i = 0; i < 256; i++) {
     dst[i] = 0xffff;
   }
   // Small LITERAL entries.  We store (len-1) in the top 6 bits.
   for (unsigned int len = 1; len <= 60; len++) {
     dst[LITERAL | ((len-1) << 2)] = MakeEntry(0, len, 0);
     assigned++;
   }
   // Large LITERAL entries.  We use 60..63 in the high 6 bits to
   // encode the number of bytes of length info that follow the opcode.
   for (unsigned int extra_bytes = 1; extra_bytes <= 4; extra_bytes++) {
     // We set the length field in the lookup table to 1 because extra
     // bytes encode len-1.
     dst[LITERAL | ((extra_bytes+59) << 2)] = MakeEntry(extra_bytes, 1, 0);
     assigned++;
   }
   // COPY_1_BYTE_OFFSET.
   //
   // The tag byte in the compressed data stores len-4 in 3 bits, and
   // offset/256 in 5 bits.  offset%256 is stored in the next byte.
   //
   // This format is used for length in range [4..11] and offset in
   // range [0..2047]
   for (unsigned int len = 4; len < 12; len++) {
     for (unsigned int offset = 0; offset < 2048; offset += 256) {
       dst[COPY_1_BYTE_OFFSET | ((len-4)<<2) | ((offset>>8)<<5)] =
         MakeEntry(1, len, offset>>8);
       assigned++;
     }
   }
   // COPY_2_BYTE_OFFSET.
   // Tag contains len-1 in top 6 bits, and offset in next two bytes.
   for (unsigned int len = 1; len <= 64; len++) {
     dst[COPY_2_BYTE_OFFSET | ((len-1)<<2)] = MakeEntry(2, len, 0);
     assigned++;
   }
   // COPY_4_BYTE_OFFSET.
   // Tag contents len-1 in top 6 bits, and offset in next four bytes.
   for (unsigned int len = 1; len <= 64; len++) {
     dst[COPY_4_BYTE_OFFSET | ((len-1)<<2)] = MakeEntry(4, len, 0);
     assigned++;
   }
   // Check that each entry was initialized exactly once.
   CHECK_EQ(assigned, 256);
   for (int i = 0; i < 256; i++) {
     CHECK_NE(dst[i], 0xffff);
   }
   if (FLAGS_snappy_dump_decompression_table) {
     printf("static const uint16 char_table[256] = {\n  ");
     for (int i = 0; i < 256; i++) {
       printf("0x%04x%s",
              dst[i],
              ((i == 255) ? "\n" : (((i%8) == 7) ? ",\n  " : ", ")));
     }
     printf("};\n");
   }
   // Check that computed table matched recorded table
   for (int i = 0; i < 256; i++) {
     CHECK_EQ(dst[i], char_table[i]);
   }
 }
 #endif /* !NDEBUG */
 // Helper class for decompression
 class SnappyDecompressor {
  private:
   Source*       reader_;         // Underlying source of bytes to decompress
   const char*   ip_;             // Points to next buffered byte
   const char*   ip_limit_;       // Points just past buffered bytes
   uint32        peeked_;         // Bytes peeked from reader (need to skip)
   bool          eof_;            // Hit end of input without an error?
   char          scratch_[5];     // Temporary buffer for PeekFast() boundaries
   // Ensure that all of the tag metadata for the next tag is available
   // in [ip_..ip_limit_-1].  Also ensures that [ip,ip+4] is readable even
   // if (ip_limit_ - ip_ < 5).
   //
   // Returns true on success, false on error or end of input.
   bool RefillTag();
  public:
   explicit SnappyDecompressor(Source* reader)
       : reader_(reader),
         ip_(NULL),
         ip_limit_(NULL),
         peeked_(0),
         eof_(false) {
   }
   ~SnappyDecompressor() {
     // Advance past any bytes we peeked at from the reader
     reader_->Skip(peeked_);
   }
   // Returns true iff we have hit the end of the input without an error.
   bool eof() const {
     return eof_;
   }
   // Read the uncompressed length stored at the start of the compressed data.
   // On succcess, stores the length in *result and returns true.
   // On failure, returns false.
   bool ReadUncompressedLength(uint32* result) {
     DCHECK(ip_ == NULL);       // Must not have read anything yet
     // Length is encoded in 1..5 bytes
     *result = 0;
     uint32 shift = 0;
     while (true) {
       if (shift >= 32) return false;
       size_t n;
       const char* ip = reader_->Peek(&n);
       if (n == 0) return false;
       const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip));
       reader_->Skip(1);
       *result |= static_cast<uint32>(c & 0x7f) << shift;
       if (c < 128) {
         break;
       }
       shift += 7;
     }
     return true;
   }
   // Process the next item found in the input.
   // Returns true if successful, false on error or end of input.
   template <class Writer>
   void DecompressAllTags(Writer* writer) {
     const char* ip = ip_;
     // 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 < 5) { \
           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)) {
           DCHECK_LT(literal_length, 61);
           ip += literal_length;
           MAYBE_REFILL();
           continue;
         }
         if (PREDICT_FALSE(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 entry = char_table[c];
         const uint32 trailer = LittleEndian::Load32(ip) & wordmask[entry >> 11];
         const uint32 length = entry & 0xff;
         ip += entry >> 11;
         // copy_offset/256 is encoded in bits 8..10.  By just fetching
         // those bits, we get copy_offset (since the bit-field starts at
         // bit 8).
         const uint32 copy_offset = entry & 0x700;
         if (!writer->AppendFromSelf(copy_offset + trailer, length)) {
           return;
         }
         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
   DCHECK_LT(ip, ip_limit_);
   const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip));
   const uint32 entry = char_table[c];
   const uint32 needed = (entry >> 11) + 1;  // +1 byte for 'c'
   DCHECK_LE(needed, sizeof(scratch_));
   // Read more bytes from reader if needed
   uint32 nbuf = ip_limit_ - ip;
   if (nbuf < needed) {
     // Stitch together bytes from ip and reader to form the word
     // contents.  We store the needed bytes in "scratch_".  They
     // will be consumed immediately by the caller since we do not
     // read more than we need.
     memmove(scratch_, ip, nbuf);
     reader_->Skip(peeked_);  // All peeked bytes are used up
     peeked_ = 0;
     while (nbuf < needed) {
       size_t length;
       const char* src = reader_->Peek(&length);
       if (length == 0) return false;
       uint32 to_add = min<uint32>(needed - nbuf, length);
       memcpy(scratch_ + nbuf, src, to_add);
       nbuf += to_add;
       reader_->Skip(to_add);
     }
     DCHECK_EQ(nbuf, needed);
     ip_ = scratch_;
     ip_limit_ = scratch_ + needed;
   } else if (nbuf < 5) {
     // Have enough bytes, but move into scratch_ so that we do not
     // read past end of input
     memmove(scratch_, ip, nbuf);
     reader_->Skip(peeked_);  // All peeked bytes are used up
     peeked_ = 0;
     ip_ = scratch_;
     ip_limit_ = scratch_ + nbuf;
   } else {
     // Pass pointer to buffer returned by reader_.
     ip_ = ip;
   }
   return true;
 }
 template <typename Writer>
 static bool InternalUncompress(Source* r,
                                Writer* writer,
                                uint32 max_len) {
   // Read the uncompressed length from the front of the compressed input
   SnappyDecompressor decompressor(r);
   uint32 uncompressed_len = 0;
   if (!decompressor.ReadUncompressedLength(&uncompressed_len)) return false;
   // Protect against possible DoS attack
   if (static_cast<uint64>(uncompressed_len) > max_len) {
     return false;
   }
   writer->SetExpectedLength(uncompressed_len);
   // Process the entire input
   decompressor.DecompressAllTags(writer);
   return (decompressor.eof() && writer->CheckLength());
 }
 bool GetUncompressedLength(Source* source, uint32* result) {
   SnappyDecompressor decompressor(source);
   return decompressor.ReadUncompressedLength(result);
 }
 size_t Compress(Source* reader, Sink* writer) {
   size_t written = 0;
   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);
     DCHECK_NE(fragment_size, 0) << ": premature end of input";
     const size_t num_to_read = 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 = min<size_t>(fragment_size, num_to_read - bytes_read);
         memcpy(scratch + bytes_read, fragment, n);
         bytes_read += n;
         reader->Skip(n);
       }
       DCHECK_EQ(bytes_read, num_to_read);
       fragment = scratch;
       fragment_size = num_to_read;
     }
     DCHECK_EQ(fragment_size, num_to_read);
     // Get encoding table for compression
     int table_size;
     uint16* table = wmem.GetHashTable(num_to_read, &table_size);
     // Compress input_fragment and append to dest
     const int max_output = MaxCompressedLength(num_to_read);
     // Need a scratch buffer for the output, in case the byte sink doesn't
     // have room for us directly.
     if (scratch_output == NULL) {
       scratch_output = new char[max_output];
     } else {
       // Since we encode kBlockSize regions followed by a region
       // which is <= kBlockSize in length, a previously allocated
       // scratch_output[] region is big enough for this iteration.
     }
     char* dest = writer->GetAppendBuffer(max_output, scratch_output);
     char* end = internal::CompressFragment(fragment, fragment_size,
                                            dest, table, table_size);
     writer->Append(dest, end - dest);
     written += (end - dest);
     N -= num_to_read;
     reader->Skip(pending_advance);
   }
   delete[] scratch;
   delete[] scratch_output;
   return written;
 }
 // -----------------------------------------------------------------------
 // Flat array interfaces
 // -----------------------------------------------------------------------
 // A type that writes to a flat array.
 // Note that this is not a "ByteSink", but a type that matches the
 // Writer template argument to SnappyDecompressor::DecompressAllTags().
 class SnappyArrayWriter {
  private:
   char* base_;
   char* op_;
   char* op_limit_;
  public:
   inline explicit SnappyArrayWriter(char* dst)
       : base_(dst),
         op_(dst) {
   }
   inline void SetExpectedLength(size_t len) {
     op_limit_ = op_ + len;
   }
   inline bool CheckLength() const {
     return op_ == op_limit_;
   }
   inline bool Append(const char* ip, 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 && 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_ = 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;
     if (op - base_ <= offset - 1u) {  // -1u catches offset==0
       return false;
     }
     if (len <= 16 && offset >= 8 && space_left >= 16) {
       // Fast path, used for the majority (70-80%) of dynamic invocations.
       UNALIGNED_STORE64(op, UNALIGNED_LOAD64(op - offset));
       UNALIGNED_STORE64(op + 8, UNALIGNED_LOAD64(op - offset + 8));
     } else {
       if (space_left >= len + kMaxIncrementCopyOverflow) {
         IncrementalCopyFastPath(op - offset, op, len);
       } else {
         if (space_left < len) {
           return false;
         }
         IncrementalCopy(op - offset, op, len);
       }
     }
     op_ = op + len;
     return true;
   }
 };
 bool RawUncompress(const char* compressed, size_t n, char* uncompressed) {
   ByteArraySource reader(compressed, n);
   return RawUncompress(&reader, uncompressed);
 }
 bool RawUncompress(Source* compressed, char* uncompressed) {
   SnappyArrayWriter output(uncompressed);
   return InternalUncompress(compressed, &output, kuint32max);
 }
 bool Uncompress(const char* compressed, size_t n, string* uncompressed) {
   size_t ulength;
   if (!GetUncompressedLength(compressed, n, &ulength)) {
     return false;
   }
   // Protect against possible DoS attack
   if ((static_cast<uint64>(ulength) + uncompressed->size()) >
       uncompressed->max_size()) {
     return false;
   }
   STLStringResizeUninitialized(uncompressed, ulength);
   return RawUncompress(compressed, n, string_as_array(uncompressed));
 }
 // A Writer that drops everything on the floor and just does validation
 class SnappyDecompressionValidator {
  private:
   size_t expected_;
   size_t produced_;
  public:
   inline SnappyDecompressionValidator() : produced_(0) { }
   inline void SetExpectedLength(size_t len) {
     expected_ = len;
   }
   inline bool CheckLength() const {
     return expected_ == produced_;
   }
   inline bool Append(const char* ip, 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) {
     if (produced_ <= offset - 1u) return false;  // -1u catches offset==0
     produced_ += len;
     return produced_ <= expected_;
   }
 };
 bool IsValidCompressedBuffer(const char* compressed, size_t n) {
   ByteArraySource reader(compressed, n);
   SnappyDecompressionValidator writer;
   return InternalUncompress(&reader, &writer, kuint32max);
 }
 void RawCompress(const char* input,
                  size_t input_length,
                  char* compressed,
                  size_t* compressed_length) {
   ByteArraySource reader(input, input_length);
   UncheckedByteArraySink writer(compressed);
   Compress(&reader, &writer);
   // Compute how many bytes were added
   *compressed_length = (writer.CurrentDestination() - compressed);
 }
 size_t Compress(const char* input, size_t input_length, string* compressed) {
   // Pre-grow the buffer to the max length of the compressed output
   compressed->resize(MaxCompressedLength(input_length));
   size_t compressed_length;
   RawCompress(input, input_length, string_as_array(compressed),
               &compressed_length);
   compressed->resize(compressed_length);
   return compressed_length;
 }
 } // end namespace snappy

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