LCOV - code coverage report
Current view: top level - lunchbox/compressor/snappy - snappy.cc (source / functions) Hit Total Coverage
Test: lcov2.info Lines: 308 477 64.6 %
Date: 2014-10-01 Functions: 30 56 53.6 %

          Line data    Source code
       1             : // Copyright 2005 Google Inc. All Rights Reserved.
       2             : //
       3             : // Redistribution and use in source and binary forms, with or without
       4             : // modification, are permitted provided that the following conditions are
       5             : // met:
       6             : //
       7             : //     * Redistributions of source code must retain the above copyright
       8             : // notice, this list of conditions and the following disclaimer.
       9             : //     * Redistributions in binary form must reproduce the above
      10             : // copyright notice, this list of conditions and the following disclaimer
      11             : // in the documentation and/or other materials provided with the
      12             : // distribution.
      13             : //     * Neither the name of Google Inc. nor the names of its
      14             : // contributors may be used to endorse or promote products derived from
      15             : // this software without specific prior written permission.
      16             : //
      17             : // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
      18             : // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
      19             : // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
      20             : // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
      21             : // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
      22             : // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
      23             : // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
      24             : // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
      25             : // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
      26             : // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
      27             : // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
      28             : 
      29             : #include "snappy.h"
      30             : #include "snappy-internal.h"
      31             : #include "snappy-sinksource.h"
      32             : 
      33             : #include <stdio.h>
      34             : 
      35             : #include <algorithm>
      36             : #include <string>
      37             : #include <vector>
      38             : 
      39             : #pragma warning( disable: 4267 ) // Conversion from .. to .. loss of data
      40             : 
      41             : namespace snappy {
      42             : 
      43             : // Any hash function will produce a valid compressed bitstream, but a good
      44             : // hash function reduces the number of collisions and thus yields better
      45             : // compression for compressible input, and more speed for incompressible
      46             : // input. Of course, it doesn't hurt if the hash function is reasonably fast
      47             : // either, as it gets called a lot.
      48   112450908 : static inline uint32 HashBytes(uint32 bytes, int shift) {
      49   112450908 :   uint32 kMul = 0x1e35a7bd;
      50   112450908 :   return (bytes * kMul) >> shift;
      51             : }
      52    75879384 : static inline uint32 Hash(const char* p, int shift) {
      53    75879384 :   return HashBytes(UNALIGNED_LOAD32(p), shift);
      54             : }
      55             : 
      56        5402 : size_t MaxCompressedLength(size_t source_len) {
      57             :   // Compressed data can be defined as:
      58             :   //    compressed := item* literal*
      59             :   //    item       := literal* copy
      60             :   //
      61             :   // The trailing literal sequence has a space blowup of at most 62/60
      62             :   // since a literal of length 60 needs one tag byte + one extra byte
      63             :   // for length information.
      64             :   //
      65             :   // Item blowup is trickier to measure.  Suppose the "copy" op copies
      66             :   // 4 bytes of data.  Because of a special check in the encoding code,
      67             :   // we produce a 4-byte copy only if the offset is < 65536.  Therefore
      68             :   // the copy op takes 3 bytes to encode, and this type of item leads
      69             :   // to at most the 62/60 blowup for representing literals.
      70             :   //
      71             :   // Suppose the "copy" op copies 5 bytes of data.  If the offset is big
      72             :   // enough, it will take 5 bytes to encode the copy op.  Therefore the
      73             :   // worst case here is a one-byte literal followed by a five-byte copy.
      74             :   // I.e., 6 bytes of input turn into 7 bytes of "compressed" data.
      75             :   //
      76             :   // This last factor dominates the blowup, so the final estimate is:
      77        5402 :   return 32 + source_len + source_len/6;
      78             : }
      79             : 
      80             : enum {
      81             :   LITERAL = 0,
      82             :   COPY_1_BYTE_OFFSET = 1,  // 3 bit length + 3 bits of offset in opcode
      83             :   COPY_2_BYTE_OFFSET = 2,
      84             :   COPY_4_BYTE_OFFSET = 3
      85             : };
      86             : static const ssize_t kMaximumTagLength = 5;  // COPY_4_BYTE_OFFSET plus the actual offset.
      87             : 
      88             : // Copy "len" bytes from "src" to "op", one byte at a time.  Used for
      89             : // handling COPY operations where the input and output regions may
      90             : // overlap.  For example, suppose:
      91             : //    src    == "ab"
      92             : //    op     == src + 2
      93             : //    len    == 20
      94             : // After IncrementalCopy(src, op, len), the result will have
      95             : // eleven copies of "ab"
      96             : //    ababababababababababab
      97             : // Note that this does not match the semantics of either memcpy()
      98             : // or memmove().
      99          18 : static inline void IncrementalCopy(const char* src, char* op, ssize_t len) {
     100          18 :   assert(len > 0);
     101         502 :   do {
     102         502 :     *op++ = *src++;
     103             :   } while (--len > 0);
     104          18 : }
     105             : 
     106             : // Equivalent to IncrementalCopy except that it can write up to ten extra
     107             : // bytes after the end of the copy, and that it is faster.
     108             : //
     109             : // The main part of this loop is a simple copy of eight bytes at a time until
     110             : // we've copied (at least) the requested amount of bytes.  However, if op and
     111             : // src are less than eight bytes apart (indicating a repeating pattern of
     112             : // length < 8), we first need to expand the pattern in order to get the correct
     113             : // results. For instance, if the buffer looks like this, with the eight-byte
     114             : // <src> and <op> patterns marked as intervals:
     115             : //
     116             : //    abxxxxxxxxxxxx
     117             : //    [------]           src
     118             : //      [------]         op
     119             : //
     120             : // a single eight-byte copy from <src> to <op> will repeat the pattern once,
     121             : // after which we can move <op> two bytes without moving <src>:
     122             : //
     123             : //    ababxxxxxxxxxx
     124             : //    [------]           src
     125             : //        [------]       op
     126             : //
     127             : // and repeat the exercise until the two no longer overlap.
     128             : //
     129             : // This allows us to do very well in the special case of one single byte
     130             : // repeated many times, without taking a big hit for more general cases.
     131             : //
     132             : // The worst case of extra writing past the end of the match occurs when
     133             : // op - src == 1 and len == 1; the last copy will read from byte positions
     134             : // [0..7] and write to [4..11], whereas it was only supposed to write to
     135             : // position 1. Thus, ten excess bytes.
     136             : 
     137             : namespace {
     138             : 
     139             : const int kMaxIncrementCopyOverflow = 10;
     140             : 
     141     1501832 : inline void IncrementalCopyFastPath(const char* src, char* op, ssize_t len) {
     142     3595492 :   while (op - src < 8) {
     143      591828 :     UnalignedCopy64(src, op);
     144      591828 :     len -= op - src;
     145      591828 :     op += op - src;
     146             :   }
     147     8080346 :   while (len > 0) {
     148     5076682 :     UnalignedCopy64(src, op);
     149     5076682 :     src += 8;
     150     5076682 :     op += 8;
     151     5076682 :     len -= 8;
     152             :   }
     153     1501832 : }
     154             : 
     155             : }  // namespace
     156             : 
     157     8071462 : static inline char* EmitLiteral(char* op,
     158             :                                 const char* literal,
     159             :                                 int len,
     160             :                                 bool allow_fast_path) {
     161     8071462 :   int n = len - 1;      // Zero-length literals are disallowed
     162     8071462 :   if (n < 60) {
     163             :     // Fits in tag byte
     164     8022884 :     *op++ = LITERAL | (n << 2);
     165             : 
     166             :     // The vast majority of copies are below 16 bytes, for which a
     167             :     // call to memcpy is overkill. This fast path can sometimes
     168             :     // copy up to 15 bytes too much, but that is okay in the
     169             :     // main loop, since we have a bit to go on for both sides:
     170             :     //
     171             :     //   - The input will always have kInputMarginBytes = 15 extra
     172             :     //     available bytes, as long as we're in the main loop, and
     173             :     //     if not, allow_fast_path = false.
     174             :     //   - The output will always have 32 spare bytes (see
     175             :     //     MaxCompressedLength).
     176     8022884 :     if (allow_fast_path && len <= 16) {
     177     7896424 :       UnalignedCopy64(literal, op);
     178     7896424 :       UnalignedCopy64(literal + 8, op + 8);
     179     7896424 :       return op + len;
     180             :     }
     181             :   } else {
     182             :     // Encode in upcoming bytes
     183       48578 :     char* base = op;
     184       48578 :     int count = 0;
     185       48578 :     op++;
     186      149288 :     while (n > 0) {
     187       52132 :       *op++ = n & 0xff;
     188       52132 :       n >>= 8;
     189       52132 :       count++;
     190             :     }
     191       48578 :     assert(count >= 1);
     192       48578 :     assert(count <= 4);
     193       48578 :     *base = LITERAL | ((59+count) << 2);
     194             :   }
     195      175038 :   memcpy(op, literal, len);
     196      175038 :   return op + len;
     197             : }
     198             : 
     199    14400398 : static inline char* EmitCopyLessThan64(char* op, size_t offset, int len) {
     200    14400398 :   assert(len <= 64);
     201    14400398 :   assert(len >= 4);
     202    14400398 :   assert(offset < 65536);
     203             : 
     204    14400398 :   if ((len < 12) && (offset < 2048)) {
     205     8695382 :     size_t len_minus_4 = len - 4;
     206     8695382 :     assert(len_minus_4 < 8);            // Must fit in 3 bits
     207     8695382 :     *op++ = COPY_1_BYTE_OFFSET + ((len_minus_4) << 2) + ((offset >> 8) << 5);
     208     8695382 :     *op++ = offset & 0xff;
     209             :   } else {
     210     5705016 :     *op++ = COPY_2_BYTE_OFFSET + ((len-1) << 2);
     211     5705016 :     LittleEndian::Store16(op, offset);
     212     5705016 :     op += 2;
     213             :   }
     214    14400398 :   return op;
     215             : }
     216             : 
     217    14255596 : static inline char* EmitCopy(char* op, size_t offset, int len) {
     218             :   // Emit 64 byte copies but make sure to keep at least four bytes reserved
     219    28647844 :   while (len >= 68) {
     220      136652 :     op = EmitCopyLessThan64(op, offset, 64);
     221      136652 :     len -= 64;
     222             :   }
     223             : 
     224             :   // Emit an extra 60 byte copy if have too much data to fit in one copy
     225    14255596 :   if (len > 64) {
     226        8150 :     op = EmitCopyLessThan64(op, offset, 60);
     227        8150 :     len -= 60;
     228             :   }
     229             : 
     230             :   // Emit remainder
     231    14255596 :   op = EmitCopyLessThan64(op, offset, len);
     232    14255596 :   return op;
     233             : }
     234             : 
     235             : 
     236           0 : bool GetUncompressedLength(const char* start, size_t n, size_t* result) {
     237           0 :   uint32 v = 0;
     238           0 :   const char* limit = start + n;
     239           0 :   if (Varint::Parse32WithLimit(start, limit, &v) != NULL) {
     240           0 :     *result = v;
     241           0 :     return true;
     242             :   } else {
     243           0 :     return false;
     244             :   }
     245             : }
     246             : 
     247             : namespace internal {
     248        5314 : uint16* WorkingMemory::GetHashTable(size_t input_size, int* table_size) {
     249             :   // Use smaller hash table when input.size() is smaller, since we
     250             :   // fill the table, incurring O(hash table size) overhead for
     251             :   // compression, and if the input is short, we won't need that
     252             :   // many hash table entries anyway.
     253             :   assert(kMaxHashTableSize >= 256);
     254        5314 :   size_t htsize = 256;
     255       42424 :   while (htsize < kMaxHashTableSize && htsize < input_size) {
     256       31796 :     htsize <<= 1;
     257             :   }
     258             : 
     259             :   uint16* table;
     260        5314 :   if (htsize <= ARRAYSIZE(small_table_)) {
     261          12 :     table = small_table_;
     262             :   } else {
     263        5302 :     if (large_table_ == NULL) {
     264          76 :       large_table_ = new uint16[kMaxHashTableSize];
     265             :     }
     266        5302 :     table = large_table_;
     267             :   }
     268             : 
     269        5314 :   *table_size = htsize;
     270        5314 :   memset(table, 0, htsize * sizeof(*table));
     271        5314 :   return table;
     272             : }
     273             : }  // end namespace internal
     274             : 
     275             : // For 0 <= offset <= 4, GetUint32AtOffset(GetEightBytesAt(p), offset) will
     276             : // equal UNALIGNED_LOAD32(p + offset).  Motivation: On x86-64 hardware we have
     277             : // empirically found that overlapping loads such as
     278             : //  UNALIGNED_LOAD32(p) ... UNALIGNED_LOAD32(p+1) ... UNALIGNED_LOAD32(p+2)
     279             : // are slower than UNALIGNED_LOAD64(p) followed by shifts and casts to uint32.
     280             : //
     281             : // We have different versions for 64- and 32-bit; ideally we would avoid the
     282             : // two functions and just inline the UNALIGNED_LOAD64 call into
     283             : // GetUint32AtOffset, but GCC (at least not as of 4.6) is seemingly not clever
     284             : // enough to avoid loading the value multiple times then. For 64-bit, the load
     285             : // is done when GetEightBytesAt() is called, whereas for 32-bit, the load is
     286             : // done at GetUint32AtOffset() time.
     287             : 
     288             : #ifdef ARCH_K8
     289             : 
     290             : typedef uint64 EightBytesReference;
     291             : 
     292    14253530 : static inline EightBytesReference GetEightBytesAt(const char* ptr) {
     293    14253530 :   return UNALIGNED_LOAD64(ptr);
     294             : }
     295             : 
     296    50825054 : static inline uint32 GetUint32AtOffset(uint64 v, int offset) {
     297    50825054 :   assert(offset >= 0);
     298    50825054 :   assert(offset <= 4);
     299    50825054 :   return v >> (LittleEndian::IsLittleEndian() ? 8 * offset : 32 - 8 * offset);
     300             : }
     301             : 
     302             : #else
     303             : 
     304             : typedef const char* EightBytesReference;
     305             : 
     306             : static inline EightBytesReference GetEightBytesAt(const char* ptr) {
     307             :   return ptr;
     308             : }
     309             : 
     310             : static inline uint32 GetUint32AtOffset(const char* v, int offset) {
     311             :   assert(offset >= 0);
     312             :   assert(offset <= 4);
     313             :   return UNALIGNED_LOAD32(v + offset);
     314             : }
     315             : 
     316             : #endif
     317             : 
     318             : // Flat array compression that does not emit the "uncompressed length"
     319             : // prefix. Compresses "input" string to the "*op" buffer.
     320             : //
     321             : // REQUIRES: "input" is at most "kBlockSize" bytes long.
     322             : // REQUIRES: "op" points to an array of memory that is at least
     323             : // "MaxCompressedLength(input.size())" in size.
     324             : // REQUIRES: All elements in "table[0..table_size-1]" are initialized to zero.
     325             : // REQUIRES: "table_size" is a power of two
     326             : //
     327             : // Returns an "end" pointer into "op" buffer.
     328             : // "end - op" is the compressed size of "input".
     329             : namespace internal {
     330        5314 : char* CompressFragment(const char* input,
     331             :                        size_t input_size,
     332             :                        char* op,
     333             :                        uint16* table,
     334             :                        const int table_size) {
     335             :   // "ip" is the input pointer, and "op" is the output pointer.
     336        5314 :   const char* ip = input;
     337        5314 :   assert(input_size <= kBlockSize);
     338        5314 :   assert((table_size & (table_size - 1)) == 0); // table must be power of two
     339        5314 :   const int shift = 32 - Bits::Log2Floor(table_size);
     340        5314 :   assert(static_cast<int>(kuint32max >> shift) == table_size - 1);
     341        5314 :   const char* ip_end = input + input_size;
     342        5314 :   const char* base_ip = ip;
     343             :   // Bytes in [next_emit, ip) will be emitted as literal bytes.  Or
     344             :   // [next_emit, ip_end) after the main loop.
     345        5314 :   const char* next_emit = ip;
     346             : 
     347        5314 :   const size_t kInputMarginBytes = 15;
     348        5314 :   if (PREDICT_TRUE(input_size >= kInputMarginBytes)) {
     349        5310 :     const char* ip_limit = input + input_size - kInputMarginBytes;
     350             : 
     351        5310 :     for (uint32 next_hash = Hash(++ip, shift); ; ) {
     352     8069774 :       assert(next_emit < ip);
     353             :       // The body of this loop calls EmitLiteral once and then EmitCopy one or
     354             :       // more times.  (The exception is that when we're close to exhausting
     355             :       // the input we goto emit_remainder.)
     356             :       //
     357             :       // In the first iteration of this loop we're just starting, so
     358             :       // there's nothing to copy, so calling EmitLiteral once is
     359             :       // necessary.  And we only start a new iteration when the
     360             :       // current iteration has determined that a call to EmitLiteral will
     361             :       // precede the next call to EmitCopy (if any).
     362             :       //
     363             :       // Step 1: Scan forward in the input looking for a 4-byte-long match.
     364             :       // If we get close to exhausting the input then goto emit_remainder.
     365             :       //
     366             :       // Heuristic match skipping: If 32 bytes are scanned with no matches
     367             :       // found, start looking only at every other byte. If 32 more bytes are
     368             :       // scanned, look at every third byte, etc.. When a match is found,
     369             :       // immediately go back to looking at every byte. This is a small loss
     370             :       // (~5% performance, ~0.1% density) for compressible data due to more
     371             :       // bookkeeping, but for non-compressible data (such as JPEG) it's a huge
     372             :       // win since the compressor quickly "realizes" the data is incompressible
     373             :       // and doesn't bother looking for matches everywhere.
     374             :       //
     375             :       // The "skip" variable keeps track of how many bytes there are since the
     376             :       // last match; dividing it by 32 (ie. right-shifting by five) gives the
     377             :       // number of bytes to move ahead for each iteration.
     378     8069774 :       uint32 skip = 32;
     379             : 
     380     8069774 :       const char* next_ip = ip;
     381             :       const char* candidate;
     382    37935415 :       do {
     383    37938659 :         ip = next_ip;
     384    37938659 :         uint32 hash = next_hash;
     385    37938659 :         assert(hash == Hash(ip, shift));
     386    37938659 :         uint32 bytes_between_hash_lookups = skip++ >> 5;
     387    37938659 :         next_ip = ip + bytes_between_hash_lookups;
     388    37938659 :         if (PREDICT_FALSE(next_ip > ip_limit)) {
     389        3244 :           goto emit_remainder;
     390             :         }
     391    37935415 :         next_hash = Hash(next_ip, shift);
     392    37935415 :         candidate = base_ip + table[hash];
     393    37935415 :         assert(candidate >= base_ip);
     394    37935415 :         assert(candidate < ip);
     395             : 
     396    37935415 :         table[hash] = ip - base_ip;
     397    37935415 :       } while (PREDICT_TRUE(UNALIGNED_LOAD32(ip) !=
     398             :                             UNALIGNED_LOAD32(candidate)));
     399             : 
     400             :       // Step 2: A 4-byte match has been found.  We'll later see if more
     401             :       // than 4 bytes match.  But, prior to the match, input
     402             :       // bytes [next_emit, ip) are unmatched.  Emit them as "literal bytes."
     403     8066530 :       assert(next_emit + 16 <= ip_end);
     404     8066530 :       op = EmitLiteral(op, next_emit, ip - next_emit, true);
     405             : 
     406             :       // Step 3: Call EmitCopy, and then see if another EmitCopy could
     407             :       // be our next move.  Repeat until we find no match for the
     408             :       // input immediately after what was consumed by the last EmitCopy call.
     409             :       //
     410             :       // If we exit this loop normally then we need to call EmitLiteral next,
     411             :       // though we don't yet know how big the literal will be.  We handle that
     412             :       // by proceeding to the next iteration of the main loop.  We also can exit
     413             :       // this loop via goto if we get close to exhausting the input.
     414             :       EightBytesReference input_bytes;
     415     8066530 :       uint32 candidate_bytes = 0;
     416             : 
     417    14253530 :       do {
     418             :         // We have a 4-byte match at ip, and no need to emit any
     419             :         // "literal bytes" prior to ip.
     420    14255596 :         const char* base = ip;
     421    14255596 :         int matched = 4 + FindMatchLength(candidate + 4, ip + 4, ip_end);
     422    14255596 :         ip += matched;
     423    14255596 :         size_t offset = base - candidate;
     424    14255596 :         assert(0 == memcmp(base, candidate, matched));
     425    14255596 :         op = EmitCopy(op, offset, matched);
     426             :         // We could immediately start working at ip now, but to improve
     427             :         // compression we first update table[Hash(ip - 1, ...)].
     428    14255596 :         const char* insert_tail = ip - 1;
     429    14255596 :         next_emit = ip;
     430    14255596 :         if (PREDICT_FALSE(ip >= ip_limit)) {
     431        2066 :           goto emit_remainder;
     432             :         }
     433    14253530 :         input_bytes = GetEightBytesAt(insert_tail);
     434    14253530 :         uint32 prev_hash = HashBytes(GetUint32AtOffset(input_bytes, 0), shift);
     435    14253530 :         table[prev_hash] = ip - base_ip - 1;
     436    14253530 :         uint32 cur_hash = HashBytes(GetUint32AtOffset(input_bytes, 1), shift);
     437    14253530 :         candidate = base_ip + table[cur_hash];
     438    14253530 :         candidate_bytes = UNALIGNED_LOAD32(candidate);
     439    14253530 :         table[cur_hash] = ip - base_ip;
     440    14253530 :       } while (GetUint32AtOffset(input_bytes, 1) == candidate_bytes);
     441             : 
     442     8064464 :       next_hash = HashBytes(GetUint32AtOffset(input_bytes, 2), shift);
     443     8064464 :       ++ip;
     444     8064464 :     }
     445             :   }
     446             : 
     447             :  emit_remainder:
     448             :   // Emit the remaining bytes as a literal
     449        5314 :   if (next_emit < ip_end) {
     450        4932 :     op = EmitLiteral(op, next_emit, ip_end - next_emit, false);
     451             :   }
     452             : 
     453        5314 :   return op;
     454             : }
     455             : }  // end namespace internal
     456             : 
     457             : // Signature of output types needed by decompression code.
     458             : // The decompression code is templatized on a type that obeys this
     459             : // signature so that we do not pay virtual function call overhead in
     460             : // the middle of a tight decompression loop.
     461             : //
     462             : // class DecompressionWriter {
     463             : //  public:
     464             : //   // Called before decompression
     465             : //   void SetExpectedLength(size_t length);
     466             : //
     467             : //   // Called after decompression
     468             : //   bool CheckLength() const;
     469             : //
     470             : //   // Called repeatedly during decompression
     471             : //   bool Append(const char* ip, size_t length);
     472             : //   bool AppendFromSelf(uint32 offset, size_t length);
     473             : //
     474             : //   // The rules for how TryFastAppend differs from Append are somewhat
     475             : //   // convoluted:
     476             : //   //
     477             : //   //  - TryFastAppend is allowed to decline (return false) at any
     478             : //   //    time, for any reason -- just "return false" would be
     479             : //   //    a perfectly legal implementation of TryFastAppend.
     480             : //   //    The intention is for TryFastAppend to allow a fast path
     481             : //   //    in the common case of a small append.
     482             : //   //  - TryFastAppend is allowed to read up to <available> bytes
     483             : //   //    from the input buffer, whereas Append is allowed to read
     484             : //   //    <length>. However, if it returns true, it must leave
     485             : //   //    at least five (kMaximumTagLength) bytes in the input buffer
     486             : //   //    afterwards, so that there is always enough space to read the
     487             : //   //    next tag without checking for a refill.
     488             : //   //  - TryFastAppend must always return decline (return false)
     489             : //   //    if <length> is 61 or more, as in this case the literal length is not
     490             : //   //    decoded fully. In practice, this should not be a big problem,
     491             : //   //    as it is unlikely that one would implement a fast path accepting
     492             : //   //    this much data.
     493             : //   //
     494             : //   bool TryFastAppend(const char* ip, size_t available, size_t length);
     495             : // };
     496             : 
     497             : // -----------------------------------------------------------------------
     498             : // Lookup table for decompression code.  Generated by ComputeTable() below.
     499             : // -----------------------------------------------------------------------
     500             : 
     501             : // Mapping from i in range [0,4] to a mask to extract the bottom 8*i bits
     502             : static const uint32 wordmask[] = {
     503             :   0u, 0xffu, 0xffffu, 0xffffffu, 0xffffffffu
     504             : };
     505             : 
     506             : // Data stored per entry in lookup table:
     507             : //      Range   Bits-used       Description
     508             : //      ------------------------------------
     509             : //      1..64   0..7            Literal/copy length encoded in opcode byte
     510             : //      0..7    8..10           Copy offset encoded in opcode byte / 256
     511             : //      0..4    11..13          Extra bytes after opcode
     512             : //
     513             : // We use eight bits for the length even though 7 would have sufficed
     514             : // because of efficiency reasons:
     515             : //      (1) Extracting a byte is faster than a bit-field
     516             : //      (2) It properly aligns copy offset so we do not need a <<8
     517             : static const uint16 char_table[256] = {
     518             :   0x0001, 0x0804, 0x1001, 0x2001, 0x0002, 0x0805, 0x1002, 0x2002,
     519             :   0x0003, 0x0806, 0x1003, 0x2003, 0x0004, 0x0807, 0x1004, 0x2004,
     520             :   0x0005, 0x0808, 0x1005, 0x2005, 0x0006, 0x0809, 0x1006, 0x2006,
     521             :   0x0007, 0x080a, 0x1007, 0x2007, 0x0008, 0x080b, 0x1008, 0x2008,
     522             :   0x0009, 0x0904, 0x1009, 0x2009, 0x000a, 0x0905, 0x100a, 0x200a,
     523             :   0x000b, 0x0906, 0x100b, 0x200b, 0x000c, 0x0907, 0x100c, 0x200c,
     524             :   0x000d, 0x0908, 0x100d, 0x200d, 0x000e, 0x0909, 0x100e, 0x200e,
     525             :   0x000f, 0x090a, 0x100f, 0x200f, 0x0010, 0x090b, 0x1010, 0x2010,
     526             :   0x0011, 0x0a04, 0x1011, 0x2011, 0x0012, 0x0a05, 0x1012, 0x2012,
     527             :   0x0013, 0x0a06, 0x1013, 0x2013, 0x0014, 0x0a07, 0x1014, 0x2014,
     528             :   0x0015, 0x0a08, 0x1015, 0x2015, 0x0016, 0x0a09, 0x1016, 0x2016,
     529             :   0x0017, 0x0a0a, 0x1017, 0x2017, 0x0018, 0x0a0b, 0x1018, 0x2018,
     530             :   0x0019, 0x0b04, 0x1019, 0x2019, 0x001a, 0x0b05, 0x101a, 0x201a,
     531             :   0x001b, 0x0b06, 0x101b, 0x201b, 0x001c, 0x0b07, 0x101c, 0x201c,
     532             :   0x001d, 0x0b08, 0x101d, 0x201d, 0x001e, 0x0b09, 0x101e, 0x201e,
     533             :   0x001f, 0x0b0a, 0x101f, 0x201f, 0x0020, 0x0b0b, 0x1020, 0x2020,
     534             :   0x0021, 0x0c04, 0x1021, 0x2021, 0x0022, 0x0c05, 0x1022, 0x2022,
     535             :   0x0023, 0x0c06, 0x1023, 0x2023, 0x0024, 0x0c07, 0x1024, 0x2024,
     536             :   0x0025, 0x0c08, 0x1025, 0x2025, 0x0026, 0x0c09, 0x1026, 0x2026,
     537             :   0x0027, 0x0c0a, 0x1027, 0x2027, 0x0028, 0x0c0b, 0x1028, 0x2028,
     538             :   0x0029, 0x0d04, 0x1029, 0x2029, 0x002a, 0x0d05, 0x102a, 0x202a,
     539             :   0x002b, 0x0d06, 0x102b, 0x202b, 0x002c, 0x0d07, 0x102c, 0x202c,
     540             :   0x002d, 0x0d08, 0x102d, 0x202d, 0x002e, 0x0d09, 0x102e, 0x202e,
     541             :   0x002f, 0x0d0a, 0x102f, 0x202f, 0x0030, 0x0d0b, 0x1030, 0x2030,
     542             :   0x0031, 0x0e04, 0x1031, 0x2031, 0x0032, 0x0e05, 0x1032, 0x2032,
     543             :   0x0033, 0x0e06, 0x1033, 0x2033, 0x0034, 0x0e07, 0x1034, 0x2034,
     544             :   0x0035, 0x0e08, 0x1035, 0x2035, 0x0036, 0x0e09, 0x1036, 0x2036,
     545             :   0x0037, 0x0e0a, 0x1037, 0x2037, 0x0038, 0x0e0b, 0x1038, 0x2038,
     546             :   0x0039, 0x0f04, 0x1039, 0x2039, 0x003a, 0x0f05, 0x103a, 0x203a,
     547             :   0x003b, 0x0f06, 0x103b, 0x203b, 0x003c, 0x0f07, 0x103c, 0x203c,
     548             :   0x0801, 0x0f08, 0x103d, 0x203d, 0x1001, 0x0f09, 0x103e, 0x203e,
     549             :   0x1801, 0x0f0a, 0x103f, 0x203f, 0x2001, 0x0f0b, 0x1040, 0x2040
     550             : };
     551             : 
     552             : // In debug mode, allow optional computation of the table at startup.
     553             : // Also, check that the decompression table is correct.
     554             : #if 0
     555             : DEFINE_bool(snappy_dump_decompression_table, false,
     556             :             "If true, we print the decompression table at startup.");
     557             : 
     558             : static uint16 MakeEntry(unsigned int extra,
     559             :                         unsigned int len,
     560             :                         unsigned int copy_offset) {
     561             :   // Check that all of the fields fit within the allocated space
     562             :   assert(extra       == (extra & 0x7));          // At most 3 bits
     563             :   assert(copy_offset == (copy_offset & 0x7));    // At most 3 bits
     564             :   assert(len         == (len & 0x7f));           // At most 7 bits
     565             :   return len | (copy_offset << 8) | (extra << 11);
     566             : }
     567             : 
     568             : static void ComputeTable() {
     569             :   uint16 dst[256];
     570             : 
     571             :   // Place invalid entries in all places to detect missing initialization
     572             :   int assigned = 0;
     573             :   for (int i = 0; i < 256; i++) {
     574             :     dst[i] = 0xffff;
     575             :   }
     576             : 
     577             :   // Small LITERAL entries.  We store (len-1) in the top 6 bits.
     578             :   for (unsigned int len = 1; len <= 60; len++) {
     579             :     dst[LITERAL | ((len-1) << 2)] = MakeEntry(0, len, 0);
     580             :     assigned++;
     581             :   }
     582             : 
     583             :   // Large LITERAL entries.  We use 60..63 in the high 6 bits to
     584             :   // encode the number of bytes of length info that follow the opcode.
     585             :   for (unsigned int extra_bytes = 1; extra_bytes <= 4; extra_bytes++) {
     586             :     // We set the length field in the lookup table to 1 because extra
     587             :     // bytes encode len-1.
     588             :     dst[LITERAL | ((extra_bytes+59) << 2)] = MakeEntry(extra_bytes, 1, 0);
     589             :     assigned++;
     590             :   }
     591             : 
     592             :   // COPY_1_BYTE_OFFSET.
     593             :   //
     594             :   // The tag byte in the compressed data stores len-4 in 3 bits, and
     595             :   // offset/256 in 5 bits.  offset%256 is stored in the next byte.
     596             :   //
     597             :   // This format is used for length in range [4..11] and offset in
     598             :   // range [0..2047]
     599             :   for (unsigned int len = 4; len < 12; len++) {
     600             :     for (unsigned int offset = 0; offset < 2048; offset += 256) {
     601             :       dst[COPY_1_BYTE_OFFSET | ((len-4)<<2) | ((offset>>8)<<5)] =
     602             :         MakeEntry(1, len, offset>>8);
     603             :       assigned++;
     604             :     }
     605             :   }
     606             : 
     607             :   // COPY_2_BYTE_OFFSET.
     608             :   // Tag contains len-1 in top 6 bits, and offset in next two bytes.
     609             :   for (unsigned int len = 1; len <= 64; len++) {
     610             :     dst[COPY_2_BYTE_OFFSET | ((len-1)<<2)] = MakeEntry(2, len, 0);
     611             :     assigned++;
     612             :   }
     613             : 
     614             :   // COPY_4_BYTE_OFFSET.
     615             :   // Tag contents len-1 in top 6 bits, and offset in next four bytes.
     616             :   for (unsigned int len = 1; len <= 64; len++) {
     617             :     dst[COPY_4_BYTE_OFFSET | ((len-1)<<2)] = MakeEntry(4, len, 0);
     618             :     assigned++;
     619             :   }
     620             : 
     621             :   // Check that each entry was initialized exactly once.
     622             :   if (assigned != 256) {
     623             :     fprintf(stderr, "ComputeTable: assigned only %d of 256\n", assigned);
     624             :     abort();
     625             :   }
     626             :   for (int i = 0; i < 256; i++) {
     627             :     if (dst[i] == 0xffff) {
     628             :       fprintf(stderr, "ComputeTable: did not assign byte %d\n", i);
     629             :       abort();
     630             :     }
     631             :   }
     632             : 
     633             :   if (FLAGS_snappy_dump_decompression_table) {
     634             :     printf("static const uint16 char_table[256] = {\n  ");
     635             :     for (int i = 0; i < 256; i++) {
     636             :       printf("0x%04x%s",
     637             :              dst[i],
     638             :              ((i == 255) ? "\n" : (((i%8) == 7) ? ",\n  " : ", ")));
     639             :     }
     640             :     printf("};\n");
     641             :   }
     642             : 
     643             :   // Check that computed table matched recorded table
     644             :   for (int i = 0; i < 256; i++) {
     645             :     if (dst[i] != char_table[i]) {
     646             :       fprintf(stderr, "ComputeTable: byte %d: computed (%x), expect (%x)\n",
     647             :               i, static_cast<int>(dst[i]), static_cast<int>(char_table[i]));
     648             :       abort();
     649             :     }
     650             :   }
     651             : }
     652             : #endif /* !NDEBUG */
     653             : 
     654             : // Helper class for decompression
     655             : class SnappyDecompressor {
     656             :  private:
     657             :   Source*       reader_;         // Underlying source of bytes to decompress
     658             :   const char*   ip_;             // Points to next buffered byte
     659             :   const char*   ip_limit_;       // Points just past buffered bytes
     660             :   uint32        peeked_;         // Bytes peeked from reader (need to skip)
     661             :   bool          eof_;            // Hit end of input without an error?
     662             :   char          scratch_[kMaximumTagLength];  // See RefillTag().
     663             : 
     664             :   // Ensure that all of the tag metadata for the next tag is available
     665             :   // in [ip_..ip_limit_-1].  Also ensures that [ip,ip+4] is readable even
     666             :   // if (ip_limit_ - ip_ < 5).
     667             :   //
     668             :   // Returns true on success, false on error or end of input.
     669             :   bool RefillTag();
     670             : 
     671             :  public:
     672             :   // cppcheck-suppress uninitMemberVar
     673          76 :   explicit SnappyDecompressor(Source* reader)
     674             :       : reader_(reader),
     675             :         ip_(NULL),
     676             :         ip_limit_(NULL),
     677             :         peeked_(0),
     678          76 :         eof_(false) {
     679          76 :   }
     680             : 
     681          76 :   ~SnappyDecompressor() {
     682             :     // Advance past any bytes we peeked at from the reader
     683          76 :     reader_->Skip(peeked_);
     684          76 :   }
     685             : 
     686             :   // Returns true iff we have hit the end of the input without an error.
     687          76 :   bool eof() const {
     688          76 :     return eof_;
     689             :   }
     690             : 
     691             :   // Read the uncompressed length stored at the start of the compressed data.
     692             :   // On succcess, stores the length in *result and returns true.
     693             :   // On failure, returns false.
     694          76 :   bool ReadUncompressedLength(uint32* result) {
     695          76 :     assert(ip_ == NULL);       // Must not have read anything yet
     696             :     // Length is encoded in 1..5 bytes
     697          76 :     *result = 0;
     698          76 :     uint32 shift = 0;
     699             :     while (true) {
     700         260 :       if (shift >= 32) return false;
     701             :       size_t n;
     702         260 :       const char* ip = reader_->Peek(&n);
     703         260 :       if (n == 0) return false;
     704         260 :       const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip));
     705         260 :       reader_->Skip(1);
     706         260 :       *result |= static_cast<uint32>(c & 0x7f) << shift;
     707         260 :       if (c < 128) {
     708          76 :         break;
     709             :       }
     710         184 :       shift += 7;
     711             :     }
     712         260 :     return true;
     713             :   }
     714             : 
     715             :   // Process the next item found in the input.
     716             :   // Returns true if successful, false on error or end of input.
     717             :   template <class Writer>
     718          76 :   void DecompressAllTags(Writer* writer) {
     719          76 :     const char* ip = ip_;
     720             : 
     721             :     // We could have put this refill fragment only at the beginning of the loop.
     722             :     // However, duplicating it at the end of each branch gives the compiler more
     723             :     // scope to optimize the <ip_limit_ - ip> expression based on the local
     724             :     // context, which overall increases speed.
     725             :     #define MAYBE_REFILL() \
     726             :         if (ip_limit_ - ip < kMaximumTagLength) { \
     727             :           ip_ = ip; \
     728             :           if (!RefillTag()) return; \
     729             :           ip = ip_; \
     730             :         }
     731             : 
     732          76 :     MAYBE_REFILL();
     733    22471688 :     for ( ;; ) {
     734    22471764 :       const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip++));
     735             : 
     736    22471764 :       if ((c & 0x3) == LITERAL) {
     737     8071410 :         size_t literal_length = (c >> 2) + 1u;
     738     8071410 :         if (writer->TryFastAppend(ip, ip_limit_ - ip, literal_length)) {
     739     7898108 :           assert(literal_length < 61);
     740     7898108 :           ip += literal_length;
     741             :           // NOTE(user): There is no MAYBE_REFILL() here, as TryFastAppend()
     742             :           // will not return true unless there's already at least five spare
     743             :           // bytes in addition to the literal.
     744     7898108 :           continue;
     745             :         }
     746      173302 :         if (PREDICT_FALSE(literal_length >= 61)) {
     747             :           // Long literal.
     748       48578 :           const size_t literal_length_length = literal_length - 60;
     749       48578 :           literal_length =
     750       48578 :               (LittleEndian::Load32(ip) & wordmask[literal_length_length]) + 1;
     751       48578 :           ip += literal_length_length;
     752             :         }
     753             : 
     754      173302 :         size_t avail = ip_limit_ - ip;
     755      346604 :         while (avail < literal_length) {
     756           0 :           if (!writer->Append(ip, avail)) return;
     757           0 :           literal_length -= avail;
     758           0 :           reader_->Skip(peeked_);
     759             :           size_t n;
     760           0 :           ip = reader_->Peek(&n);
     761           0 :           avail = n;
     762           0 :           peeked_ = avail;
     763           0 :           if (avail == 0) return;  // Premature end of input
     764           0 :           ip_limit_ = ip + avail;
     765             :         }
     766      173302 :         if (!writer->Append(ip, literal_length)) {
     767           0 :           return;
     768             :         }
     769      173302 :         ip += literal_length;
     770      173302 :         MAYBE_REFILL();
     771             :       } else {
     772    14400354 :         const uint32 entry = char_table[c];
     773    14400354 :         const uint32 trailer = LittleEndian::Load32(ip) & wordmask[entry >> 11];
     774    14400354 :         const uint32 length = entry & 0xff;
     775    14400354 :         ip += entry >> 11;
     776             : 
     777             :         // copy_offset/256 is encoded in bits 8..10.  By just fetching
     778             :         // those bits, we get copy_offset (since the bit-field starts at
     779             :         // bit 8).
     780    14400354 :         const uint32 copy_offset = entry & 0x700;
     781    14400354 :         if (!writer->AppendFromSelf(copy_offset + trailer, length)) {
     782           0 :           return;
     783             :         }
     784    14400354 :         MAYBE_REFILL();
     785             :       }
     786             :     }
     787             : 
     788             : #undef MAYBE_REFILL
     789             :   }
     790             : };
     791             : 
     792         156 : bool SnappyDecompressor::RefillTag() {
     793         156 :   const char* ip = ip_;
     794         156 :   if (ip == ip_limit_) {
     795             :     // Fetch a new fragment from the reader
     796         152 :     reader_->Skip(peeked_);   // All peeked bytes are used up
     797             :     size_t n;
     798         152 :     ip = reader_->Peek(&n);
     799         152 :     peeked_ = n;
     800         152 :     if (n == 0) {
     801          76 :       eof_ = true;
     802          76 :       return false;
     803             :     }
     804          76 :     ip_limit_ = ip + n;
     805             :   }
     806             : 
     807             :   // Read the tag character
     808          80 :   assert(ip < ip_limit_);
     809          80 :   const unsigned char c = *(reinterpret_cast<const unsigned char*>(ip));
     810          80 :   const uint32 entry = char_table[c];
     811          80 :   const uint32 needed = (entry >> 11) + 1;  // +1 byte for 'c'
     812          80 :   assert(needed <= sizeof(scratch_));
     813             : 
     814             :   // Read more bytes from reader if needed
     815          80 :   uint32 nbuf = ip_limit_ - ip;
     816          80 :   if (nbuf < needed) {
     817             :     // Stitch together bytes from ip and reader to form the word
     818             :     // contents.  We store the needed bytes in "scratch_".  They
     819             :     // will be consumed immediately by the caller since we do not
     820             :     // read more than we need.
     821           0 :     memmove(scratch_, ip, nbuf);
     822           0 :     reader_->Skip(peeked_);  // All peeked bytes are used up
     823           0 :     peeked_ = 0;
     824           0 :     while (nbuf < needed) {
     825             :       size_t length;
     826           0 :       const char* src = reader_->Peek(&length);
     827           0 :       if (length == 0) return false;
     828           0 :       uint32 to_add = min<uint32>(needed - nbuf, length);
     829           0 :       memcpy(scratch_ + nbuf, src, to_add);
     830           0 :       nbuf += to_add;
     831           0 :       reader_->Skip(to_add);
     832             :     }
     833           0 :     assert(nbuf == needed);
     834           0 :     ip_ = scratch_;
     835           0 :     ip_limit_ = scratch_ + needed;
     836          80 :   } else if (nbuf < kMaximumTagLength) {
     837             :     // Have enough bytes, but move into scratch_ so that we do not
     838             :     // read past end of input
     839           4 :     memmove(scratch_, ip, nbuf);
     840           4 :     reader_->Skip(peeked_);  // All peeked bytes are used up
     841           4 :     peeked_ = 0;
     842           4 :     ip_ = scratch_;
     843           4 :     ip_limit_ = scratch_ + nbuf;
     844             :   } else {
     845             :     // Pass pointer to buffer returned by reader_.
     846          76 :     ip_ = ip;
     847             :   }
     848          80 :   return true;
     849             : }
     850             : 
     851             : template <typename Writer>
     852          76 : static bool InternalUncompress(Source* r, Writer* writer) {
     853             :   // Read the uncompressed length from the front of the compressed input
     854          76 :   SnappyDecompressor decompressor(r);
     855          76 :   uint32 uncompressed_len = 0;
     856          76 :   if (!decompressor.ReadUncompressedLength(&uncompressed_len)) return false;
     857          76 :   return InternalUncompressAllTags(&decompressor, writer, uncompressed_len);
     858             : }
     859             : 
     860             : template <typename Writer>
     861          76 : static bool InternalUncompressAllTags(SnappyDecompressor* decompressor,
     862             :                                       Writer* writer,
     863             :                                       uint32 uncompressed_len) {
     864          76 :   writer->SetExpectedLength(uncompressed_len);
     865             : 
     866             :   // Process the entire input
     867          76 :   decompressor->DecompressAllTags(writer);
     868          76 :   return (decompressor->eof() && writer->CheckLength());
     869             : }
     870             : 
     871           0 : bool GetUncompressedLength(Source* source, uint32* result) {
     872           0 :   SnappyDecompressor decompressor(source);
     873           0 :   return decompressor.ReadUncompressedLength(result);
     874             : }
     875             : 
     876          88 : size_t Compress(Source* reader, Sink* writer) {
     877          88 :   size_t written = 0;
     878          88 :   size_t N = reader->Available();
     879             :   char ulength[Varint::kMax32];
     880          88 :   char* p = Varint::Encode32(ulength, N);
     881          88 :   writer->Append(ulength, p-ulength);
     882          88 :   written += (p - ulength);
     883             : 
     884          88 :   internal::WorkingMemory wmem;
     885          88 :   char* scratch = NULL;
     886          88 :   char* scratch_output = NULL;
     887             : 
     888        5490 :   while (N > 0) {
     889             :     // Get next block to compress (without copying if possible)
     890             :     size_t fragment_size;
     891        5314 :     const char* fragment = reader->Peek(&fragment_size);
     892        5314 :     assert(fragment_size != 0);  // premature end of input
     893        5314 :     const size_t num_to_read = min(N, kBlockSize);
     894        5314 :     size_t bytes_read = fragment_size;
     895             : 
     896        5314 :     size_t pending_advance = 0;
     897        5314 :     if (bytes_read >= num_to_read) {
     898             :       // Buffer returned by reader is large enough
     899        5314 :       pending_advance = num_to_read;
     900        5314 :       fragment_size = num_to_read;
     901             :     } else {
     902             :       // Read into scratch buffer
     903           0 :       if (scratch == NULL) {
     904             :         // If this is the last iteration, we want to allocate N bytes
     905             :         // of space, otherwise the max possible kBlockSize space.
     906             :         // num_to_read contains exactly the correct value
     907           0 :         scratch = new char[num_to_read];
     908             :       }
     909           0 :       memcpy(scratch, fragment, bytes_read);
     910           0 :       reader->Skip(bytes_read);
     911             : 
     912           0 :       while (bytes_read < num_to_read) {
     913           0 :         fragment = reader->Peek(&fragment_size);
     914           0 :         size_t n = min<size_t>(fragment_size, num_to_read - bytes_read);
     915           0 :         memcpy(scratch + bytes_read, fragment, n);
     916           0 :         bytes_read += n;
     917           0 :         reader->Skip(n);
     918             :       }
     919           0 :       assert(bytes_read == num_to_read);
     920           0 :       fragment = scratch;
     921           0 :       fragment_size = num_to_read;
     922             :     }
     923        5314 :     assert(fragment_size == num_to_read);
     924             : 
     925             :     // Get encoding table for compression
     926             :     int table_size;
     927        5314 :     uint16* table = wmem.GetHashTable(num_to_read, &table_size);
     928             : 
     929             :     // Compress input_fragment and append to dest
     930        5314 :     const int max_output = MaxCompressedLength(num_to_read);
     931             : 
     932             :     // Need a scratch buffer for the output, in case the byte sink doesn't
     933             :     // have room for us directly.
     934        5314 :     if (scratch_output == NULL) {
     935          88 :       scratch_output = new char[max_output];
     936             :     } else {
     937             :       // Since we encode kBlockSize regions followed by a region
     938             :       // which is <= kBlockSize in length, a previously allocated
     939             :       // scratch_output[] region is big enough for this iteration.
     940             :     }
     941        5314 :     char* dest = writer->GetAppendBuffer(max_output, scratch_output);
     942             :     char* end = internal::CompressFragment(fragment, fragment_size,
     943        5314 :                                            dest, table, table_size);
     944        5314 :     writer->Append(dest, end - dest);
     945        5314 :     written += (end - dest);
     946             : 
     947        5314 :     N -= num_to_read;
     948        5314 :     reader->Skip(pending_advance);
     949             :   }
     950             : 
     951          88 :   delete[] scratch;
     952          88 :   delete[] scratch_output;
     953             : 
     954          88 :   return written;
     955             : }
     956             : 
     957             : // -----------------------------------------------------------------------
     958             : // IOVec interfaces
     959             : // -----------------------------------------------------------------------
     960             : 
     961             : // A type that writes to an iovec.
     962             : // Note that this is not a "ByteSink", but a type that matches the
     963             : // Writer template argument to SnappyDecompressor::DecompressAllTags().
     964             : class SnappyIOVecWriter {
     965             :  private:
     966             :   const struct iovec* output_iov_;
     967             :   const size_t output_iov_count_;
     968             : 
     969             :   // We are currently writing into output_iov_[curr_iov_index_].
     970             :   size_t curr_iov_index_;
     971             : 
     972             :   // Bytes written to output_iov_[curr_iov_index_] so far.
     973             :   size_t curr_iov_written_;
     974             : 
     975             :   // Total bytes decompressed into output_iov_ so far.
     976             :   size_t total_written_;
     977             : 
     978             :   // Maximum number of bytes that will be decompressed into output_iov_.
     979             :   size_t output_limit_;
     980             : 
     981           0 :   inline char* GetIOVecPointer(const size_t index, const size_t offset) {
     982           0 :     return reinterpret_cast<char*>(output_iov_[index].iov_base) +
     983           0 :         offset;
     984             :   }
     985             : 
     986             :  public:
     987             :   // Does not take ownership of iov. iov must be valid during the
     988             :   // entire lifetime of the SnappyIOVecWriter.
     989           0 :   inline SnappyIOVecWriter(const struct iovec* iov, size_t iov_count)
     990             :       : output_iov_(iov),
     991             :         output_iov_count_(iov_count),
     992             :         curr_iov_index_(0),
     993             :         curr_iov_written_(0),
     994             :         total_written_(0),
     995           0 :         output_limit_(-1) {
     996           0 :   }
     997             : 
     998           0 :   inline void SetExpectedLength(size_t len) {
     999           0 :     output_limit_ = len;
    1000           0 :   }
    1001             : 
    1002           0 :   inline bool CheckLength() const {
    1003           0 :     return total_written_ == output_limit_;
    1004             :   }
    1005             : 
    1006           0 :   inline bool Append(const char* ip, size_t len) {
    1007           0 :     if (total_written_ + len > output_limit_) {
    1008           0 :       return false;
    1009             :     }
    1010             : 
    1011           0 :     while (len > 0) {
    1012           0 :       assert(curr_iov_written_ <= output_iov_[curr_iov_index_].iov_len);
    1013           0 :       if (curr_iov_written_ >= output_iov_[curr_iov_index_].iov_len) {
    1014             :         // This iovec is full. Go to the next one.
    1015           0 :         if (curr_iov_index_ + 1 >= output_iov_count_) {
    1016           0 :           return false;
    1017             :         }
    1018           0 :         curr_iov_written_ = 0;
    1019           0 :         ++curr_iov_index_;
    1020             :       }
    1021             : 
    1022             :       const size_t to_write = std::min(
    1023           0 :           len, output_iov_[curr_iov_index_].iov_len - curr_iov_written_);
    1024           0 :       memcpy(GetIOVecPointer(curr_iov_index_, curr_iov_written_),
    1025             :              ip,
    1026           0 :              to_write);
    1027           0 :       curr_iov_written_ += to_write;
    1028           0 :       total_written_ += to_write;
    1029           0 :       ip += to_write;
    1030           0 :       len -= to_write;
    1031             :     }
    1032             : 
    1033           0 :     return true;
    1034             :   }
    1035             : 
    1036           0 :   inline bool TryFastAppend(const char* ip, size_t available, size_t len) {
    1037           0 :     const size_t space_left = output_limit_ - total_written_;
    1038           0 :     if (len <= 16 && available >= 16 + kMaximumTagLength && space_left >= 16 &&
    1039           0 :         output_iov_[curr_iov_index_].iov_len - curr_iov_written_ >= 16) {
    1040             :       // Fast path, used for the majority (about 95%) of invocations.
    1041           0 :       char* ptr = GetIOVecPointer(curr_iov_index_, curr_iov_written_);
    1042           0 :       UnalignedCopy64(ip, ptr);
    1043           0 :       UnalignedCopy64(ip + 8, ptr + 8);
    1044           0 :       curr_iov_written_ += len;
    1045           0 :       total_written_ += len;
    1046           0 :       return true;
    1047             :     }
    1048             : 
    1049           0 :     return false;
    1050             :   }
    1051             : 
    1052           0 :   inline bool AppendFromSelf(size_t offset, size_t len) {
    1053           0 :     if (offset > total_written_ || offset == 0) {
    1054           0 :       return false;
    1055             :     }
    1056           0 :     const size_t space_left = output_limit_ - total_written_;
    1057           0 :     if (len > space_left) {
    1058           0 :       return false;
    1059             :     }
    1060             : 
    1061             :     // Locate the iovec from which we need to start the copy.
    1062           0 :     size_t from_iov_index = curr_iov_index_;
    1063           0 :     size_t from_iov_offset = curr_iov_written_;
    1064           0 :     while (offset > 0) {
    1065           0 :       if (from_iov_offset >= offset) {
    1066           0 :         from_iov_offset -= offset;
    1067           0 :         break;
    1068             :       }
    1069             : 
    1070           0 :       offset -= from_iov_offset;
    1071           0 :       --from_iov_index;
    1072           0 :       from_iov_offset = output_iov_[from_iov_index].iov_len;
    1073             :     }
    1074             : 
    1075             :     // Copy <len> bytes starting from the iovec pointed to by from_iov_index to
    1076             :     // the current iovec.
    1077           0 :     while (len > 0) {
    1078           0 :       assert(from_iov_index <= curr_iov_index_);
    1079           0 :       if (from_iov_index != curr_iov_index_) {
    1080             :         const size_t to_copy = std::min(
    1081           0 :             output_iov_[from_iov_index].iov_len - from_iov_offset,
    1082           0 :             len);
    1083           0 :         Append(GetIOVecPointer(from_iov_index, from_iov_offset), to_copy);
    1084           0 :         len -= to_copy;
    1085           0 :         if (len > 0) {
    1086           0 :           ++from_iov_index;
    1087           0 :           from_iov_offset = 0;
    1088             :         }
    1089             :       } else {
    1090           0 :         assert(curr_iov_written_ <= output_iov_[curr_iov_index_].iov_len);
    1091           0 :         size_t to_copy = std::min(output_iov_[curr_iov_index_].iov_len -
    1092             :                                       curr_iov_written_,
    1093           0 :                                   len);
    1094           0 :         if (to_copy == 0) {
    1095             :           // This iovec is full. Go to the next one.
    1096           0 :           if (curr_iov_index_ + 1 >= output_iov_count_) {
    1097           0 :             return false;
    1098             :           }
    1099           0 :           ++curr_iov_index_;
    1100           0 :           curr_iov_written_ = 0;
    1101           0 :           continue;
    1102             :         }
    1103           0 :         if (to_copy > len) {
    1104           0 :           to_copy = len;
    1105             :         }
    1106           0 :         IncrementalCopy(GetIOVecPointer(from_iov_index, from_iov_offset),
    1107             :                         GetIOVecPointer(curr_iov_index_, curr_iov_written_),
    1108           0 :                         to_copy);
    1109           0 :         curr_iov_written_ += to_copy;
    1110           0 :         from_iov_offset += to_copy;
    1111           0 :         total_written_ += to_copy;
    1112           0 :         len -= to_copy;
    1113             :       }
    1114             :     }
    1115             : 
    1116           0 :     return true;
    1117             :   }
    1118             : 
    1119             : };
    1120             : 
    1121           0 : bool RawUncompressToIOVec(const char* compressed, size_t compressed_length,
    1122             :                           const struct iovec* iov, size_t iov_cnt) {
    1123           0 :   ByteArraySource reader(compressed, compressed_length);
    1124           0 :   return RawUncompressToIOVec(&reader, iov, iov_cnt);
    1125             : }
    1126             : 
    1127           0 : bool RawUncompressToIOVec(Source* compressed, const struct iovec* iov,
    1128             :                           size_t iov_cnt) {
    1129           0 :   SnappyIOVecWriter output(iov, iov_cnt);
    1130           0 :   return InternalUncompress(compressed, &output);
    1131             : }
    1132             : 
    1133             : // -----------------------------------------------------------------------
    1134             : // Flat array interfaces
    1135             : // -----------------------------------------------------------------------
    1136             : 
    1137             : // A type that writes to a flat array.
    1138             : // Note that this is not a "ByteSink", but a type that matches the
    1139             : // Writer template argument to SnappyDecompressor::DecompressAllTags().
    1140             : class SnappyArrayWriter {
    1141             :  private:
    1142             :   char* base_;
    1143             :   char* op_;
    1144             :   char* op_limit_;
    1145             : 
    1146             :  public:
    1147          76 :   inline explicit SnappyArrayWriter(char* dst)
    1148             :       : base_(dst),
    1149             :         op_(dst),
    1150          76 :         op_limit_(0) {
    1151          76 :   }
    1152             : 
    1153          76 :   inline void SetExpectedLength(size_t len) {
    1154          76 :     op_limit_ = op_ + len;
    1155          76 :   }
    1156             : 
    1157          76 :   inline bool CheckLength() const {
    1158          76 :     return op_ == op_limit_;
    1159             :   }
    1160             : 
    1161      173302 :   inline bool Append(const char* ip, size_t len) {
    1162      173302 :     char* op = op_;
    1163      173302 :     const size_t space_left = op_limit_ - op;
    1164      173302 :     if (space_left < len) {
    1165           0 :       return false;
    1166             :     }
    1167      173302 :     memcpy(op, ip, len);
    1168      173302 :     op_ = op + len;
    1169      173302 :     return true;
    1170             :   }
    1171             : 
    1172     8071410 :   inline bool TryFastAppend(const char* ip, size_t available, size_t len) {
    1173     8071410 :     char* op = op_;
    1174     8071410 :     const size_t space_left = op_limit_ - op;
    1175     8071410 :     if (len <= 16 && available >= 16 + kMaximumTagLength && space_left >= 16) {
    1176             :       // Fast path, used for the majority (about 95%) of invocations.
    1177     7898108 :       UnalignedCopy64(ip, op);
    1178     7898108 :       UnalignedCopy64(ip + 8, op + 8);
    1179     7898108 :       op_ = op + len;
    1180     7898108 :       return true;
    1181             :     } else {
    1182      173302 :       return false;
    1183             :     }
    1184             :   }
    1185             : 
    1186    14400354 :   inline bool AppendFromSelf(size_t offset, size_t len) {
    1187    14400354 :     char* op = op_;
    1188    14400354 :     const size_t space_left = op_limit_ - op;
    1189             : 
    1190             :     // Check if we try to append from before the start of the buffer.
    1191             :     // Normally this would just be a check for "produced < offset",
    1192             :     // but "produced <= offset - 1u" is equivalent for every case
    1193             :     // except the one where offset==0, where the right side will wrap around
    1194             :     // to a very big number. This is convenient, as offset==0 is another
    1195             :     // invalid case that we also want to catch, so that we do not go
    1196             :     // into an infinite loop.
    1197    14400354 :     assert(op >= base_);
    1198    14400354 :     size_t produced = op - base_;
    1199    14400354 :     if (produced <= offset - 1u) {
    1200           0 :       return false;
    1201             :     }
    1202    14400354 :     if (len <= 16 && offset >= 8 && space_left >= 16) {
    1203             :       // Fast path, used for the majority (70-80%) of dynamic invocations.
    1204    12898504 :       UnalignedCopy64(op - offset, op);
    1205    12898504 :       UnalignedCopy64(op - offset + 8, op + 8);
    1206             :     } else {
    1207     1501850 :       if (space_left >= len + kMaxIncrementCopyOverflow) {
    1208     1501832 :         IncrementalCopyFastPath(op - offset, op, len);
    1209             :       } else {
    1210          18 :         if (space_left < len) {
    1211           0 :           return false;
    1212             :         }
    1213          18 :         IncrementalCopy(op - offset, op, len);
    1214             :       }
    1215             :     }
    1216             : 
    1217    14400354 :     op_ = op + len;
    1218    14400354 :     return true;
    1219             :   }
    1220             : };
    1221             : 
    1222          76 : bool RawUncompress(const char* compressed, size_t n, char* uncompressed) {
    1223          76 :   ByteArraySource reader(compressed, n);
    1224          76 :   return RawUncompress(&reader, uncompressed);
    1225             : }
    1226             : 
    1227          76 : bool RawUncompress(Source* compressed, char* uncompressed) {
    1228          76 :   SnappyArrayWriter output(uncompressed);
    1229          76 :   return InternalUncompress(compressed, &output);
    1230             : }
    1231             : 
    1232           0 : bool Uncompress(const char* compressed, size_t n, string* uncompressed) {
    1233             :   size_t ulength;
    1234           0 :   if (!GetUncompressedLength(compressed, n, &ulength)) {
    1235           0 :     return false;
    1236             :   }
    1237             :   // On 32-bit builds: max_size() < kuint32max.  Check for that instead
    1238             :   // of crashing (e.g., consider externally specified compressed data).
    1239           0 :   if (ulength > uncompressed->max_size()) {
    1240           0 :     return false;
    1241             :   }
    1242           0 :   STLStringResizeUninitialized(uncompressed, ulength);
    1243           0 :   return RawUncompress(compressed, n, string_as_array(uncompressed));
    1244             : }
    1245             : 
    1246             : 
    1247             : // A Writer that drops everything on the floor and just does validation
    1248             : class SnappyDecompressionValidator {
    1249             :  private:
    1250             :   size_t expected_;
    1251             :   size_t produced_;
    1252             : 
    1253             :  public:
    1254           0 :   inline SnappyDecompressionValidator() : expected_(0), produced_(0) { }
    1255           0 :   inline void SetExpectedLength(size_t len) {
    1256           0 :     expected_ = len;
    1257           0 :   }
    1258           0 :   inline bool CheckLength() const {
    1259           0 :     return expected_ == produced_;
    1260             :   }
    1261           0 :   inline bool Append(const char*, size_t len) {
    1262           0 :     produced_ += len;
    1263           0 :     return produced_ <= expected_;
    1264             :   }
    1265           0 :   inline bool TryFastAppend(const char*, size_t, size_t) {
    1266           0 :     return false;
    1267             :   }
    1268           0 :   inline bool AppendFromSelf(size_t offset, size_t len) {
    1269             :     // See SnappyArrayWriter::AppendFromSelf for an explanation of
    1270             :     // the "offset - 1u" trick.
    1271           0 :     if (produced_ <= offset - 1u) return false;
    1272           0 :     produced_ += len;
    1273           0 :     return produced_ <= expected_;
    1274             :   }
    1275             : };
    1276             : 
    1277           0 : bool IsValidCompressedBuffer(const char* compressed, size_t n) {
    1278           0 :   ByteArraySource reader(compressed, n);
    1279           0 :   SnappyDecompressionValidator writer;
    1280           0 :   return InternalUncompress(&reader, &writer);
    1281             : }
    1282             : 
    1283          88 : void RawCompress(const char* input,
    1284             :                  size_t input_length,
    1285             :                  char* compressed,
    1286             :                  size_t* compressed_length) {
    1287          88 :   ByteArraySource reader(input, input_length);
    1288         176 :   UncheckedByteArraySink writer(compressed);
    1289          88 :   Compress(&reader, &writer);
    1290             : 
    1291             :   // Compute how many bytes were added
    1292         176 :   *compressed_length = (writer.CurrentDestination() - compressed);
    1293          88 : }
    1294             : 
    1295           0 : size_t Compress(const char* input, size_t input_length, string* compressed) {
    1296             :   // Pre-grow the buffer to the max length of the compressed output
    1297           0 :   compressed->resize(MaxCompressedLength(input_length));
    1298             : 
    1299             :   size_t compressed_length;
    1300             :   RawCompress(input, input_length, string_as_array(compressed),
    1301           0 :               &compressed_length);
    1302           0 :   compressed->resize(compressed_length);
    1303           0 :   return compressed_length;
    1304             : }
    1305             : 
    1306             : 
    1307             : } // end namespace snappy

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