text: // Copyright 2008 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.
  //
  // Internals shared between the Snappy implementation and its unittest.
  
  #ifndef SNAPPY_INTERNAL_H_
  #define SNAPPY_INTERNAL_H_
  
  #include "snappy-stubs-internal.h"
  
  namespace snappy {
  namespace internal {
  
  class WorkingMemory {
   public:
    WorkingMemory() : large_table_(NULL) { }
    ~WorkingMemory() { delete[] large_table_; }
  
    // Allocates and clears a hash table using memory in "*this",
    // stores the number of buckets in "*table_size" and returns a pointer to
    // the base of the hash table.
    uint16* GetHashTable(size_t input_size, int* table_size);
  
   private:
    uint16 small_table_[1<<10];    // 2KB
    uint16* large_table_;          // Allocated only when needed
  
    // No copying
    WorkingMemory(const WorkingMemory&);
    void operator=(const WorkingMemory&);
  };
  
  // Flat array compression that does not emit the "uncompressed length"
  // prefix. Compresses "input" string to the "*op" buffer.
  //
  // REQUIRES: "input_length <= kBlockSize"
  // REQUIRES: "op" points to an array of memory that is at least
  // "MaxCompressedLength(input_length)" 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".
  char* CompressFragment(const char* input,
                         size_t input_length,
                         char* op,
                         uint16* table,
                         const int table_size);
  
  // Find the largest n such that
  //
  //   s1[0,n-1] == s2[0,n-1]
  //   and n <= (s2_limit - s2).
  //
  // Return make_pair(n, n < 8).
  // Does not read *s2_limit or beyond.
  // Does not read *(s1 + (s2_limit - s2)) or beyond.
  // Requires that s2_limit >= s2.
  //
  // Separate implementation for 64-bit, little-endian cpus.
  #if !defined(SNAPPY_IS_BIG_ENDIAN) && \
      (defined(ARCH_K8) || defined(ARCH_PPC) || defined(ARCH_ARM))
  static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
                                                        const char* s2,
                                                        const char* s2_limit) {
    assert(s2_limit >= s2);
    size_t matched = 0;
  
    // This block isn't necessary for correctness; we could just start looping
    // immediately.  As an optimization though, it is useful.  It creates some not
    // uncommon code paths that determine, without extra effort, whether the match
    // length is less than 8.  In short, we are hoping to avoid a conditional
    // branch, and perhaps get better code layout from the C++ compiler.
    if (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 8)) {
      uint64 a1 = UNALIGNED_LOAD64(s1);
      uint64 a2 = UNALIGNED_LOAD64(s2);
      if (a1 != a2) {
        return std::pair<size_t, bool>(Bits::FindLSBSetNonZero64(a1 ^ a2) >> 3,
                                       true);
      } else {
        matched = 8;
        s2 += 8;
      }
    }
  
    // Find out how long the match is. We loop over the data 64 bits at a
    // time until we find a 64-bit block that doesn't match; then we find
    // the first non-matching bit and use that to calculate the total
    // length of the match.
    while (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 8)) {
      if (UNALIGNED_LOAD64(s2) == UNALIGNED_LOAD64(s1 + matched)) {
        s2 += 8;
        matched += 8;
      } else {
        uint64 x = UNALIGNED_LOAD64(s2) ^ UNALIGNED_LOAD64(s1 + matched);
        int matching_bits = Bits::FindLSBSetNonZero64(x);
        matched += matching_bits >> 3;
        assert(matched >= 8);
        return std::pair<size_t, bool>(matched, false);
      }
    }
    while (SNAPPY_PREDICT_TRUE(s2 < s2_limit)) {
      if (s1[matched] == *s2) {
        ++s2;
        ++matched;
      } else {
        return std::pair<size_t, bool>(matched, matched < 8);
      }
    }
    return std::pair<size_t, bool>(matched, matched < 8);
  }
  #else
  static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
                                                        const char* s2,
                                                        const char* s2_limit) {
    // Implementation based on the x86-64 version, above.
    assert(s2_limit >= s2);
    int matched = 0;
  
    while (s2 <= s2_limit - 4 &&
           UNALIGNED_LOAD32(s2) == UNALIGNED_LOAD32(s1 + matched)) {
      s2 += 4;
      matched += 4;
    }
    if (LittleEndian::IsLittleEndian() && s2 <= s2_limit - 4) {
      uint32 x = UNALIGNED_LOAD32(s2) ^ UNALIGNED_LOAD32(s1 + matched);
      int matching_bits = Bits::FindLSBSetNonZero(x);
      matched += matching_bits >> 3;
    } else {
      while ((s2 < s2_limit) && (s1[matched] == *s2)) {
        ++s2;
        ++matched;
      }
    }
    return std::pair<size_t, bool>(matched, matched < 8);
  }
  #endif
  
  // Lookup tables for decompression code.  Give --snappy_dump_decompression_table
  // to the unit test to recompute char_table.
  
  enum {
    LITERAL = 0,
    COPY_1_BYTE_OFFSET = 1,  // 3 bit length + 3 bits of offset in opcode
    COPY_2_BYTE_OFFSET = 2,
    COPY_4_BYTE_OFFSET = 3
  };
  static const int kMaximumTagLength = 5;  // COPY_4_BYTE_OFFSET plus the actual offset.
  
  // Data stored per entry in lookup table:
  //      Range   Bits-used       Description
  //      ------------------------------------
  //      1..64   0..7            Literal/copy length encoded in opcode byte
  //      0..7    8..10           Copy offset encoded in opcode byte / 256
  //      0..4    11..13          Extra bytes after opcode
  //
  // We use eight bits for the length even though 7 would have sufficed
  // because of efficiency reasons:
  //      (1) Extracting a byte is faster than a bit-field
  //      (2) It properly aligns copy offset so we do not need a <<8
  static const uint16 char_table[256] = {
    0x0001, 0x0804, 0x1001, 0x2001, 0x0002, 0x0805, 0x1002, 0x2002,
    0x0003, 0x0806, 0x1003, 0x2003, 0x0004, 0x0807, 0x1004, 0x2004,
    0x0005, 0x0808, 0x1005, 0x2005, 0x0006, 0x0809, 0x1006, 0x2006,
    0x0007, 0x080a, 0x1007, 0x2007, 0x0008, 0x080b, 0x1008, 0x2008,
    0x0009, 0x0904, 0x1009, 0x2009, 0x000a, 0x0905, 0x100a, 0x200a,
    0x000b, 0x0906, 0x100b, 0x200b, 0x000c, 0x0907, 0x100c, 0x200c,
    0x000d, 0x0908, 0x100d, 0x200d, 0x000e, 0x0909, 0x100e, 0x200e,
    0x000f, 0x090a, 0x100f, 0x200f, 0x0010, 0x090b, 0x1010, 0x2010,
    0x0011, 0x0a04, 0x1011, 0x2011, 0x0012, 0x0a05, 0x1012, 0x2012,
    0x0013, 0x0a06, 0x1013, 0x2013, 0x0014, 0x0a07, 0x1014, 0x2014,
    0x0015, 0x0a08, 0x1015, 0x2015, 0x0016, 0x0a09, 0x1016, 0x2016,
    0x0017, 0x0a0a, 0x1017, 0x2017, 0x0018, 0x0a0b, 0x1018, 0x2018,
    0x0019, 0x0b04, 0x1019, 0x2019, 0x001a, 0x0b05, 0x101a, 0x201a,
    0x001b, 0x0b06, 0x101b, 0x201b, 0x001c, 0x0b07, 0x101c, 0x201c,
    0x001d, 0x0b08, 0x101d, 0x201d, 0x001e, 0x0b09, 0x101e, 0x201e,
    0x001f, 0x0b0a, 0x101f, 0x201f, 0x0020, 0x0b0b, 0x1020, 0x2020,
    0x0021, 0x0c04, 0x1021, 0x2021, 0x0022, 0x0c05, 0x1022, 0x2022,
    0x0023, 0x0c06, 0x1023, 0x2023, 0x0024, 0x0c07, 0x1024, 0x2024,
    0x0025, 0x0c08, 0x1025, 0x2025, 0x0026, 0x0c09, 0x1026, 0x2026,
    0x0027, 0x0c0a, 0x1027, 0x2027, 0x0028, 0x0c0b, 0x1028, 0x2028,
    0x0029, 0x0d04, 0x1029, 0x2029, 0x002a, 0x0d05, 0x102a, 0x202a,
    0x002b, 0x0d06, 0x102b, 0x202b, 0x002c, 0x0d07, 0x102c, 0x202c,
    0x002d, 0x0d08, 0x102d, 0x202d, 0x002e, 0x0d09, 0x102e, 0x202e,
    0x002f, 0x0d0a, 0x102f, 0x202f, 0x0030, 0x0d0b, 0x1030, 0x2030,
    0x0031, 0x0e04, 0x1031, 0x2031, 0x0032, 0x0e05, 0x1032, 0x2032,
    0x0033, 0x0e06, 0x1033, 0x2033, 0x0034, 0x0e07, 0x1034, 0x2034,
    0x0035, 0x0e08, 0x1035, 0x2035, 0x0036, 0x0e09, 0x1036, 0x2036,
    0x0037, 0x0e0a, 0x1037, 0x2037, 0x0038, 0x0e0b, 0x1038, 0x2038,
    0x0039, 0x0f04, 0x1039, 0x2039, 0x003a, 0x0f05, 0x103a, 0x203a,
    0x003b, 0x0f06, 0x103b, 0x203b, 0x003c, 0x0f07, 0x103c, 0x203c,
    0x0801, 0x0f08, 0x103d, 0x203d, 0x1001, 0x0f09, 0x103e, 0x203e,
    0x1801, 0x0f0a, 0x103f, 0x203f, 0x2001, 0x0f0b, 0x1040, 0x2040
  };
  
  }  // end namespace internal
  }  // end namespace snappy
  
  #endif  // SNAPPY_SNAPPY_INTERNAL_H_
  