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29 // Internals shared between the Snappy implementation and its unittest.
31 #ifndef UTIL_SNAPPY_SNAPPY_INTERNAL_H_
32 #define UTIL_SNAPPY_SNAPPY_INTERNAL_H_
34 #include "snappy-stubs-internal.h"
41 WorkingMemory() : large_table_(NULL) { }
42 ~WorkingMemory() { delete[] large_table_; }
44 // Allocates and clears a hash table using memory in "*this",
45 // stores the number of buckets in "*table_size" and returns a pointer to
46 // the base of the hash table.
47 uint16* GetHashTable(size_t input_size, int* table_size);
50 uint16 small_table_[1<<10]; // 2KB
51 uint16* large_table_; // Allocated only when needed
53 DISALLOW_COPY_AND_ASSIGN(WorkingMemory);
56 // Flat array compression that does not emit the "uncompressed length"
57 // prefix. Compresses "input" string to the "*op" buffer.
59 // REQUIRES: "input_length <= kBlockSize"
60 // REQUIRES: "op" points to an array of memory that is at least
61 // "MaxCompressedLength(input_length)" in size.
62 // REQUIRES: All elements in "table[0..table_size-1]" are initialized to zero.
63 // REQUIRES: "table_size" is a power of two
65 // Returns an "end" pointer into "op" buffer.
66 // "end - op" is the compressed size of "input".
67 char* CompressFragment(const char* input,
71 const int table_size);
73 // Return the largest n such that
75 // s1[0,n-1] == s2[0,n-1]
76 // and n <= (s2_limit - s2).
78 // Does not read *s2_limit or beyond.
79 // Does not read *(s1 + (s2_limit - s2)) or beyond.
80 // Requires that s2_limit >= s2.
82 // Separate implementation for x86_64, for speed. Uses the fact that
83 // x86_64 is little endian.
85 static inline int FindMatchLength(const char* s1,
87 const char* s2_limit) {
88 DCHECK_GE(s2_limit, s2);
91 // Find out how long the match is. We loop over the data 64 bits at a
92 // time until we find a 64-bit block that doesn't match; then we find
93 // the first non-matching bit and use that to calculate the total
94 // length of the match.
95 while (PREDICT_TRUE(s2 <= s2_limit - 8)) {
96 if (PREDICT_FALSE(UNALIGNED_LOAD64(s2) == UNALIGNED_LOAD64(s1 + matched))) {
100 // On current (mid-2008) Opteron models there is a 3% more
101 // efficient code sequence to find the first non-matching byte.
102 // However, what follows is ~10% better on Intel Core 2 and newer,
103 // and we expect AMD's bsf instruction to improve.
104 uint64 x = UNALIGNED_LOAD64(s2) ^ UNALIGNED_LOAD64(s1 + matched);
105 int matching_bits = Bits::FindLSBSetNonZero64(x);
106 matched += matching_bits >> 3;
110 while (PREDICT_TRUE(s2 < s2_limit)) {
111 if (PREDICT_TRUE(s1[matched] == *s2)) {
121 static inline int FindMatchLength(const char* s1,
123 const char* s2_limit) {
124 // Implementation based on the x86-64 version, above.
125 DCHECK_GE(s2_limit, s2);
128 while (s2 <= s2_limit - 4 &&
129 UNALIGNED_LOAD32(s2) == UNALIGNED_LOAD32(s1 + matched)) {
133 if (LittleEndian::IsLittleEndian() && s2 <= s2_limit - 4) {
134 uint32 x = UNALIGNED_LOAD32(s2) ^ UNALIGNED_LOAD32(s1 + matched);
135 int matching_bits = Bits::FindLSBSetNonZero(x);
136 matched += matching_bits >> 3;
138 while ((s2 < s2_limit) && (s1[matched] == *s2)) {
147 } // end namespace internal
148 } // end namespace snappy
150 #endif // UTIL_SNAPPY_SNAPPY_INTERNAL_H_