1// Copyright 2008 Google Inc. All Rights Reserved.
2//
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4// modification, are permitted provided that the following conditions are
5// met:
6//
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11// in the documentation and/or other materials provided with the
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16//
17// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
18// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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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// Internals shared between the Snappy implementation and its unittest.
30
31#ifndef THIRD_PARTY_SNAPPY_SNAPPY_INTERNAL_H_
32#define THIRD_PARTY_SNAPPY_SNAPPY_INTERNAL_H_
33
34#include "snappy-stubs-internal.h"
35
36namespace snappy {
37namespace internal {
38
39// Working memory performs a single allocation to hold all scratch space
40// required for compression.
41class WorkingMemory {
42 public:
43 explicit WorkingMemory(size_t input_size);
44 ~WorkingMemory();
45
46 // Allocates and clears a hash table using memory in "*this",
47 // stores the number of buckets in "*table_size" and returns a pointer to
48 // the base of the hash table.
49 uint16* GetHashTable(size_t fragment_size, int* table_size) const;
50 char* GetScratchInput() const { return input_; }
51 char* GetScratchOutput() const { return output_; }
52
53 private:
54 char* mem_; // the allocated memory, never nullptr
55 size_t size_; // the size of the allocated memory, never 0
56 uint16* table_; // the pointer to the hashtable
57 char* input_; // the pointer to the input scratch buffer
58 char* output_; // the pointer to the output scratch buffer
59
60 // No copying
61 WorkingMemory(const WorkingMemory&);
62 void operator=(const WorkingMemory&);
63};
64
65// Flat array compression that does not emit the "uncompressed length"
66// prefix. Compresses "input" string to the "*op" buffer.
67//
68// REQUIRES: "input_length <= kBlockSize"
69// REQUIRES: "op" points to an array of memory that is at least
70// "MaxCompressedLength(input_length)" in size.
71// REQUIRES: All elements in "table[0..table_size-1]" are initialized to zero.
72// REQUIRES: "table_size" is a power of two
73//
74// Returns an "end" pointer into "op" buffer.
75// "end - op" is the compressed size of "input".
76char* CompressFragment(const char* input,
77 size_t input_length,
78 char* op,
79 uint16* table,
80 const int table_size);
81
82// Find the largest n such that
83//
84// s1[0,n-1] == s2[0,n-1]
85// and n <= (s2_limit - s2).
86//
87// Return make_pair(n, n < 8).
88// Does not read *s2_limit or beyond.
89// Does not read *(s1 + (s2_limit - s2)) or beyond.
90// Requires that s2_limit >= s2.
91//
92// Separate implementation for 64-bit, little-endian cpus.
93#if !defined(SNAPPY_IS_BIG_ENDIAN) && \
94 (defined(ARCH_K8) || defined(ARCH_PPC) || defined(ARCH_ARM))
95static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
96 const char* s2,
97 const char* s2_limit) {
98 assert(s2_limit >= s2);
99 size_t matched = 0;
100
101 // This block isn't necessary for correctness; we could just start looping
102 // immediately. As an optimization though, it is useful. It creates some not
103 // uncommon code paths that determine, without extra effort, whether the match
104 // length is less than 8. In short, we are hoping to avoid a conditional
105 // branch, and perhaps get better code layout from the C++ compiler.
106 if (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 8)) {
107 uint64 a1 = UNALIGNED_LOAD64(s1);
108 uint64 a2 = UNALIGNED_LOAD64(s2);
109 if (a1 != a2) {
110 return std::pair<size_t, bool>(Bits::FindLSBSetNonZero64(a1 ^ a2) >> 3,
111 true);
112 } else {
113 matched = 8;
114 s2 += 8;
115 }
116 }
117
118 // Find out how long the match is. We loop over the data 64 bits at a
119 // time until we find a 64-bit block that doesn't match; then we find
120 // the first non-matching bit and use that to calculate the total
121 // length of the match.
122 while (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 8)) {
123 if (UNALIGNED_LOAD64(s2) == UNALIGNED_LOAD64(s1 + matched)) {
124 s2 += 8;
125 matched += 8;
126 } else {
127 uint64 x = UNALIGNED_LOAD64(s2) ^ UNALIGNED_LOAD64(s1 + matched);
128 int matching_bits = Bits::FindLSBSetNonZero64(x);
129 matched += matching_bits >> 3;
130 assert(matched >= 8);
131 return std::pair<size_t, bool>(matched, false);
132 }
133 }
134 while (SNAPPY_PREDICT_TRUE(s2 < s2_limit)) {
135 if (s1[matched] == *s2) {
136 ++s2;
137 ++matched;
138 } else {
139 return std::pair<size_t, bool>(matched, matched < 8);
140 }
141 }
142 return std::pair<size_t, bool>(matched, matched < 8);
143}
144#else
145static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
146 const char* s2,
147 const char* s2_limit) {
148 // Implementation based on the x86-64 version, above.
149 assert(s2_limit >= s2);
150 int matched = 0;
151
152 while (s2 <= s2_limit - 4 &&
153 UNALIGNED_LOAD32(s2) == UNALIGNED_LOAD32(s1 + matched)) {
154 s2 += 4;
155 matched += 4;
156 }
157 if (LittleEndian::IsLittleEndian() && s2 <= s2_limit - 4) {
158 uint32 x = UNALIGNED_LOAD32(s2) ^ UNALIGNED_LOAD32(s1 + matched);
159 int matching_bits = Bits::FindLSBSetNonZero(x);
160 matched += matching_bits >> 3;
161 } else {
162 while ((s2 < s2_limit) && (s1[matched] == *s2)) {
163 ++s2;
164 ++matched;
165 }
166 }
167 return std::pair<size_t, bool>(matched, matched < 8);
168}
169#endif
170
171// Lookup tables for decompression code. Give --snappy_dump_decompression_table
172// to the unit test to recompute char_table.
173
174enum {
175 LITERAL = 0,
176 COPY_1_BYTE_OFFSET = 1, // 3 bit length + 3 bits of offset in opcode
177 COPY_2_BYTE_OFFSET = 2,
178 COPY_4_BYTE_OFFSET = 3
179};
180static const int kMaximumTagLength = 5; // COPY_4_BYTE_OFFSET plus the actual offset.
181
182// Data stored per entry in lookup table:
183// Range Bits-used Description
184// ------------------------------------
185// 1..64 0..7 Literal/copy length encoded in opcode byte
186// 0..7 8..10 Copy offset encoded in opcode byte / 256
187// 0..4 11..13 Extra bytes after opcode
188//
189// We use eight bits for the length even though 7 would have sufficed
190// because of efficiency reasons:
191// (1) Extracting a byte is faster than a bit-field
192// (2) It properly aligns copy offset so we do not need a <<8
193static const uint16 char_table[256] = {
194 0x0001, 0x0804, 0x1001, 0x2001, 0x0002, 0x0805, 0x1002, 0x2002,
195 0x0003, 0x0806, 0x1003, 0x2003, 0x0004, 0x0807, 0x1004, 0x2004,
196 0x0005, 0x0808, 0x1005, 0x2005, 0x0006, 0x0809, 0x1006, 0x2006,
197 0x0007, 0x080a, 0x1007, 0x2007, 0x0008, 0x080b, 0x1008, 0x2008,
198 0x0009, 0x0904, 0x1009, 0x2009, 0x000a, 0x0905, 0x100a, 0x200a,
199 0x000b, 0x0906, 0x100b, 0x200b, 0x000c, 0x0907, 0x100c, 0x200c,
200 0x000d, 0x0908, 0x100d, 0x200d, 0x000e, 0x0909, 0x100e, 0x200e,
201 0x000f, 0x090a, 0x100f, 0x200f, 0x0010, 0x090b, 0x1010, 0x2010,
202 0x0011, 0x0a04, 0x1011, 0x2011, 0x0012, 0x0a05, 0x1012, 0x2012,
203 0x0013, 0x0a06, 0x1013, 0x2013, 0x0014, 0x0a07, 0x1014, 0x2014,
204 0x0015, 0x0a08, 0x1015, 0x2015, 0x0016, 0x0a09, 0x1016, 0x2016,
205 0x0017, 0x0a0a, 0x1017, 0x2017, 0x0018, 0x0a0b, 0x1018, 0x2018,
206 0x0019, 0x0b04, 0x1019, 0x2019, 0x001a, 0x0b05, 0x101a, 0x201a,
207 0x001b, 0x0b06, 0x101b, 0x201b, 0x001c, 0x0b07, 0x101c, 0x201c,
208 0x001d, 0x0b08, 0x101d, 0x201d, 0x001e, 0x0b09, 0x101e, 0x201e,
209 0x001f, 0x0b0a, 0x101f, 0x201f, 0x0020, 0x0b0b, 0x1020, 0x2020,
210 0x0021, 0x0c04, 0x1021, 0x2021, 0x0022, 0x0c05, 0x1022, 0x2022,
211 0x0023, 0x0c06, 0x1023, 0x2023, 0x0024, 0x0c07, 0x1024, 0x2024,
212 0x0025, 0x0c08, 0x1025, 0x2025, 0x0026, 0x0c09, 0x1026, 0x2026,
213 0x0027, 0x0c0a, 0x1027, 0x2027, 0x0028, 0x0c0b, 0x1028, 0x2028,
214 0x0029, 0x0d04, 0x1029, 0x2029, 0x002a, 0x0d05, 0x102a, 0x202a,
215 0x002b, 0x0d06, 0x102b, 0x202b, 0x002c, 0x0d07, 0x102c, 0x202c,
216 0x002d, 0x0d08, 0x102d, 0x202d, 0x002e, 0x0d09, 0x102e, 0x202e,
217 0x002f, 0x0d0a, 0x102f, 0x202f, 0x0030, 0x0d0b, 0x1030, 0x2030,
218 0x0031, 0x0e04, 0x1031, 0x2031, 0x0032, 0x0e05, 0x1032, 0x2032,
219 0x0033, 0x0e06, 0x1033, 0x2033, 0x0034, 0x0e07, 0x1034, 0x2034,
220 0x0035, 0x0e08, 0x1035, 0x2035, 0x0036, 0x0e09, 0x1036, 0x2036,
221 0x0037, 0x0e0a, 0x1037, 0x2037, 0x0038, 0x0e0b, 0x1038, 0x2038,
222 0x0039, 0x0f04, 0x1039, 0x2039, 0x003a, 0x0f05, 0x103a, 0x203a,
223 0x003b, 0x0f06, 0x103b, 0x203b, 0x003c, 0x0f07, 0x103c, 0x203c,
224 0x0801, 0x0f08, 0x103d, 0x203d, 0x1001, 0x0f09, 0x103e, 0x203e,
225 0x1801, 0x0f0a, 0x103f, 0x203f, 0x2001, 0x0f0b, 0x1040, 0x2040
226};
227
228} // end namespace internal
229} // end namespace snappy
230
231#endif // THIRD_PARTY_SNAPPY_SNAPPY_INTERNAL_H_
232