1 // Copyright 2013 Google Inc. All Rights Reserved. 2 // 3 // Licensed under the Apache License, Version 2.0 (the "License"); 4 // you may not use this file except in compliance with the License. 5 // You may obtain a copy of the License at 6 // 7 // http://www.apache.org/licenses/LICENSE-2.0 8 // 9 // Unless required by applicable law or agreed to in writing, software 10 // distributed under the License is distributed on an "AS IS" BASIS, 11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12 // See the License for the specific language governing permissions and 13 // limitations under the License. 14 // 15 // Functions for encoding of integers into prefix codes the amount of extra 16 // bits, and the actual values of the extra bits. 17 18 #include "./prefix.h" 19 20 #include "./fast_log.h" 21 22 namespace brotli { 23 24 // Represents the range of values belonging to a prefix code: 25 // [offset, offset + 2^nbits) 26 struct PrefixCodeRange { 27 int offset; 28 int nbits; 29 }; 30 31 static const PrefixCodeRange kBlockLengthPrefixCode[kNumBlockLenPrefixes] = { 32 { 1, 2}, { 5, 2}, { 9, 2}, { 13, 2}, 33 { 17, 3}, { 25, 3}, { 33, 3}, { 41, 3}, 34 { 49, 4}, { 65, 4}, { 81, 4}, { 97, 4}, 35 { 113, 5}, { 145, 5}, { 177, 5}, { 209, 5}, 36 { 241, 6}, { 305, 6}, { 369, 7}, { 497, 8}, 37 { 753, 9}, { 1265, 10}, {2289, 11}, {4337, 12}, 38 {8433, 13}, {16625, 24} 39 }; 40 41 static const PrefixCodeRange kInsertLengthPrefixCode[kNumInsertLenPrefixes] = { 42 { 0, 0}, { 1, 0}, { 2, 0}, { 3, 0}, 43 { 4, 0}, { 5, 0}, { 6, 1}, { 8, 1}, 44 { 10, 2}, { 14, 2}, { 18, 3}, { 26, 3}, 45 { 34, 4}, { 50, 4}, { 66, 5}, { 98, 5}, 46 { 130, 6}, { 194, 7}, { 322, 8}, { 578, 9}, 47 {1090, 10}, {2114, 12}, {6210, 14}, {22594, 24}, 48 }; 49 50 static const PrefixCodeRange kCopyLengthPrefixCode[kNumCopyLenPrefixes] = { 51 { 2, 0}, { 3, 0}, { 4, 0}, { 5, 0}, 52 { 6, 0}, { 7, 0}, { 8, 0}, { 9, 0}, 53 { 10, 1}, { 12, 1}, { 14, 2}, { 18, 2}, 54 { 22, 3}, { 30, 3}, { 38, 4}, { 54, 4}, 55 { 70, 5}, { 102, 5}, { 134, 6}, { 198, 7}, 56 {326, 8}, { 582, 9}, {1094, 10}, {2118, 24}, 57 }; 58 59 static const int kInsertAndCopyRangeLut[9] = { 60 0, 1, 4, 2, 3, 6, 5, 7, 8, 61 }; 62 63 static const int kInsertRangeLut[9] = { 64 0, 0, 1, 1, 0, 2, 1, 2, 2, 65 }; 66 67 static const int kCopyRangeLut[9] = { 68 0, 1, 0, 1, 2, 0, 2, 1, 2, 69 }; 70 71 int InsertLengthPrefix(int length) { 72 for (int i = 0; i < kNumInsertLenPrefixes; ++i) { 73 const PrefixCodeRange& range = kInsertLengthPrefixCode[i]; 74 if (length >= range.offset && length < range.offset + (1 << range.nbits)) { 75 return i; 76 } 77 } 78 return -1; 79 } 80 81 int CopyLengthPrefix(int length) { 82 for (int i = 0; i < kNumCopyLenPrefixes; ++i) { 83 const PrefixCodeRange& range = kCopyLengthPrefixCode[i]; 84 if (length >= range.offset && length < range.offset + (1 << range.nbits)) { 85 return i; 86 } 87 } 88 return -1; 89 } 90 91 int CommandPrefix(int insert_length, int copy_length) { 92 if (copy_length == 0) { 93 copy_length = 4; 94 } 95 int insert_prefix = InsertLengthPrefix(insert_length); 96 int copy_prefix = CopyLengthPrefix(copy_length); 97 int range_idx = 3 * (insert_prefix >> 3) + (copy_prefix >> 3); 98 return ((kInsertAndCopyRangeLut[range_idx] << 6) + 99 ((insert_prefix & 7) << 3) + (copy_prefix & 7)); 100 } 101 102 int InsertLengthExtraBits(int code) { 103 int insert_code = (kInsertRangeLut[code >> 6] << 3) + ((code >> 3) & 7); 104 return kInsertLengthPrefixCode[insert_code].nbits; 105 } 106 107 int InsertLengthOffset(int code) { 108 int insert_code = (kInsertRangeLut[code >> 6] << 3) + ((code >> 3) & 7); 109 return kInsertLengthPrefixCode[insert_code].offset; 110 } 111 112 int CopyLengthExtraBits(int code) { 113 int copy_code = (kCopyRangeLut[code >> 6] << 3) + (code & 7); 114 return kCopyLengthPrefixCode[copy_code].nbits; 115 } 116 117 int CopyLengthOffset(int code) { 118 int copy_code = (kCopyRangeLut[code >> 6] << 3) + (code & 7); 119 return kCopyLengthPrefixCode[copy_code].offset; 120 } 121 122 void PrefixEncodeCopyDistance(int distance_code, 123 int num_direct_codes, 124 int postfix_bits, 125 uint16_t* code, 126 int* nbits, 127 uint32_t* extra_bits) { 128 distance_code -= 1; 129 if (distance_code < kNumDistanceShortCodes + num_direct_codes) { 130 *code = distance_code; 131 *nbits = 0; 132 *extra_bits = 0; 133 return; 134 } 135 distance_code -= kNumDistanceShortCodes + num_direct_codes; 136 distance_code += (1 << (postfix_bits + 2)); 137 int bucket = Log2Floor(distance_code) - 1; 138 int postfix_mask = (1 << postfix_bits) - 1; 139 int postfix = distance_code & postfix_mask; 140 int prefix = (distance_code >> bucket) & 1; 141 int offset = (2 + prefix) << bucket; 142 *nbits = bucket - postfix_bits; 143 *code = kNumDistanceShortCodes + num_direct_codes + 144 ((2 * (*nbits - 1) + prefix) << postfix_bits) + postfix; 145 *extra_bits = (distance_code - offset) >> postfix_bits; 146 } 147 148 int BlockLengthPrefix(int length) { 149 for (int i = 0; i < kNumBlockLenPrefixes; ++i) { 150 const PrefixCodeRange& range = kBlockLengthPrefixCode[i]; 151 if (length >= range.offset && length < range.offset + (1 << range.nbits)) { 152 return i; 153 } 154 } 155 return -1; 156 } 157 158 int BlockLengthExtraBits(int length_code) { 159 return kBlockLengthPrefixCode[length_code].nbits; 160 } 161 162 int BlockLengthOffset(int length_code) { 163 return kBlockLengthPrefixCode[length_code].offset; 164 } 165 166 } // namespace brotli 167
