mirror of
https://github.com/google/brotli.git
synced 2024-11-22 19:50:06 +00:00
80 lines
2.8 KiB
C++
80 lines
2.8 KiB
C++
/* Copyright 2013 Google Inc. All Rights Reserved.
|
|
|
|
Distributed under MIT license.
|
|
See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
|
|
*/
|
|
|
|
// Functions for encoding of integers into prefix codes the amount of extra
|
|
// bits, and the actual values of the extra bits.
|
|
|
|
#ifndef BROTLI_ENC_PREFIX_H_
|
|
#define BROTLI_ENC_PREFIX_H_
|
|
|
|
#include "./fast_log.h"
|
|
#include "./types.h"
|
|
|
|
namespace brotli {
|
|
|
|
static const uint32_t kNumInsertLenPrefixes = 24;
|
|
static const uint32_t kNumCopyLenPrefixes = 24;
|
|
static const uint32_t kNumCommandPrefixes = 704;
|
|
static const uint32_t kNumBlockLenPrefixes = 26;
|
|
static const uint32_t kNumDistanceShortCodes = 16;
|
|
static const uint32_t kNumDistancePrefixes = 520;
|
|
|
|
// Represents the range of values belonging to a prefix code:
|
|
// [offset, offset + 2^nbits)
|
|
struct PrefixCodeRange {
|
|
uint32_t offset;
|
|
uint32_t nbits;
|
|
};
|
|
|
|
static const PrefixCodeRange kBlockLengthPrefixCode[kNumBlockLenPrefixes] = {
|
|
{ 1, 2}, { 5, 2}, { 9, 2}, { 13, 2},
|
|
{ 17, 3}, { 25, 3}, { 33, 3}, { 41, 3},
|
|
{ 49, 4}, { 65, 4}, { 81, 4}, { 97, 4},
|
|
{ 113, 5}, { 145, 5}, { 177, 5}, { 209, 5},
|
|
{ 241, 6}, { 305, 6}, { 369, 7}, { 497, 8},
|
|
{ 753, 9}, { 1265, 10}, {2289, 11}, {4337, 12},
|
|
{8433, 13}, {16625, 24}
|
|
};
|
|
|
|
inline void GetBlockLengthPrefixCode(uint32_t len, uint32_t* code,
|
|
uint32_t* n_extra, uint32_t* extra) {
|
|
*code = 0;
|
|
while (*code < 25 && len >= kBlockLengthPrefixCode[*code + 1].offset) {
|
|
++(*code);
|
|
}
|
|
*n_extra = kBlockLengthPrefixCode[*code].nbits;
|
|
*extra = len - kBlockLengthPrefixCode[*code].offset;
|
|
}
|
|
|
|
inline void PrefixEncodeCopyDistance(size_t distance_code,
|
|
size_t num_direct_codes,
|
|
size_t postfix_bits,
|
|
uint16_t* code,
|
|
uint32_t* extra_bits) {
|
|
if (distance_code < kNumDistanceShortCodes + num_direct_codes) {
|
|
*code = static_cast<uint16_t>(distance_code);
|
|
*extra_bits = 0;
|
|
return;
|
|
}
|
|
distance_code -= kNumDistanceShortCodes + num_direct_codes; /* >= 0 */
|
|
distance_code += (1u << (postfix_bits + 2u)); /* > 0 */
|
|
size_t bucket = Log2FloorNonZero(distance_code) - 1;
|
|
size_t postfix_mask = (1 << postfix_bits) - 1;
|
|
size_t postfix = distance_code & postfix_mask;
|
|
size_t prefix = (distance_code >> bucket) & 1;
|
|
size_t offset = (2 + prefix) << bucket;
|
|
size_t nbits = bucket - postfix_bits;
|
|
*code = static_cast<uint16_t>(
|
|
(kNumDistanceShortCodes + num_direct_codes +
|
|
((2 * (nbits - 1) + prefix) << postfix_bits) + postfix));
|
|
*extra_bits = static_cast<uint32_t>(
|
|
(nbits << 24) | ((distance_code - offset) >> postfix_bits));
|
|
}
|
|
|
|
} // namespace brotli
|
|
|
|
#endif // BROTLI_ENC_PREFIX_H_
|