prefix.h revision 26fa459c 
1/* Copyright 2013 Google Inc. All Rights Reserved.
2
3   Distributed under MIT license.
4   See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
5*/
6
7/* Functions for encoding of integers into prefix codes the amount of extra
8   bits, and the actual values of the extra bits. */
9
10#ifndef BROTLI_ENC_PREFIX_H_
11#define BROTLI_ENC_PREFIX_H_
12
13#include "../common/constants.h"
14#include "../common/platform.h"
15#include <brotli/types.h>
16#include "./fast_log.h"
17
18#if defined(__cplusplus) || defined(c_plusplus)
19extern "C" {
20#endif
21
22/* Here distance_code is an intermediate code, i.e. one of the special codes or
23   the actual distance increased by BROTLI_NUM_DISTANCE_SHORT_CODES - 1. */
24static BROTLI_INLINE void PrefixEncodeCopyDistance(size_t distance_code,
25                                                   size_t num_direct_codes,
26                                                   size_t postfix_bits,
27                                                   uint16_t* code,
28                                                   uint32_t* extra_bits) {
29  if (distance_code < BROTLI_NUM_DISTANCE_SHORT_CODES + num_direct_codes) {
30    *code = (uint16_t)distance_code;
31    *extra_bits = 0;
32    return;
33  } else {
34    size_t dist = ((size_t)1 << (postfix_bits + 2u)) +
35        (distance_code - BROTLI_NUM_DISTANCE_SHORT_CODES - num_direct_codes);
36    size_t bucket = Log2FloorNonZero(dist) - 1;
37    size_t postfix_mask = (1u << postfix_bits) - 1;
38    size_t postfix = dist & postfix_mask;
39    size_t prefix = (dist >> bucket) & 1;
40    size_t offset = (2 + prefix) << bucket;
41    size_t nbits = bucket - postfix_bits;
42    *code = (uint16_t)((nbits << 10) |
43        (BROTLI_NUM_DISTANCE_SHORT_CODES + num_direct_codes +
44         ((2 * (nbits - 1) + prefix) << postfix_bits) + postfix));
45    *extra_bits = (uint32_t)((dist - offset) >> postfix_bits);
46  }
47}
48
49#if defined(__cplusplus) || defined(c_plusplus)
50}  /* extern "C" */
51#endif
52
53#endif  /* BROTLI_ENC_PREFIX_H_ */
54