1/* Copyright 2010 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/* Write bits into a byte array. */
8
9#ifndef BROTLI_ENC_WRITE_BITS_H_
10#define BROTLI_ENC_WRITE_BITS_H_
11
12#include "../common/platform.h"
13#include <brotli/types.h>
14
15#if defined(__cplusplus) || defined(c_plusplus)
16extern "C" {
17#endif
18
19/*#define BIT_WRITER_DEBUG */
20
21/* This function writes bits into bytes in increasing addresses, and within
22 a byte least-significant-bit first.
23
24 The function can write up to 56 bits in one go with WriteBits
25 Example: let's assume that 3 bits (Rs below) have been written already:
26
27 BYTE-0 BYTE+1 BYTE+2
28
29 0000 0RRR 0000 0000 0000 0000
30
31 Now, we could write 5 or less bits in MSB by just sifting by 3
32 and OR'ing to BYTE-0.
33
34 For n bits, we take the last 5 bits, OR that with high bits in BYTE-0,
35 and locate the rest in BYTE+1, BYTE+2, etc. */
36static BROTLI_INLINE void BrotliWriteBits(size_t n_bits,
37 uint64_t bits,
38 size_t* BROTLI_RESTRICT pos,
39 uint8_t* BROTLI_RESTRICT array) {
40#if defined(BROTLI_LITTLE_ENDIAN)
41 /* This branch of the code can write up to 56 bits at a time,
42 7 bits are lost by being perhaps already in *p and at least
43 1 bit is needed to initialize the bit-stream ahead (i.e. if 7
44 bits are in *p and we write 57 bits, then the next write will
45 access a byte that was never initialized). */
46 uint8_t* p = &array[*pos >> 3];
47 uint64_t v = (uint64_t)(*p); /* Zero-extend 8 to 64 bits. */
48 BROTLI_LOG(("WriteBits %2d 0x%08x%08x %10d\n", (int)n_bits,
49 (uint32_t)(bits >> 32), (uint32_t)(bits & 0xFFFFFFFF),
50 (int)*pos));
51 BROTLI_DCHECK((bits >> n_bits) == 0);
52 BROTLI_DCHECK(n_bits <= 56);
53 v |= bits << (*pos & 7);
54 BROTLI_UNALIGNED_STORE64LE(p, v); /* Set some bits. */
55 *pos += n_bits;
56#else
57 /* implicit & 0xFF is assumed for uint8_t arithmetics */
58 uint8_t* array_pos = &array[*pos >> 3];
59 const size_t bits_reserved_in_first_byte = (*pos & 7);
60 size_t bits_left_to_write;
61 bits <<= bits_reserved_in_first_byte;
62 *array_pos++ |= (uint8_t)bits;
63 for (bits_left_to_write = n_bits + bits_reserved_in_first_byte;
64 bits_left_to_write >= 9;
65 bits_left_to_write -= 8) {
66 bits >>= 8;
67 *array_pos++ = (uint8_t)bits;
68 }
69 *array_pos = 0;
70 *pos += n_bits;
71#endif
72}
73
74static BROTLI_INLINE void BrotliWriteBitsPrepareStorage(
75 size_t pos, uint8_t* array) {
76 BROTLI_LOG(("WriteBitsPrepareStorage %10d\n", (int)pos));
77 BROTLI_DCHECK((pos & 7) == 0);
78 array[pos >> 3] = 0;
79}
80
81#if defined(__cplusplus) || defined(c_plusplus)
82} /* extern "C" */
83#endif
84
85#endif /* BROTLI_ENC_WRITE_BITS_H_ */
86