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)
16 extern "C" {
17 #endif
18
19 /* This function writes bits into bytes in increasing addresses, and within
20 a byte least-significant-bit first.
21
22 The function can write up to 56 bits in one go with WriteBits
23 Example: let's assume that 3 bits (Rs below) have been written already:
24
25 BYTE-0 BYTE+1 BYTE+2
26
27 0000 0RRR 0000 0000 0000 0000
28
29 Now, we could write 5 or less bits in MSB by just shifting by 3
30 and OR'ing to BYTE-0.
31
32 For n bits, we take the last 5 bits, OR that with high bits in BYTE-0,
33 and locate the rest in BYTE+1, BYTE+2, etc. */
BrotliWriteBits(size_t n_bits,uint64_t bits,size_t * BROTLI_RESTRICT pos,uint8_t * BROTLI_RESTRICT array)34 static BROTLI_INLINE void BrotliWriteBits(size_t n_bits,
35 uint64_t bits,
36 size_t* BROTLI_RESTRICT pos,
37 uint8_t* BROTLI_RESTRICT array) {
38 BROTLI_LOG(("WriteBits %2d 0x%08x%08x %10d\n", (int)n_bits,
39 (uint32_t)(bits >> 32), (uint32_t)(bits & 0xFFFFFFFF),
40 (int)*pos));
41 BROTLI_DCHECK((bits >> n_bits) == 0);
42 BROTLI_DCHECK(n_bits <= 56);
43 #if defined(BROTLI_LITTLE_ENDIAN)
44 /* This branch of the code can write up to 56 bits at a time,
45 7 bits are lost by being perhaps already in *p and at least
46 1 bit is needed to initialize the bit-stream ahead (i.e. if 7
47 bits are in *p and we write 57 bits, then the next write will
48 access a byte that was never initialized). */
49 {
50 uint8_t* p = &array[*pos >> 3];
51 uint64_t v = (uint64_t)(*p); /* Zero-extend 8 to 64 bits. */
52 v |= bits << (*pos & 7);
53 BROTLI_UNALIGNED_STORE64LE(p, v); /* Set some bits. */
54 *pos += n_bits;
55 }
56 #else
57 /* implicit & 0xFF is assumed for uint8_t arithmetics */
58 {
59 uint8_t* array_pos = &array[*pos >> 3];
60 const size_t bits_reserved_in_first_byte = (*pos & 7);
61 size_t bits_left_to_write;
62 bits <<= bits_reserved_in_first_byte;
63 *array_pos++ |= (uint8_t)bits;
64 for (bits_left_to_write = n_bits + bits_reserved_in_first_byte;
65 bits_left_to_write >= 9;
66 bits_left_to_write -= 8) {
67 bits >>= 8;
68 *array_pos++ = (uint8_t)bits;
69 }
70 *array_pos = 0;
71 *pos += n_bits;
72 }
73 #endif
74 }
75
BrotliWriteBitsPrepareStorage(size_t pos,uint8_t * array)76 static BROTLI_INLINE void BrotliWriteBitsPrepareStorage(
77 size_t pos, uint8_t* array) {
78 BROTLI_LOG(("WriteBitsPrepareStorage %10d\n", (int)pos));
79 BROTLI_DCHECK((pos & 7) == 0);
80 array[pos >> 3] = 0;
81 }
82
83 #if defined(__cplusplus) || defined(c_plusplus)
84 } /* extern "C" */
85 #endif
86
87 #endif /* BROTLI_ENC_WRITE_BITS_H_ */
88