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 | /*#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. */ |
36 | static 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 | |
74 | static 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 | |