1 // Copyright 2010 Google Inc. All Rights Reserved.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // http://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 //
15 // Write bits into a byte array.
16
17 #ifndef BROTLI_ENC_WRITE_BITS_H_
18 #define BROTLI_ENC_WRITE_BITS_H_
19
20 #include <assert.h>
21 #if defined(OS_MACOSX)
22 #include <machine/endian.h>
23 #else
24 #include <endian.h>
25 #endif
26 #include <stdint.h>
27 #include <stdio.h>
28
29 #include "./port.h"
30
31 namespace brotli {
32
33 //#define BIT_WRITER_DEBUG
34
35 // This function writes bits into bytes in increasing addresses, and within
36 // a byte least-significant-bit first.
37 //
38 // The function can write up to 56 bits in one go with WriteBits
39 // Example: let's assume that 3 bits (Rs below) have been written already:
40 //
41 // BYTE-0 BYTE+1 BYTE+2
42 //
43 // 0000 0RRR 0000 0000 0000 0000
44 //
45 // Now, we could write 5 or less bits in MSB by just sifting by 3
46 // and OR'ing to BYTE-0.
47 //
48 // For n bits, we take the last 5 bits, OR that with high bits in BYTE-0,
49 // and locate the rest in BYTE+1, BYTE+2, etc.
WriteBits(int n_bits,uint64_t bits,int * __restrict pos,uint8_t * __restrict array)50 inline void WriteBits(int n_bits,
51 uint64_t bits,
52 int * __restrict pos,
53 uint8_t * __restrict array) {
54 #ifdef BIT_WRITER_DEBUG
55 printf("WriteBits %2d 0x%016llx %10d\n", n_bits, bits, *pos);
56 #endif
57 #ifdef IS_LITTLE_ENDIAN
58 // This branch of the code can write up to 56 bits at a time,
59 // 7 bits are lost by being perhaps already in *p and at least
60 // 1 bit is needed to initialize the bit-stream ahead (i.e. if 7
61 // bits are in *p and we write 57 bits, then the next write will
62 // access a byte that was never initialized).
63 uint8_t *p = &array[*pos >> 3];
64 uint64_t v = *p;
65 v |= bits << (*pos & 7);
66 BROTLI_UNALIGNED_STORE64(p, v); // Set some bits.
67 *pos += n_bits;
68 #else
69 // implicit & 0xff is assumed for uint8_t arithmetics
70 uint8_t *array_pos = &array[*pos >> 3];
71 const int bits_reserved_in_first_byte = (*pos & 7);
72 bits <<= bits_reserved_in_first_byte;
73 *array_pos++ |= bits;
74 for (int bits_left_to_write = n_bits - 8 + bits_reserved_in_first_byte;
75 bits_left_to_write >= 1;
76 bits_left_to_write -= 8) {
77 bits >>= 8;
78 *array_pos++ = bits;
79 }
80 *array_pos = 0;
81 *pos += n_bits;
82 #endif
83 }
84
WriteBitsPrepareStorage(int pos,uint8_t * array)85 inline void WriteBitsPrepareStorage(int pos, uint8_t *array) {
86 #ifdef BIT_WRITER_DEBUG
87 printf("WriteBitsPrepareStorage %10d\n", pos);
88 #endif
89 assert((pos & 7) == 0);
90 array[pos >> 3] = 0;
91 }
92
93 } // namespace brotli
94
95 #endif // BROTLI_ENC_WRITE_BITS_H_
96