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
