write_bits.h (2835B)
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 14 #if defined(__cplusplus) || defined(c_plusplus) 15 extern "C" { 16 #endif 17 18 /* This function writes bits into bytes in increasing addresses, and within 19 a byte least-significant-bit first. 20 21 The function can write up to 56 bits in one go with WriteBits 22 Example: let's assume that 3 bits (Rs below) have been written already: 23 24 BYTE-0 BYTE+1 BYTE+2 25 26 0000 0RRR 0000 0000 0000 0000 27 28 Now, we could write 5 or less bits in MSB by just shifting by 3 29 and OR'ing to BYTE-0. 30 31 For n bits, we take the last 5 bits, OR that with high bits in BYTE-0, 32 and locate the rest in BYTE+1, BYTE+2, etc. */ 33 static BROTLI_INLINE void BrotliWriteBits(size_t n_bits, 34 uint64_t bits, 35 size_t* BROTLI_RESTRICT pos, 36 uint8_t* BROTLI_RESTRICT array) { 37 BROTLI_LOG(("WriteBits %2d 0x%08x%08x %10d\n", (int)n_bits, 38 (uint32_t)(bits >> 32), (uint32_t)(bits & 0xFFFFFFFF), 39 (int)*pos)); 40 BROTLI_DCHECK((bits >> n_bits) == 0); 41 BROTLI_DCHECK(n_bits <= 56); 42 #if defined(BROTLI_LITTLE_ENDIAN) 43 /* This branch of the code can write up to 56 bits at a time, 44 7 bits are lost by being perhaps already in *p and at least 45 1 bit is needed to initialize the bit-stream ahead (i.e. if 7 46 bits are in *p and we write 57 bits, then the next write will 47 access a byte that was never initialized). */ 48 { 49 uint8_t* p = &array[*pos >> 3]; 50 uint64_t v = (uint64_t)(*p); /* Zero-extend 8 to 64 bits. */ 51 v |= bits << (*pos & 7); 52 BROTLI_UNALIGNED_STORE64LE(p, v); /* Set some bits. */ 53 *pos += n_bits; 54 } 55 #else 56 /* implicit & 0xFF is assumed for uint8_t arithmetics */ 57 { 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 } 72 #endif 73 } 74 75 static BROTLI_INLINE void BrotliWriteBitsPrepareStorage( 76 size_t pos, uint8_t* array) { 77 BROTLI_LOG(("WriteBitsPrepareStorage %10d\n", (int)pos)); 78 BROTLI_DCHECK((pos & 7) == 0); 79 array[pos >> 3] = 0; 80 } 81 82 #if defined(__cplusplus) || defined(c_plusplus) 83 } /* extern "C" */ 84 #endif 85 86 #endif /* BROTLI_ENC_WRITE_BITS_H_ */