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Diffstat (limited to 'c/enc/write_bits.h')
-rw-r--r-- | c/enc/write_bits.h | 90 |
1 files changed, 90 insertions, 0 deletions
diff --git a/c/enc/write_bits.h b/c/enc/write_bits.h new file mode 100644 index 0000000..3999c91 --- /dev/null +++ b/c/enc/write_bits.h @@ -0,0 +1,90 @@ +/* Copyright 2010 Google Inc. All Rights Reserved. + + Distributed under MIT license. + See file LICENSE for detail or copy at https://opensource.org/licenses/MIT +*/ + +/* Write bits into a byte array. */ + +#ifndef BROTLI_ENC_WRITE_BITS_H_ +#define BROTLI_ENC_WRITE_BITS_H_ + +#include <assert.h> +#include <stdio.h> /* printf */ + +#include <brotli/types.h> +#include "./port.h" + +#if defined(__cplusplus) || defined(c_plusplus) +extern "C" { +#endif + +/*#define BIT_WRITER_DEBUG */ + +/* This function writes bits into bytes in increasing addresses, and within + a byte least-significant-bit first. + + The function can write up to 56 bits in one go with WriteBits + Example: let's assume that 3 bits (Rs below) have been written already: + + BYTE-0 BYTE+1 BYTE+2 + + 0000 0RRR 0000 0000 0000 0000 + + Now, we could write 5 or less bits in MSB by just sifting by 3 + and OR'ing to BYTE-0. + + For n bits, we take the last 5 bits, OR that with high bits in BYTE-0, + and locate the rest in BYTE+1, BYTE+2, etc. */ +static BROTLI_INLINE void BrotliWriteBits(size_t n_bits, + uint64_t bits, + size_t * BROTLI_RESTRICT pos, + uint8_t * BROTLI_RESTRICT array) { +#ifdef IS_LITTLE_ENDIAN + /* This branch of the code can write up to 56 bits at a time, + 7 bits are lost by being perhaps already in *p and at least + 1 bit is needed to initialize the bit-stream ahead (i.e. if 7 + bits are in *p and we write 57 bits, then the next write will + access a byte that was never initialized). */ + uint8_t *p = &array[*pos >> 3]; + uint64_t v = *p; +#ifdef BIT_WRITER_DEBUG + printf("WriteBits %2d 0x%016llx %10d\n", n_bits, bits, *pos); +#endif + assert((bits >> n_bits) == 0); + assert(n_bits <= 56); + v |= bits << (*pos & 7); + BROTLI_UNALIGNED_STORE64(p, v); /* Set some bits. */ + *pos += n_bits; +#else + /* implicit & 0xff is assumed for uint8_t arithmetics */ + uint8_t *array_pos = &array[*pos >> 3]; + const size_t bits_reserved_in_first_byte = (*pos & 7); + size_t bits_left_to_write; + bits <<= bits_reserved_in_first_byte; + *array_pos++ |= (uint8_t)bits; + for (bits_left_to_write = n_bits + bits_reserved_in_first_byte; + bits_left_to_write >= 9; + bits_left_to_write -= 8) { + bits >>= 8; + *array_pos++ = (uint8_t)bits; + } + *array_pos = 0; + *pos += n_bits; +#endif +} + +static BROTLI_INLINE void BrotliWriteBitsPrepareStorage( + size_t pos, uint8_t *array) { +#ifdef BIT_WRITER_DEBUG + printf("WriteBitsPrepareStorage %10d\n", pos); +#endif + assert((pos & 7) == 0); + array[pos >> 3] = 0; +} + +#if defined(__cplusplus) || defined(c_plusplus) +} /* extern "C" */ +#endif + +#endif /* BROTLI_ENC_WRITE_BITS_H_ */ |