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