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92 lines
2.8 KiB
92 lines
2.8 KiB
2 years ago
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// Copyright 2010 Google Inc. All Rights Reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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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 <stdint.h>
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#include <stdio.h>
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#include "./port.h"
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namespace brotli {
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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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//
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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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//
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// BYTE-0 BYTE+1 BYTE+2
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//
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// 0000 0RRR 0000 0000 0000 0000
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//
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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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//
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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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inline void WriteBits(int n_bits,
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uint64_t bits,
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int * __restrict pos,
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uint8_t * __restrict array) {
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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 < 1ULL << n_bits);
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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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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 int bits_reserved_in_first_byte = (*pos & 7);
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bits <<= bits_reserved_in_first_byte;
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*array_pos++ |= bits;
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for (int bits_left_to_write = n_bits - 8 + bits_reserved_in_first_byte;
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bits_left_to_write >= 1;
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bits_left_to_write -= 8) {
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bits >>= 8;
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*array_pos++ = 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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inline void WriteBitsPrepareStorage(int 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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} // namespace brotli
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#endif // BROTLI_ENC_WRITE_BITS_H_
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