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lavc: add a new bitstream reader to replace get_bits
The new bit reader features a simpler API and an implementation without stacks of nested macros. Signed-off-by: Anton Khirnov <anton@khirnov.net>
This commit is contained in:
Alexandra Hájková
Anton Khirnov
parent
e18ba2dfd2
commit
dd3d7ddf2a
387
libavcodec/bitstream.h
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387
libavcodec/bitstream.h
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/*
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* Copyright (c) 2016 Alexandra Hájková
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*
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* This file is part of Libav.
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*
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* Libav is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2.1 of the License, or (at your option) any later version.
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*
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* Libav is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with Libav; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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*/
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/**
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* @file
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* functions for reading bits from a buffer
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*/
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#ifndef AVCODEC_BITSTREAM_H
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#define AVCODEC_BITSTREAM_H
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#include <stdint.h>
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#include "libavutil/common.h"
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#include "libavutil/intreadwrite.h"
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#include "libavutil/log.h"
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#include "mathops.h"
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#include "vlc.h"
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typedef struct BitstreamContext {
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uint64_t bits; // stores bits read from the buffer
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const uint8_t *buffer, *buffer_end;
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const uint8_t *ptr; // position inside a buffer
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unsigned bits_left; // number of bits left in bits field
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unsigned size_in_bits;
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} BitstreamContext;
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static inline void refill_64(BitstreamContext *bc)
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{
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if (bc->ptr >= bc->buffer_end)
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return;
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#ifdef BITSTREAM_READER_LE
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bc->bits = AV_RL64(bc->ptr);
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#else
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bc->bits = AV_RB64(bc->ptr);
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#endif
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bc->ptr += 8;
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bc->bits_left = 64;
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}
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static inline void refill_32(BitstreamContext *bc)
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{
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if (bc->ptr >= bc->buffer_end)
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return;
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#ifdef BITSTREAM_READER_LE
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bc->bits = (uint64_t)AV_RL32(bc->ptr) << bc->bits_left | bc->bits;
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#else
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bc->bits = bc->bits | (uint64_t)AV_RB32(bc->ptr) << (32 - bc->bits_left);
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#endif
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bc->ptr += 4;
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bc->bits_left += 32;
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}
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/* Initialize BitstreamContext. Input buffer must have an additional zero
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* padding of AV_INPUT_BUFFER_PADDING_SIZE bytes at the end. */
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static inline int bitstream_init(BitstreamContext *bc, const uint8_t *buffer,
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unsigned bit_size)
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{
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unsigned buffer_size;
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if (bit_size > INT_MAX - 7 || !buffer) {
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buffer =
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bc->buffer =
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bc->ptr = NULL;
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bc->bits_left = 0;
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return AVERROR_INVALIDDATA;
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}
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buffer_size = (bit_size + 7) >> 3;
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bc->buffer = buffer;
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bc->buffer_end = buffer + buffer_size;
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bc->ptr = bc->buffer;
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bc->size_in_bits = bit_size;
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bc->bits_left = 0;
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bc->bits = 0;
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refill_64(bc);
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return 0;
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}
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/* Initialize BitstreamContext with buffer size in bytes instead of bits. */
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static inline int bitstream_init8(BitstreamContext *bc, const uint8_t *buffer,
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unsigned byte_size)
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{
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if (byte_size > INT_MAX / 8)
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return AVERROR_INVALIDDATA;
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return bitstream_init(bc, buffer, byte_size * 8);
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}
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/* Return number of bits already read. */
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static inline int bitstream_tell(const BitstreamContext *bc)
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{
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return (bc->ptr - bc->buffer) * 8 - bc->bits_left;
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}
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/* Return buffer size in bits. */
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static inline int bitstream_tell_size(const BitstreamContext *bc)
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{
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return bc->size_in_bits;
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}
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/* Return the number of the bits left in a buffer. */
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static inline int bitstream_bits_left(const BitstreamContext *bc)
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{
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return (bc->buffer - bc->ptr) * 8 + bc->size_in_bits + bc->bits_left;
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}
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static inline uint64_t get_val(BitstreamContext *bc, unsigned n)
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{
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#ifdef BITSTREAM_READER_LE
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uint64_t ret = bc->bits & ((UINT64_C(1) << n) - 1);
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bc->bits >>= n;
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#else
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uint64_t ret = bc->bits >> (64 - n);
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bc->bits <<= n;
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#endif
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bc->bits_left -= n;
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return ret;
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}
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/* Return one bit from the buffer. */
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static inline unsigned bitstream_read_bit(BitstreamContext *bc)
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{
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if (!bc->bits_left)
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refill_64(bc);
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return get_val(bc, 1);
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}
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/* Return n bits from the buffer. n has to be in the 0-63 range. */
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static inline uint64_t bitstream_read_63(BitstreamContext *bc, unsigned n)
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{
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uint64_t ret = 0;
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#ifdef BITSTREAM_READER_LE
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uint64_t left = 0;
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#endif
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if (!n)
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return 0;
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if (n > bc->bits_left) {
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n -= bc->bits_left;
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#ifdef BITSTREAM_READER_LE
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left = bc->bits_left;
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#endif
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ret = get_val(bc, bc->bits_left);
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refill_64(bc);
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}
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#ifdef BITSTREAM_READER_LE
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ret = get_val(bc, n) << left | ret;
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#else
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ret = get_val(bc, n) | ret << n;
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#endif
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return ret;
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}
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/* Return n bits from the buffer. n has to be in the 0-32 range. */
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static inline uint32_t bitstream_read(BitstreamContext *bc, unsigned n)
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{
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if (!n)
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return 0;
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if (n > bc->bits_left) {
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refill_32(bc);
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if (bc->bits_left < 32)
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bc->bits_left = n;
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}
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return get_val(bc, n);
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}
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/* Return n bits from the buffer as a signed integer.
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* n has to be in the 0-32 range. */
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static inline int32_t bitstream_read_signed(BitstreamContext *bc, unsigned n)
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{
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return sign_extend(bitstream_read(bc, n), n);
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}
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static inline unsigned show_val(const BitstreamContext *bc, unsigned n)
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{
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#ifdef BITSTREAM_READER_LE
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return bc->bits & ((UINT64_C(1) << n) - 1);
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#else
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return bc->bits >> (64 - n);
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#endif
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}
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/* Return n bits from the buffer, but do not change the buffer state.
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* n has to be in the 0-32 range. */
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static inline unsigned bitstream_peek(BitstreamContext *bc, unsigned n)
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{
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if (n > bc->bits_left)
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refill_32(bc);
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return show_val(bc, n);
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}
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/* Return n bits from the buffer as a signed integer, but do not change the
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* buffer state. n has to be in the 0-32 range. */
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static inline int bitstream_peek_signed(BitstreamContext *bc, unsigned n)
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{
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return sign_extend(bitstream_peek(bc, n), n);
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}
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static inline void skip_remaining(BitstreamContext *bc, unsigned n)
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{
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#ifdef BITSTREAM_READER_LE
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bc->bits >>= n;
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#else
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bc->bits <<= n;
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#endif
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bc->bits_left -= n;
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}
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/* Skip n bits in the buffer. */
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static inline void bitstream_skip(BitstreamContext *bc, unsigned n)
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{
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if (n <= bc->bits_left)
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skip_remaining(bc, n);
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else {
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n -= bc->bits_left;
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skip_remaining(bc, bc->bits_left);
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if (n >= 64) {
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unsigned skip = n / 8;
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n -= skip * 8;
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bc->ptr += skip;
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}
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refill_64(bc);
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if (n)
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skip_remaining(bc, n);
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}
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}
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/* Seek to the given bit position. */
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static inline void bitstream_seek(BitstreamContext *bc, unsigned pos)
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{
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bc->ptr = bc->buffer;
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bc->bits = 0;
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bc->bits_left = 0;
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bitstream_skip(bc, pos);
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}
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/* Skip bits to a byte boundary. */
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static inline const uint8_t *bitstream_align(BitstreamContext *bc)
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{
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unsigned n = -bitstream_tell(bc) & 7;
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if (n)
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bitstream_skip(bc, n);
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return bc->buffer + (bitstream_tell(bc) >> 3);
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}
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/* Read MPEG-1 dc-style VLC (sign bit + mantissa with no MSB).
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* If MSB not set it is negative. */
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static inline int bitstream_read_xbits(BitstreamContext *bc, unsigned length)
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{
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int32_t cache = bitstream_peek(bc, 32);
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int sign = ~cache >> 31;
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skip_remaining(bc, length);
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return ((((uint32_t)(sign ^ cache)) >> (32 - length)) ^ sign) - sign;
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}
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/* Return the LUT element for the given bitstream configuration. */
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static inline int set_idx(BitstreamContext *bc, int code, int *n, int *nb_bits,
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VLC_TYPE (*table)[2])
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{
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unsigned idx;
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*nb_bits = -*n;
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idx = bitstream_peek(bc, *nb_bits) + code;
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*n = table[idx][1];
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return table[idx][0];
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}
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/**
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* Parse a VLC code.
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* @param bits is the number of bits which will be read at once, must be
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* identical to nb_bits in init_vlc()
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* @param max_depth is the number of times bits bits must be read to completely
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* read the longest VLC code
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* = (max_vlc_length + bits - 1) / bits
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* If the VLC code is invalid and max_depth = 1, then no bits will be removed.
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* If the VLC code is invalid and max_depth > 1, then the number of bits removed
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* is undefined. */
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static inline int bitstream_read_vlc(BitstreamContext *bc, VLC_TYPE (*table)[2],
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int bits, int max_depth)
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{
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int nb_bits;
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unsigned idx = bitstream_peek(bc, bits);
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int code = table[idx][0];
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int n = table[idx][1];
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if (max_depth > 1 && n < 0) {
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skip_remaining(bc, bits);
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code = set_idx(bc, code, &n, &nb_bits, table);
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if (max_depth > 2 && n < 0) {
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skip_remaining(bc, nb_bits);
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code = set_idx(bc, code, &n, &nb_bits, table);
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}
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}
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skip_remaining(bc, n);
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return code;
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}
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#define BITSTREAM_RL_VLC(level, run, bc, table, bits, max_depth) \
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do { \
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int n, nb_bits; \
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unsigned index = bitstream_peek(bc, bits); \
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level = table[index].level; \
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n = table[index].len; \
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\
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if (max_depth > 1 && n < 0) { \
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bitstream_skip(bc, bits); \
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\
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nb_bits = -n; \
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\
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index = bitstream_peek(bc, nb_bits) + level; \
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level = table[index].level; \
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n = table[index].len; \
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if (max_depth > 2 && n < 0) { \
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bitstream_skip(bc, nb_bits); \
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nb_bits = -n; \
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\
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index = bitstream_peek(bc, nb_bits) + level; \
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level = table[index].level; \
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n = table[index].len; \
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} \
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} \
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run = table[index].run; \
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bitstream_skip(bc, n); \
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} while (0)
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/* Return decoded truncated unary code for the values 0, 1, 2. */
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static inline int bitstream_decode012(BitstreamContext *bc)
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{
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if (!bitstream_read_bit(bc))
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return 0;
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else
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return bitstream_read_bit(bc) + 1;
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}
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/* Return decoded truncated unary code for the values 2, 1, 0. */
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static inline int bitstream_decode210(BitstreamContext *bc)
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{
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if (bitstream_read_bit(bc))
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return 0;
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else
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return 2 - bitstream_read_bit(bc);
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}
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/* Read sign bit and flip the sign of the provided value accordingly. */
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static inline int bitstream_apply_sign(BitstreamContext *bc, int val)
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{
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int sign = bitstream_read_signed(bc, 1);
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return (val ^ sign) - sign;
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}
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#endif /* AVCODEC_BITSTREAM_H */
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