[FFmpeg-devel] [PATCH v4 1/4] lavc: add standalone cached bitstream reader
Anton Khirnov
anton at khirnov.net
Sat Oct 22 13:32:54 EEST 2022
From: Alexandra Hájková <alexandra.khirnova at gmail.com>
The cached bitstream reader was originally written by Alexandra Hájková
for Libav, with significant input from Kostya Shishkov and Luca Barbato.
It was then committed to FFmpeg in ca079b09549, by merging it with the
implementation of the current bitstream reader.
This merge makes the code of get_bits.h significantly harder to read,
since it now contains two different bitstream readers interleaved with
#ifdefs. Additionally, the code was committed without proper authorship
attribution.
This commit re-adds the cached bitstream reader as a standalone header,
as it was originally developed. It will be made useful in following
commits.
Integration by Anton Khirnov.
Signed-off-by: Anton Khirnov <anton at khirnov.net>
---
libavcodec/bitstream.h | 529 +++++++++++++++++++++++++++++++++++++++++
1 file changed, 529 insertions(+)
create mode 100644 libavcodec/bitstream.h
diff --git a/libavcodec/bitstream.h b/libavcodec/bitstream.h
new file mode 100644
index 0000000000..6fd321dba5
--- /dev/null
+++ b/libavcodec/bitstream.h
@@ -0,0 +1,529 @@
+/*
+ * Copyright (c) 2016 Alexandra Hájková
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file
+ * bitstream reader API header.
+ */
+
+#ifndef AVCODEC_BITSTREAM_H
+#define AVCODEC_BITSTREAM_H
+
+#include <stdint.h>
+
+#include "config.h"
+
+#include "libavutil/avassert.h"
+#include "libavutil/common.h"
+#include "libavutil/intreadwrite.h"
+#include "libavutil/log.h"
+
+#include "mathops.h"
+#include "vlc.h"
+
+#ifndef UNCHECKED_BITSTREAM_READER
+#define UNCHECKED_BITSTREAM_READER !CONFIG_SAFE_BITSTREAM_READER
+#endif
+
+typedef struct BitstreamContext {
+ uint64_t bits; // stores bits read from the buffer
+ const uint8_t *buffer, *buffer_end;
+ const uint8_t *ptr; // pointer to the position inside a buffer
+ unsigned bits_valid; // number of bits left in bits field
+ unsigned size_in_bits;
+} BitstreamContext;
+
+/**
+ * @return
+ * - 0 on successful refill
+ * - a negative number when bitstream end is hit
+ *
+ * Always succeeds when UNCHECKED_BITSTREAM_READER is enabled.
+ */
+static inline int bits_priv_refill_64(BitstreamContext *bc)
+{
+#if !UNCHECKED_BITSTREAM_READER
+ if (bc->ptr >= bc->buffer_end)
+ return -1;
+#endif
+
+#ifdef BITSTREAM_READER_LE
+ bc->bits = AV_RL64(bc->ptr);
+#else
+ bc->bits = AV_RB64(bc->ptr);
+#endif
+ bc->ptr += 8;
+ bc->bits_valid = 64;
+
+ return 0;
+}
+
+/**
+ * @return
+ * - 0 on successful refill
+ * - a negative number when bitstream end is hit
+ *
+ * Always succeeds when UNCHECKED_BITSTREAM_READER is enabled.
+ */
+static inline int bits_priv_refill_32(BitstreamContext *bc)
+{
+#if !UNCHECKED_BITSTREAM_READER
+ if (bc->ptr >= bc->buffer_end)
+ return -1;
+#endif
+
+#ifdef BITSTREAM_READER_LE
+ bc->bits |= (uint64_t)AV_RL32(bc->ptr) << bc->bits_valid;
+#else
+ bc->bits |= (uint64_t)AV_RB32(bc->ptr) << (32 - bc->bits_valid);
+#endif
+ bc->ptr += 4;
+ bc->bits_valid += 32;
+
+ return 0;
+}
+
+/**
+ * Initialize BitstreamContext.
+ * @param buffer bitstream buffer, must be AV_INPUT_BUFFER_PADDING_SIZE bytes
+ * larger than the actual read bits because some optimized bitstream
+ * readers read 32 or 64 bits at once and could read over the end
+ * @param bit_size the size of the buffer in bits
+ * @return 0 on success, AVERROR_INVALIDDATA if the buffer_size would overflow.
+ */
+static inline int bits_init(BitstreamContext *bc, const uint8_t *buffer,
+ unsigned int bit_size)
+{
+ unsigned int buffer_size;
+
+ if (bit_size > INT_MAX - 7 || !buffer) {
+ bc->buffer = NULL;
+ bc->ptr = NULL;
+ bc->bits_valid = 0;
+ return AVERROR_INVALIDDATA;
+ }
+
+ buffer_size = (bit_size + 7) >> 3;
+
+ bc->buffer = buffer;
+ bc->buffer_end = buffer + buffer_size;
+ bc->ptr = bc->buffer;
+ bc->size_in_bits = bit_size;
+ bc->bits_valid = 0;
+ bc->bits = 0;
+
+ bits_priv_refill_64(bc);
+
+ return 0;
+}
+
+/**
+ * Initialize BitstreamContext.
+ * @param buffer bitstream buffer, must be AV_INPUT_BUFFER_PADDING_SIZE bytes
+ * larger than the actual read bits because some optimized bitstream
+ * readers read 32 or 64 bits at once and could read over the end
+ * @param byte_size the size of the buffer in bytes
+ * @return 0 on success, AVERROR_INVALIDDATA if the buffer_size would overflow
+ */
+static inline int bits_init8(BitstreamContext *bc, const uint8_t *buffer,
+ unsigned int byte_size)
+{
+ if (byte_size > INT_MAX / 8)
+ return AVERROR_INVALIDDATA;
+ return bits_init(bc, buffer, byte_size * 8);
+}
+
+/**
+ * Return number of bits already read.
+ */
+static inline int bits_tell(const BitstreamContext *bc)
+{
+ return (bc->ptr - bc->buffer) * 8 - bc->bits_valid;
+}
+
+/**
+ * Return buffer size in bits.
+ */
+static inline int bits_size(const BitstreamContext *bc)
+{
+ return bc->size_in_bits;
+}
+
+/**
+ * Return the number of the bits left in a buffer.
+ */
+static inline int bits_left(const BitstreamContext *bc)
+{
+ return (bc->buffer - bc->ptr) * 8 + bc->size_in_bits + bc->bits_valid;
+}
+
+static inline uint64_t bits_priv_val_show(BitstreamContext *bc, unsigned int n)
+{
+ av_assert2(n > 0 && n <= 64);
+
+#ifdef BITSTREAM_READER_LE
+ return bc->bits & (UINT64_MAX >> (64 - n));
+#else
+ return bc->bits >> (64 - n);
+#endif
+}
+
+static inline void bits_priv_skip_remaining(BitstreamContext *bc, unsigned int n)
+{
+#ifdef BITSTREAM_READER_LE
+ bc->bits >>= n;
+#else
+ bc->bits <<= n;
+#endif
+ bc->bits_valid -= n;
+}
+
+static inline uint64_t bits_priv_val_get(BitstreamContext *bc, unsigned int n)
+{
+ uint64_t ret;
+
+ av_assert2(n > 0 && n < 64);
+
+ ret = bits_priv_val_show(bc, n);
+ bits_priv_skip_remaining(bc, n);
+
+ return ret;
+}
+
+/**
+ * Return one bit from the buffer.
+ */
+static inline unsigned int bits_read_bit(BitstreamContext *bc)
+{
+ if (!bc->bits_valid && bits_priv_refill_64(bc) < 0)
+ return 0;
+
+ return bits_priv_val_get(bc, 1);
+}
+
+/**
+ * Return n bits from the buffer, n has to be in the 1-32 range.
+ * May be faster than bits_read() when n is not a compile-time constant and is
+ * known to be non-zero;
+ */
+static inline uint32_t bits_read_nz(BitstreamContext *bc, unsigned int n)
+{
+ av_assert2(n > 0 && n <= 32);
+
+ if (n > bc->bits_valid) {
+ if (bits_priv_refill_32(bc) < 0)
+ bc->bits_valid = n;
+ }
+
+ return bits_priv_val_get(bc, n);
+}
+
+/**
+ * Return n bits from the buffer, n has to be in the 0-32 range.
+ */
+static inline uint32_t bits_read(BitstreamContext *bc, unsigned int n)
+{
+ av_assert2(n <= 32);
+
+ if (!n)
+ return 0;
+
+ return bits_read_nz(bc, n);
+}
+
+/**
+ * Return n bits from the buffer, n has to be in the 0-63 range.
+ */
+static inline uint64_t bits_read_63(BitstreamContext *bc, unsigned int n)
+{
+ uint64_t ret = 0;
+ unsigned left = 0;
+
+ av_assert2(n <= 63);
+
+ if (!n)
+ return 0;
+
+ if (n > bc->bits_valid) {
+ left = bc->bits_valid;
+ n -= left;
+
+ if (left)
+ ret = bits_priv_val_get(bc, left);
+
+ if (bits_priv_refill_64(bc) < 0)
+ bc->bits_valid = n;
+
+ }
+
+#ifdef BITSTREAM_READER_LE
+ ret = bits_priv_val_get(bc, n) << left | ret;
+#else
+ ret = bits_priv_val_get(bc, n) | ret << n;
+#endif
+
+ return ret;
+}
+
+/**
+ * Return n bits from the buffer, n has to be in the 0-64 range.
+ */
+static inline uint64_t bits_read_64(BitstreamContext *bc, unsigned int n)
+{
+ av_assert2(n <= 64);
+
+ if (n == 64) {
+ uint64_t ret = bits_read_63(bc, 63);
+#ifdef BITSTREAM_READER_LE
+ return ret | ((uint64_t)bits_read_bit(bc) << 63);
+#else
+ return (ret << 1) | (uint64_t)bits_read_bit(bc);
+#endif
+ }
+ return bits_read_63(bc, n);
+}
+
+/**
+ * Return n bits from the buffer as a signed integer.
+ * n has to be in the 0-32 range.
+ */
+static inline int32_t bits_read_signed(BitstreamContext *bc, unsigned int n)
+{
+ return sign_extend(bits_read(bc, n), n);
+}
+
+/**
+ * Return n bits from the buffer but do not change the buffer state.
+ * n has to be in the 1-32 range. May
+ */
+static inline uint32_t bits_peek_nz(BitstreamContext *bc, unsigned int n)
+{
+ av_assert2(n > 0 && n <= 32);
+
+ if (n > bc->bits_valid)
+ bits_priv_refill_32(bc);
+
+ return bits_priv_val_show(bc, n);
+}
+
+/**
+ * Return n bits from the buffer but do not change the buffer state.
+ * n has to be in the 0-32 range.
+ */
+static inline uint32_t bits_peek(BitstreamContext *bc, unsigned int n)
+{
+ av_assert2(n <= 32);
+
+ if (!n)
+ return 0;
+
+ return bits_peek_nz(bc, n);
+}
+
+/**
+ * Return n bits from the buffer as a signed integer,
+ * do not change the buffer state.
+ * n has to be in the 0-32 range.
+ */
+static inline int bits_peek_signed(BitstreamContext *bc, unsigned int n)
+{
+ return sign_extend(bits_peek(bc, n), n);
+}
+
+/**
+ * Skip n bits in the buffer.
+ */
+static inline void bits_skip(BitstreamContext *bc, unsigned int n)
+{
+ if (n < bc->bits_valid)
+ bits_priv_skip_remaining(bc, n);
+ else {
+ n -= bc->bits_valid;
+ bc->bits = 0;
+ bc->bits_valid = 0;
+
+ if (n >= 64) {
+ unsigned int skip = n / 8;
+
+ n -= skip * 8;
+ bc->ptr += skip;
+ }
+ bits_priv_refill_64(bc);
+ if (n)
+ bits_priv_skip_remaining(bc, n);
+ }
+}
+
+/**
+ * Seek to the given bit position.
+ */
+static inline void bits_seek(BitstreamContext *bc, unsigned pos)
+{
+ bc->ptr = bc->buffer;
+ bc->bits = 0;
+ bc->bits_valid = 0;
+
+ bits_skip(bc, pos);
+}
+
+/**
+ * Skip bits to a byte boundary.
+ */
+static inline const uint8_t *bits_align(BitstreamContext *bc)
+{
+ unsigned int n = -bits_tell(bc) & 7;
+ if (n)
+ bits_skip(bc, n);
+ return bc->buffer + (bits_tell(bc) >> 3);
+}
+
+/**
+ * Read MPEG-1 dc-style VLC (sign bit + mantissa with no MSB).
+ * If MSB not set it is negative.
+ * @param n length in bits
+ */
+static inline int bits_read_xbits(BitstreamContext *bc, unsigned int n)
+{
+ int32_t cache = bits_peek(bc, 32);
+ int sign = ~cache >> 31;
+ bits_priv_skip_remaining(bc, n);
+
+ return ((((uint32_t)(sign ^ cache)) >> (32 - n)) ^ sign) - sign;
+}
+
+/**
+ * Return decoded truncated unary code for the values 0, 1, 2.
+ */
+static inline int bits_decode012(BitstreamContext *bc)
+{
+ if (!bits_read_bit(bc))
+ return 0;
+ else
+ return bits_read_bit(bc) + 1;
+}
+
+/**
+ * Return decoded truncated unary code for the values 2, 1, 0.
+ */
+static inline int bits_decode210(BitstreamContext *bc)
+{
+ if (bits_read_bit(bc))
+ return 0;
+ else
+ return 2 - bits_read_bit(bc);
+}
+
+/* Read sign bit and flip the sign of the provided value accordingly. */
+static inline int bits_apply_sign(BitstreamContext *bc, int val)
+{
+ int sign = bits_read_signed(bc, 1);
+ return (val ^ sign) - sign;
+}
+
+static inline int bits_skip_1stop_8data(BitstreamContext *s)
+{
+ if (bits_left(s) <= 0)
+ return AVERROR_INVALIDDATA;
+
+ while (bits_read_bit(s)) {
+ bits_skip(s, 8);
+ if (bits_left(s) <= 0)
+ return AVERROR_INVALIDDATA;
+ }
+
+ return 0;
+}
+
+/**
+ * Return the LUT element for the given bitstream configuration.
+ */
+static inline int bits_priv_set_idx(BitstreamContext *bc, int code, int *n, int *nb_bits,
+ const VLCElem *table)
+{
+ unsigned idx;
+
+ *nb_bits = -*n;
+ idx = bits_peek(bc, *nb_bits) + code;
+ *n = table[idx].len;
+
+ return table[idx].sym;
+}
+
+/**
+ * Parse a vlc code.
+ * @param bits is the number of bits which will be read at once, must be
+ * identical to nb_bits in init_vlc()
+ * @param max_depth is the number of times bits bits must be read to completely
+ * read the longest vlc code
+ * = (max_vlc_length + bits - 1) / bits
+ * If the vlc code is invalid and max_depth=1, then no bits will be removed.
+ * If the vlc code is invalid and max_depth>1, then the number of bits removed
+ * is undefined.
+ */
+static inline int bits_read_vlc(BitstreamContext *bc, const VLCElem *table,
+ int bits, int max_depth)
+{
+ int nb_bits;
+ unsigned idx = bits_peek(bc, bits);
+ int code = table[idx].sym;
+ int n = table[idx].len;
+
+ if (max_depth > 1 && n < 0) {
+ bits_priv_skip_remaining(bc, bits);
+ code = bits_priv_set_idx(bc, code, &n, &nb_bits, table);
+ if (max_depth > 2 && n < 0) {
+ bits_priv_skip_remaining(bc, nb_bits);
+ code = bits_priv_set_idx(bc, code, &n, &nb_bits, table);
+ }
+ }
+ bits_priv_skip_remaining(bc, n);
+
+ return code;
+}
+
+#define BITS_RL_VLC(level, run, bc, table, bits, max_depth) \
+ do { \
+ int n, nb_bits; \
+ unsigned int index = bits_peek(bc, bits); \
+ level = table[index].level; \
+ n = table[index].len; \
+ \
+ if (max_depth > 1 && n < 0) { \
+ bits_skip(bc, bits); \
+ \
+ nb_bits = -n; \
+ \
+ index = bits_peek(bc, nb_bits) + level; \
+ level = table[index].level; \
+ n = table[index].len; \
+ if (max_depth > 2 && n < 0) { \
+ bits_skip(bc, nb_bits); \
+ nb_bits = -n; \
+ \
+ index = bits_peek(bc, nb_bits) + level; \
+ level = table[index].level; \
+ n = table[index].len; \
+ } \
+ } \
+ run = table[index].run; \
+ bits_skip(bc, n); \
+ } while (0)
+
+#endif /* AVCODEC_BITSTREAM_H */
--
2.35.1
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