[FFmpeg-cvslog] lavc: add a new bitstream reader to replace get_bits
Alexandra Hájková
git at videolan.org
Mon Apr 3 21:38:34 EEST 2017
ffmpeg | branch: master | Alexandra Hájková <alexandra at khirnov.net> | Fri Apr 22 09:43:52 2016 +0200| [dd3d7ddf2a508b15877bb978c2309f777853a228] | committer: Anton Khirnov
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 at khirnov.net>
> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=dd3d7ddf2a508b15877bb978c2309f777853a228
---
libavcodec/bitstream.h | 387 +++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 387 insertions(+)
diff --git a/libavcodec/bitstream.h b/libavcodec/bitstream.h
new file mode 100644
index 0000000..996e32e
--- /dev/null
+++ b/libavcodec/bitstream.h
@@ -0,0 +1,387 @@
+/*
+ * Copyright (c) 2016 Alexandra Hájková
+ *
+ * This file is part of Libav.
+ *
+ * Libav 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.
+ *
+ * Libav 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 Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file
+ * functions for reading bits from a buffer
+ */
+
+#ifndef AVCODEC_BITSTREAM_H
+#define AVCODEC_BITSTREAM_H
+
+#include <stdint.h>
+
+#include "libavutil/common.h"
+#include "libavutil/intreadwrite.h"
+#include "libavutil/log.h"
+
+#include "mathops.h"
+#include "vlc.h"
+
+typedef struct BitstreamContext {
+ uint64_t bits; // stores bits read from the buffer
+ const uint8_t *buffer, *buffer_end;
+ const uint8_t *ptr; // position inside a buffer
+ unsigned bits_left; // number of bits left in bits field
+ unsigned size_in_bits;
+} BitstreamContext;
+
+static inline void refill_64(BitstreamContext *bc)
+{
+ if (bc->ptr >= bc->buffer_end)
+ return;
+
+#ifdef BITSTREAM_READER_LE
+ bc->bits = AV_RL64(bc->ptr);
+#else
+ bc->bits = AV_RB64(bc->ptr);
+#endif
+ bc->ptr += 8;
+ bc->bits_left = 64;
+}
+
+static inline void refill_32(BitstreamContext *bc)
+{
+ if (bc->ptr >= bc->buffer_end)
+ return;
+
+#ifdef BITSTREAM_READER_LE
+ bc->bits = (uint64_t)AV_RL32(bc->ptr) << bc->bits_left | bc->bits;
+#else
+ bc->bits = bc->bits | (uint64_t)AV_RB32(bc->ptr) << (32 - bc->bits_left);
+#endif
+ bc->ptr += 4;
+ bc->bits_left += 32;
+}
+
+/* Initialize BitstreamContext. Input buffer must have an additional zero
+ * padding of AV_INPUT_BUFFER_PADDING_SIZE bytes at the end. */
+static inline int bitstream_init(BitstreamContext *bc, const uint8_t *buffer,
+ unsigned bit_size)
+{
+ unsigned buffer_size;
+
+ if (bit_size > INT_MAX - 7 || !buffer) {
+ buffer =
+ bc->buffer =
+ bc->ptr = NULL;
+ bc->bits_left = 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_left = 0;
+ bc->bits = 0;
+
+ refill_64(bc);
+
+ return 0;
+}
+
+/* Initialize BitstreamContext with buffer size in bytes instead of bits. */
+static inline int bitstream_init8(BitstreamContext *bc, const uint8_t *buffer,
+ unsigned byte_size)
+{
+ if (byte_size > INT_MAX / 8)
+ return AVERROR_INVALIDDATA;
+ return bitstream_init(bc, buffer, byte_size * 8);
+}
+
+/* Return number of bits already read. */
+static inline int bitstream_tell(const BitstreamContext *bc)
+{
+ return (bc->ptr - bc->buffer) * 8 - bc->bits_left;
+}
+
+/* Return buffer size in bits. */
+static inline int bitstream_tell_size(const BitstreamContext *bc)
+{
+ return bc->size_in_bits;
+}
+
+/* Return the number of the bits left in a buffer. */
+static inline int bitstream_bits_left(const BitstreamContext *bc)
+{
+ return (bc->buffer - bc->ptr) * 8 + bc->size_in_bits + bc->bits_left;
+}
+
+static inline uint64_t get_val(BitstreamContext *bc, unsigned n)
+{
+#ifdef BITSTREAM_READER_LE
+ uint64_t ret = bc->bits & ((UINT64_C(1) << n) - 1);
+ bc->bits >>= n;
+#else
+ uint64_t ret = bc->bits >> (64 - n);
+ bc->bits <<= n;
+#endif
+ bc->bits_left -= n;
+
+ return ret;
+}
+
+/* Return one bit from the buffer. */
+static inline unsigned bitstream_read_bit(BitstreamContext *bc)
+{
+ if (!bc->bits_left)
+ refill_64(bc);
+
+ return get_val(bc, 1);
+}
+
+/* Return n bits from the buffer. n has to be in the 0-63 range. */
+static inline uint64_t bitstream_read_63(BitstreamContext *bc, unsigned n)
+{
+ uint64_t ret = 0;
+#ifdef BITSTREAM_READER_LE
+ uint64_t left = 0;
+#endif
+
+ if (!n)
+ return 0;
+
+ if (n > bc->bits_left) {
+ n -= bc->bits_left;
+#ifdef BITSTREAM_READER_LE
+ left = bc->bits_left;
+#endif
+ ret = get_val(bc, bc->bits_left);
+ refill_64(bc);
+ }
+
+#ifdef BITSTREAM_READER_LE
+ ret = get_val(bc, n) << left | ret;
+#else
+ ret = get_val(bc, n) | ret << n;
+#endif
+
+ return ret;
+}
+
+/* Return n bits from the buffer. n has to be in the 0-32 range. */
+static inline uint32_t bitstream_read(BitstreamContext *bc, unsigned n)
+{
+ if (!n)
+ return 0;
+
+ if (n > bc->bits_left) {
+ refill_32(bc);
+ if (bc->bits_left < 32)
+ bc->bits_left = n;
+ }
+
+ return get_val(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 bitstream_read_signed(BitstreamContext *bc, unsigned n)
+{
+ return sign_extend(bitstream_read(bc, n), n);
+}
+
+static inline unsigned show_val(const BitstreamContext *bc, unsigned n)
+{
+#ifdef BITSTREAM_READER_LE
+ return bc->bits & ((UINT64_C(1) << n) - 1);
+#else
+ return bc->bits >> (64 - n);
+#endif
+}
+
+/* Return n bits from the buffer, but do not change the buffer state.
+ * n has to be in the 0-32 range. */
+static inline unsigned bitstream_peek(BitstreamContext *bc, unsigned n)
+{
+ if (n > bc->bits_left)
+ refill_32(bc);
+
+ return show_val(bc, n);
+}
+
+/* Return n bits from the buffer as a signed integer, but do not change the
+ * buffer state. n has to be in the 0-32 range. */
+static inline int bitstream_peek_signed(BitstreamContext *bc, unsigned n)
+{
+ return sign_extend(bitstream_peek(bc, n), n);
+}
+
+static inline void skip_remaining(BitstreamContext *bc, unsigned n)
+{
+#ifdef BITSTREAM_READER_LE
+ bc->bits >>= n;
+#else
+ bc->bits <<= n;
+#endif
+ bc->bits_left -= n;
+}
+
+/* Skip n bits in the buffer. */
+static inline void bitstream_skip(BitstreamContext *bc, unsigned n)
+{
+ if (n <= bc->bits_left)
+ skip_remaining(bc, n);
+ else {
+ n -= bc->bits_left;
+ skip_remaining(bc, bc->bits_left);
+ if (n >= 64) {
+ unsigned skip = n / 8;
+
+ n -= skip * 8;
+ bc->ptr += skip;
+ }
+ refill_64(bc);
+ if (n)
+ skip_remaining(bc, n);
+ }
+}
+
+/* Seek to the given bit position. */
+static inline void bitstream_seek(BitstreamContext *bc, unsigned pos)
+{
+ bc->ptr = bc->buffer;
+ bc->bits = 0;
+ bc->bits_left = 0;
+
+ bitstream_skip(bc, pos);
+}
+
+/* Skip bits to a byte boundary. */
+static inline const uint8_t *bitstream_align(BitstreamContext *bc)
+{
+ unsigned n = -bitstream_tell(bc) & 7;
+ if (n)
+ bitstream_skip(bc, n);
+ return bc->buffer + (bitstream_tell(bc) >> 3);
+}
+
+/* Read MPEG-1 dc-style VLC (sign bit + mantissa with no MSB).
+ * If MSB not set it is negative. */
+static inline int bitstream_read_xbits(BitstreamContext *bc, unsigned length)
+{
+ int32_t cache = bitstream_peek(bc, 32);
+ int sign = ~cache >> 31;
+ skip_remaining(bc, length);
+
+ return ((((uint32_t)(sign ^ cache)) >> (32 - length)) ^ sign) - sign;
+}
+
+/* Return the LUT element for the given bitstream configuration. */
+static inline int set_idx(BitstreamContext *bc, int code, int *n, int *nb_bits,
+ VLC_TYPE (*table)[2])
+{
+ unsigned idx;
+
+ *nb_bits = -*n;
+ idx = bitstream_peek(bc, *nb_bits) + code;
+ *n = table[idx][1];
+
+ return table[idx][0];
+}
+
+/**
+ * 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 bitstream_read_vlc(BitstreamContext *bc, VLC_TYPE (*table)[2],
+ int bits, int max_depth)
+{
+ int nb_bits;
+ unsigned idx = bitstream_peek(bc, bits);
+ int code = table[idx][0];
+ int n = table[idx][1];
+
+ if (max_depth > 1 && n < 0) {
+ skip_remaining(bc, bits);
+ code = set_idx(bc, code, &n, &nb_bits, table);
+ if (max_depth > 2 && n < 0) {
+ skip_remaining(bc, nb_bits);
+ code = set_idx(bc, code, &n, &nb_bits, table);
+ }
+ }
+ skip_remaining(bc, n);
+
+ return code;
+}
+
+#define BITSTREAM_RL_VLC(level, run, bc, table, bits, max_depth) \
+ do { \
+ int n, nb_bits; \
+ unsigned index = bitstream_peek(bc, bits); \
+ level = table[index].level; \
+ n = table[index].len; \
+ \
+ if (max_depth > 1 && n < 0) { \
+ bitstream_skip(bc, bits); \
+ \
+ nb_bits = -n; \
+ \
+ index = bitstream_peek(bc, nb_bits) + level; \
+ level = table[index].level; \
+ n = table[index].len; \
+ if (max_depth > 2 && n < 0) { \
+ bitstream_skip(bc, nb_bits); \
+ nb_bits = -n; \
+ \
+ index = bitstream_peek(bc, nb_bits) + level; \
+ level = table[index].level; \
+ n = table[index].len; \
+ } \
+ } \
+ run = table[index].run; \
+ bitstream_skip(bc, n); \
+ } while (0)
+
+/* Return decoded truncated unary code for the values 0, 1, 2. */
+static inline int bitstream_decode012(BitstreamContext *bc)
+{
+ if (!bitstream_read_bit(bc))
+ return 0;
+ else
+ return bitstream_read_bit(bc) + 1;
+}
+
+/* Return decoded truncated unary code for the values 2, 1, 0. */
+static inline int bitstream_decode210(BitstreamContext *bc)
+{
+ if (bitstream_read_bit(bc))
+ return 0;
+ else
+ return 2 - bitstream_read_bit(bc);
+}
+
+/* Read sign bit and flip the sign of the provided value accordingly. */
+static inline int bitstream_apply_sign(BitstreamContext *bc, int val)
+{
+ int sign = bitstream_read_signed(bc, 1);
+ return (val ^ sign) - sign;
+}
+
+#endif /* AVCODEC_BITSTREAM_H */
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