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>

libavcodec/bitstream.h

+/*
+ * 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 */