bitstream.h

   1 /*
   2  * Extracted 2009 from mplayer 2009-02-10 libavcodec/bitstream.h.
   3  *
   4  * copyright (c) 2004 Michael Niedermayer <michaelni@gmx.at>
   5  *
   6  * Licensed under the GNU Lesser General Public License.
   7  * For licencing details see COPYING.LIB.
   8  */
   9
  10 /** \file bitstream.h Bitstream structures and inline functions. */
  11
  12 /** Structure for bistream I/O. */
  13 struct getbit_context {
  14         /* Start of the internal buffer. */
  15         const uint8_t *buffer;
  16         /* End of the internal buffer. */
  17         const uint8_t *buffer_end;
  18         /** Bit counter. */
  19         int index;
  20 };
  21
  22 #define VLC_TYPE int16_t
  23
  24 struct vlc {
  25         int bits;
  26         VLC_TYPE(*table)[2];    ///< code, bits
  27         int table_size, table_allocated;
  28 };
  29
  30 static inline uint32_t show_bits(struct getbit_context *gbc, int num)
  31 {
  32         int idx = gbc->index;
  33         const uint8_t *p = gbc->buffer + (idx >> 3);
  34         uint32_t x = ((p[0] << 24) | (p[1] << 16) | (p[2] << 8) | p[3]);
  35         return (x << (idx & 7)) >> (32 - num);
  36 }
  37
  38 static inline int get_bits_count(struct getbit_context *gbc)
  39 {
  40         return gbc->index;
  41 }
  42
  43 static inline void skip_bits(struct getbit_context *gbc, int n)
  44 {
  45         gbc->index += n;
  46 }
  47
  48 static inline unsigned int get_bits(struct getbit_context *gbc, int n)
  49 {
  50         unsigned int ret = show_bits(gbc, n);
  51         skip_bits(gbc, n);
  52         return ret;
  53 }
  54
  55 /* This is rather hot, we can do better than get_bits(gbc, 1). */
  56 static inline unsigned int get_bit(struct getbit_context *gbc)
  57 {
  58         int idx = gbc->index++;
  59         uint8_t tmp = gbc->buffer[idx >> 3], mask = (1 << (7 - (idx & 7)));
  60         return !!(tmp & mask);
  61 }
  62
  63 /**
  64  * Initialize a getbit_context structure.
  65  *
  66  * \param buffer The bitstream buffer. It must be FF_INPUT_BUFFER_PADDING_SIZE
  67  * bytes larger then the actual read bits * because some optimized bitstream
  68  * readers read 32 or 64 bit at once and could read over the end.
  69  *
  70  * \param bit_size The size of the buffer in bytes.
  71  */
  72 static inline void init_get_bits(struct getbit_context *gbc,
  73                 const uint8_t *buffer, int size)
  74 {
  75         gbc->buffer = buffer;
  76         gbc->buffer_end = buffer + size;
  77         gbc->index = 0;
  78 }
  79
  80 void init_vlc(struct vlc *vlc, int nb_bits, int nb_codes, const void *bits,
  81                 const void *codes, int codes_size);
  82
  83 void free_vlc(struct vlc *vlc);
  84
  85 /**
  86  * Parse a vlc code.
  87  *
  88  * \param bits The number of bits which will be read at once, must be
  89  * identical to nb_bits in init_vlc()
  90  *
  91  * \param max_depth The number of times bits bits must be read to completely
  92  * read the longest vlc code = (max_vlc_length + bits - 1) / bits.
  93  */
  94 static inline int get_vlc(struct getbit_context *gbc, VLC_TYPE(*table)[2],
  95                 int bits, int max_depth)
  96 {
  97         int n, idx, nb_bits, code;
  98
  99         idx = show_bits(gbc, bits);
 100         code = table[idx][0];
 101         n = table[idx][1];
 102         if (max_depth > 1 && n < 0) {
 103                 skip_bits(gbc, bits);
 104                 nb_bits = -n;
 105                 idx = show_bits(gbc, nb_bits) + code;
 106                 code = table[idx][0];
 107                 n = table[idx][1];
 108                 if (max_depth > 2 && n < 0) {
 109                         skip_bits(gbc, nb_bits);
 110                         nb_bits = -n;
 111                         idx = show_bits(gbc, nb_bits) + code;
 112                         code = table[idx][0];
 113                         n = table[idx][1];
 114                 }
 115         }
 116         skip_bits(gbc, n);
 117         return code;
 118 }