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 #define AV_RB32(x)  ((((const uint8_t*)(x))[0] << 24) | \
  13                      (((const uint8_t*)(x))[1] << 16) | \
  14                      (((const uint8_t*)(x))[2] <<  8) | \
  15                       ((const uint8_t*)(x))[3])
  16
  17 /** Structure for bistream I/O. */
  18 struct getbit_context {
  19         /* Start of the internal buffer. */
  20         const uint8_t *buffer;
  21         /* End of the internal buffer. */
  22         const uint8_t *buffer_end;
  23         /** Bit counter. */
  24         int index;
  25 };
  26
  27 #define VLC_TYPE int16_t
  28
  29 struct vlc {
  30         int bits;
  31         VLC_TYPE(*table)[2];    ///< code, bits
  32         int table_size, table_allocated;
  33 };
  34
  35 static inline uint32_t show_bits(struct getbit_context *gbc, int num)
  36 {
  37         int idx = gbc->index;
  38         uint32_t x = AV_RB32(gbc->buffer + (idx >> 3)) << (idx & 7);
  39         return x >> (32 - num);
  40 }
  41
  42 static inline int get_bits_count(struct getbit_context *gbc)
  43 {
  44         return gbc->index;
  45 }
  46
  47 static inline void skip_bits(struct getbit_context *gbc, int n)
  48 {
  49         gbc->index += n;
  50 }
  51
  52 static inline unsigned int get_bits(struct getbit_context *gbc, int n)
  53 {
  54         unsigned int ret = show_bits(gbc, n);
  55         skip_bits(gbc, n);
  56         return ret;
  57 }
  58
  59 /* This is rather hot, we can do better than get_bits(gbc, 1). */
  60 static inline unsigned int get_bits1(struct getbit_context *gbc)
  61 {
  62         int idx = gbc->index++;
  63         uint8_t tmp = gbc->buffer[idx >> 3], mask = (1 << (7 - (idx & 7)));
  64         return !!(tmp & mask);
  65 }
  66
  67 /**
  68  * Initialize a getbit_context structure.
  69  *
  70  * \param buffer The bitstream buffer. It must be FF_INPUT_BUFFER_PADDING_SIZE
  71  * bytes larger then the actual read bits * because some optimized bitstream
  72  * readers read 32 or 64 bit at once and could read over the end.
  73  *
  74  * \param bit_size The size of the buffer in bytes.
  75  */
  76 static inline void init_get_bits(struct getbit_context *gbc,
  77                 const uint8_t *buffer, int size)
  78 {
  79         gbc->buffer = buffer;
  80         gbc->buffer_end = buffer + size;
  81         gbc->index = 0;
  82 }
  83
  84 void init_vlc(struct vlc *vlc, int nb_bits, int nb_codes, const void *bits,
  85                 const void *codes, int codes_size);
  86
  87 void free_vlc(struct vlc *vlc);
  88
  89 /**
  90  * Parse a vlc code.
  91  *
  92  * \param bits The number of bits which will be read at once, must be
  93  * identical to nb_bits in init_vlc()
  94  *
  95  * \param max_depth The number of times bits bits must be read to completely
  96  * read the longest vlc code = (max_vlc_length + bits - 1) / bits.
  97  */
  98 static inline int get_vlc(struct getbit_context *gbc, VLC_TYPE(*table)[2],
  99                 int bits, int max_depth)
 100 {
 101         int n, idx, nb_bits, code;
 102
 103         idx = show_bits(gbc, bits);
 104         code = table[idx][0];
 105         n = table[idx][1];
 106         if (max_depth > 1 && n < 0) {
 107                 skip_bits(gbc, bits);
 108                 nb_bits = -n;
 109                 idx = show_bits(gbc, nb_bits) + code;
 110                 code = table[idx][0];
 111                 n = table[idx][1];
 112                 if (max_depth > 2 && n < 0) {
 113                         skip_bits(gbc, nb_bits);
 114                         nb_bits = -n;
 115                         idx = show_bits(gbc, nb_bits) + code;
 116                         code = table[idx][0];
 117                         n = table[idx][1];
 118                 }
 119         }
 120         skip_bits(gbc, n);
 121         return code;
 122 }