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, see file COPYING.LIB.
   7  */
   8
   9 /** \file bitstream.h Bitstream structures and inline functions. */
  10
  11 /** Structure for bistream I/O. */
  12 struct getbit_context {
  13         /** Start of the internal buffer. */
  14         const uint8_t *buffer;
  15         /** Length of buffer in bits (always a multiple of 8). */
  16         uint32_t num_bits;
  17         /** Bit counter. */
  18         int index;
  19 };
  20
  21 /** A variable length code table. */
  22 struct vlc {
  23         /** Number of bits of the table. */
  24         int bits;
  25         /** The code and the bits table. */
  26         int16_t (*table)[2];
  27         /** The size of the table. */
  28         int table_size;
  29         /** Amount of memory allocated so far. */
  30         int table_allocated;
  31 };
  32
  33 static inline uint32_t show_bits(struct getbit_context *gbc, int num)
  34 {
  35         int idx = gbc->index;
  36         const char *p;
  37         uint32_t x;
  38
  39         assert(idx + num <= gbc->num_bits);
  40         p = (const char *)gbc->buffer + (idx >> 3);
  41         x = read_u32_be(p);
  42         return (x << (idx & 7)) >> (32 - num);
  43 }
  44
  45 static inline int get_bits_count(struct getbit_context *gbc)
  46 {
  47         return gbc->index;
  48 }
  49
  50 static inline void skip_bits(struct getbit_context *gbc, int n)
  51 {
  52         assert(gbc->index + n <= gbc->num_bits);
  53         gbc->index += n;
  54 }
  55
  56 static inline unsigned int get_bits(struct getbit_context *gbc, int n)
  57 {
  58         unsigned int ret = show_bits(gbc, n); /* checks n */
  59         skip_bits(gbc, n);
  60         return ret;
  61 }
  62
  63 /* This is rather hot, we can do better than get_bits(gbc, 1). */
  64 static inline unsigned int get_bit(struct getbit_context *gbc)
  65 {
  66         int idx;
  67         uint8_t tmp, mask;
  68
  69         assert(gbc->index < gbc->num_bits);
  70         idx = gbc->index++;
  71         tmp = gbc->buffer[idx >> 3];
  72         mask = 1 << (7 - (idx & 7));
  73         return !!(tmp & mask);
  74 }
  75
  76 /**
  77  * Initialize a getbit_context structure.
  78  *
  79  * \param gbc The structure to initialize.
  80  * \param buffer The bitstream buffer.
  81  * \param size The size of the buffer in bytes.
  82  *
  83  * The bitstream buffer must be 4 bytes larger then the actual read bits
  84  * because the bitstream reader might read 32 bits at once and could read over
  85  * the end.
  86  */
  87 static inline void init_get_bits(struct getbit_context *gbc,
  88                 const uint8_t *buffer, int size)
  89 {
  90         gbc->buffer = buffer;
  91         gbc->num_bits = size * 8;
  92         gbc->index = 0;
  93 }
  94
  95 void init_vlc(struct vlc *vlc, int nb_bits, int nb_codes, const void *bits,
  96                 const void *codes, int codes_size);
  97 void free_vlc(struct vlc *vlc);
  98 int get_vlc(struct getbit_context *gbc, const struct vlc *vlc);