deps/libchdr/src/libchdr_bitstream.c

   1 /* license:BSD-3-Clause
   2  * copyright-holders:Aaron Giles
   3 ***************************************************************************
   4
   5     bitstream.c
   6
   7     Helper classes for reading/writing at the bit level.
   8
   9 ***************************************************************************/
  10
  11 #include <stdlib.h>
  12 #include <libchdr/bitstream.h>
  13
  14 /***************************************************************************
  15  *  INLINE FUNCTIONS
  16  ***************************************************************************
  17  */
  18
  19 int bitstream_overflow(struct bitstream* bitstream) { return ((bitstream->doffset - bitstream->bits / 8) > bitstream->dlength); }
  20
  21 /*-------------------------------------------------
  22  *  create_bitstream - constructor
  23  *-------------------------------------------------
  24  */
  25
  26 struct bitstream* create_bitstream(const void *src, uint32_t srclength)
  27 {
  28         struct bitstream* bitstream = (struct bitstream*)malloc(sizeof(struct bitstream));
  29         bitstream->buffer = 0;
  30         bitstream->bits = 0;
  31         bitstream->read = (const uint8_t*)src;
  32         bitstream->doffset = 0;
  33         bitstream->dlength = srclength;
  34         return bitstream;
  35 }
  36
  37
  38 /*-----------------------------------------------------
  39  *  bitstream_peek - fetch the requested number of bits
  40  *  but don't advance the input pointer
  41  *-----------------------------------------------------
  42  */
  43
  44 uint32_t bitstream_peek(struct bitstream* bitstream, int numbits)
  45 {
  46         if (numbits == 0)
  47                 return 0;
  48
  49         /* fetch data if we need more */
  50         if (numbits > bitstream->bits)
  51         {
  52                 while (bitstream->bits <= 24)
  53                 {
  54                         if (bitstream->doffset < bitstream->dlength)
  55                                 bitstream->buffer |= bitstream->read[bitstream->doffset] << (24 - bitstream->bits);
  56                         bitstream->doffset++;
  57                         bitstream->bits += 8;
  58                 }
  59         }
  60
  61         /* return the data */
  62         return bitstream->buffer >> (32 - numbits);
  63 }
  64
  65
  66 /*-----------------------------------------------------
  67  *  bitstream_remove - advance the input pointer by the
  68  *  specified number of bits
  69  *-----------------------------------------------------
  70  */
  71
  72 void bitstream_remove(struct bitstream* bitstream, int numbits)
  73 {
  74         bitstream->buffer <<= numbits;
  75         bitstream->bits -= numbits;
  76 }
  77
  78
  79 /*-----------------------------------------------------
  80  *  bitstream_read - fetch the requested number of bits
  81  *-----------------------------------------------------
  82  */
  83
  84 uint32_t bitstream_read(struct bitstream* bitstream, int numbits)
  85 {
  86         uint32_t result = bitstream_peek(bitstream, numbits);
  87         bitstream_remove(bitstream, numbits);
  88         return result;
  89 }
  90
  91
  92 /*-------------------------------------------------
  93  *  read_offset - return the current read offset
  94  *-------------------------------------------------
  95  */
  96
  97 uint32_t bitstream_read_offset(struct bitstream* bitstream)
  98 {
  99         uint32_t result = bitstream->doffset;
 100         int bits = bitstream->bits;
 101         while (bits >= 8)
 102         {
 103                 result--;
 104                 bits -= 8;
 105         }
 106         return result;
 107 }
 108
 109
 110 /*-------------------------------------------------
 111  *  flush - flush to the nearest byte
 112  *-------------------------------------------------
 113  */
 114
 115 uint32_t bitstream_flush(struct bitstream* bitstream)
 116 {
 117         while (bitstream->bits >= 8)
 118         {
 119                 bitstream->doffset--;
 120                 bitstream->bits -= 8;
 121         }
 122         bitstream->bits = bitstream->buffer = 0;
 123         return bitstream->doffset;
 124 }
 125