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