| 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 | |