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| Rev | Author | Line No. | Line |
|---|---|---|---|
| 1 | pmbaty | 1 | /** |
| 2 | libsmacker - A C library for decoding .smk Smacker Video files |
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| 3 | Copyright (C) 2012-2017 Greg Kennedy |
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| 4 | |||
| 5 | See smacker.h for more information. |
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| 6 | |||
| 7 | smk_bitstream.c |
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| 8 | Implements a bitstream structure, which can extract and |
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| 9 | return a bit at a time from a raw block of bytes. |
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| 10 | */ |
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| 11 | |||
| 12 | #include "smk_bitstream.h" |
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| 13 | |||
| 14 | /* malloc and friends */ |
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| 15 | #include "smk_malloc.h" |
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| 16 | |||
| 17 | /* |
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| 18 | Bitstream structure |
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| 19 | Pointer to raw block of data and a size limit. |
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| 20 | Maintains internal pointers to byte_num and bit_number. |
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| 21 | */ |
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| 22 | struct smk_bit_t |
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| 23 | { |
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| 24 | const unsigned char* buffer; |
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| 25 | unsigned long size; |
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| 26 | |||
| 27 | unsigned long byte_num; |
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| 28 | char bit_num; |
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| 29 | }; |
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| 30 | |||
| 31 | /* BITSTREAM Functions */ |
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| 32 | struct smk_bit_t* smk_bs_init(const unsigned char* b, const unsigned long size) |
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| 33 | { |
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| 34 | struct smk_bit_t* ret = NULL; |
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| 35 | |||
| 36 | /* sanity check */ |
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| 37 | smk_assert(b); |
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| 38 | |||
| 39 | /* allocate a bitstream struct */ |
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| 40 | smk_malloc(ret, sizeof(struct smk_bit_t)); |
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| 41 | |||
| 42 | /* set up the pointer to bitstream, and the size counter */ |
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| 43 | ret->buffer = b; |
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| 44 | ret->size = size; |
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| 45 | |||
| 46 | /* point to initial byte: note, smk_malloc already sets these to 0 */ |
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| 47 | /* ret->byte_num = 0; |
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| 48 | ret->bit_num = 0; */ |
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| 49 | |||
| 50 | /* return ret or NULL if error : ) */ |
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| 51 | error: |
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| 52 | return ret; |
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| 53 | } |
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| 54 | |||
| 55 | /* Reads a bit |
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| 56 | Returns -1 if error encountered */ |
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| 57 | char _smk_bs_read_1(struct smk_bit_t* bs) |
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| 58 | { |
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| 59 | unsigned char ret = -1; |
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| 60 | |||
| 61 | /* sanity check */ |
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| 62 | smk_assert(bs); |
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| 63 | |||
| 64 | /* don't die when running out of bits, but signal */ |
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| 65 | if (bs->byte_num >= bs->size) |
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| 66 | { |
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| 67 | fprintf(stderr, "libsmacker::_smk_bs_read_1(bs): ERROR: bitstream (length=%lu) exhausted.\n", bs->size); |
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| 68 | goto error; |
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| 69 | } |
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| 70 | |||
| 71 | /* get next bit and return */ |
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| 72 | ret = (((bs->buffer[bs->byte_num]) & (1 << bs->bit_num)) != 0); |
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| 73 | |||
| 74 | /* advance to next bit */ |
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| 75 | bs->bit_num ++; |
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| 76 | |||
| 77 | /* Out of bits in this byte: next! */ |
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| 78 | if (bs->bit_num > 7) |
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| 79 | { |
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| 80 | bs->byte_num ++; |
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| 81 | bs->bit_num = 0; |
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| 82 | } |
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| 83 | |||
| 84 | /* return ret, or (default) -1 if error */ |
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| 85 | error: |
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| 86 | return ret; |
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| 87 | } |
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| 88 | |||
| 89 | /* Reads a byte |
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| 90 | Returns -1 if error. */ |
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| 91 | short _smk_bs_read_8(struct smk_bit_t* bs) |
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| 92 | { |
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| 93 | unsigned char ret = -1; |
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| 94 | |||
| 95 | /* sanity check */ |
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| 96 | smk_assert(bs); |
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| 97 | |||
| 98 | /* don't die when running out of bits, but signal */ |
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| 99 | if (bs->byte_num + (bs->bit_num > 0) >= bs->size) |
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| 100 | { |
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| 101 | fprintf(stderr, "libsmacker::_smk_bs_read_8(bs): ERROR: bitstream (length=%lu) exhausted.\n", bs->size); |
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| 102 | goto error; |
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| 103 | } |
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| 104 | |||
| 105 | if (bs->bit_num) |
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| 106 | { |
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| 107 | /* unaligned read */ |
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| 108 | ret = bs->buffer[bs->byte_num] >> bs->bit_num; |
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| 109 | bs->byte_num ++; |
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| 110 | ret |= (bs->buffer[bs->byte_num] << (8 - bs->bit_num)); |
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| 111 | } else { |
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| 112 | /* aligned read */ |
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| 113 | ret = bs->buffer[bs->byte_num ++]; |
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| 114 | } |
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| 115 | |||
| 116 | /* return ret, or (default) -1 if error */ |
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| 117 | error: |
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| 118 | return ret; |
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| 119 | } |