120 lines
2.5 KiB
C
120 lines
2.5 KiB
C
/**
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libsmacker - A C library for decoding .smk Smacker Video files
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Copyright (C) 2012-2017 Greg Kennedy
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See smacker.h for more information.
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smk_bitstream.c
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Implements a bitstream structure, which can extract and
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return a bit at a time from a raw block of bytes.
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*/
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#include "smk_bitstream.h"
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/* malloc and friends */
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#include "smk_malloc.h"
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/*
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Bitstream structure
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Pointer to raw block of data and a size limit.
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Maintains internal pointers to byte_num and bit_number.
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*/
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struct smk_bit_t
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{
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const unsigned char* buffer;
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unsigned long size;
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unsigned long byte_num;
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char bit_num;
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};
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/* BITSTREAM Functions */
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struct smk_bit_t* smk_bs_init(const unsigned char* b, const unsigned long size)
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{
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struct smk_bit_t* ret = NULL;
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/* sanity check */
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smk_assert(b);
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/* allocate a bitstream struct */
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smk_malloc(ret, sizeof(struct smk_bit_t));
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/* set up the pointer to bitstream, and the size counter */
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ret->buffer = b;
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ret->size = size;
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/* point to initial byte: note, smk_malloc already sets these to 0 */
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/* ret->byte_num = 0;
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ret->bit_num = 0; */
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/* return ret or NULL if error : ) */
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error:
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return ret;
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}
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/* Reads a bit
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Returns -1 if error encountered */
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char _smk_bs_read_1(struct smk_bit_t* bs)
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{
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unsigned char ret = -1;
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/* sanity check */
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smk_assert(bs);
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/* don't die when running out of bits, but signal */
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if (bs->byte_num >= bs->size)
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{
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fprintf(stderr, "libsmacker::_smk_bs_read_1(bs): ERROR: bitstream (length=%lu) exhausted.\n", bs->size);
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goto error;
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}
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/* get next bit and return */
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ret = (((bs->buffer[bs->byte_num]) & (1 << bs->bit_num)) != 0);
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/* advance to next bit */
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bs->bit_num ++;
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/* Out of bits in this byte: next! */
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if (bs->bit_num > 7)
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{
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bs->byte_num ++;
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bs->bit_num = 0;
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}
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/* return ret, or (default) -1 if error */
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error:
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return ret;
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}
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/* Reads a byte
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Returns -1 if error. */
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short _smk_bs_read_8(struct smk_bit_t* bs)
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{
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unsigned char ret = -1;
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/* sanity check */
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smk_assert(bs);
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/* don't die when running out of bits, but signal */
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if (bs->byte_num + (bs->bit_num > 0) >= bs->size)
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{
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fprintf(stderr, "libsmacker::_smk_bs_read_8(bs): ERROR: bitstream (length=%lu) exhausted.\n", bs->size);
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goto error;
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}
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if (bs->bit_num)
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{
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/* unaligned read */
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ret = bs->buffer[bs->byte_num] >> bs->bit_num;
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bs->byte_num ++;
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ret |= (bs->buffer[bs->byte_num] << (8 - bs->bit_num));
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} else {
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/* aligned read */
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ret = bs->buffer[bs->byte_num ++];
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}
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/* return ret, or (default) -1 if error */
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error:
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return ret;
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}
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