bitreader.h

经典的MP4编解码核心库

#ifndef _DECODER_BITREADER_H_
#define _DECODER_BITREADER_H_

#include "../global.h"
#include "decoder.h"

#define MARKER()	bs_skip(bs, 1)

// header interpreter
// support B_VOP decoding
int bs_headers(Decoder * dec);

static void __inline bs_init(BitReader * const br, void * const bitstream, uint32_t length)
{
	br->buf = br->ptr = (uint32_t*)bitstream;
	br->pos = 0;
	br->length = length;

	br->curr = BSWAP(*br->ptr ++);
	br->next = BSWAP(*br->ptr ++);
	br->dec_err = 0;
}


// return the ammount of bytes used for decoding, ptr is for uint32_t *
static __inline uint32_t bs_length(BitReader * const br)
{
	return ((br->ptr - br->buf)<<2) - 4 - ((32 - br->pos) >> 3);
}


static __inline uint32_t bs_show(BitReader * const br, const uint32_t bits)
{
	int32_t nbit = (int32_t)(bits + br->pos) - 32;

	if (nbit > 0) 
	{
		return ((br->curr & (0xffffffff >> br->pos)) << nbit) |
				(br->next >> (32 - nbit));
	}
	else 
	{
		return (br->curr & (0xffffffff >> br->pos)) >> (32 - br->pos - bits);
	}
}



static __inline void bs_skip(BitReader * const br, const uint32_t bits)
{
	br->pos += bits;

	if (br->pos > 31) 
	{
		uint32_t consumed;
		br->curr = br->next;
		
		br->next = BSWAP(*br->ptr++);
		br->pos -= 32;

		consumed = bs_length(br);

		if(consumed > br->length)
		{
			br->dec_err = 1;		// set error flag for detection of decoder
		}
		
	}
}



static __inline void bs_bytealign(BitReader * const br)
{
	uint32_t remained = br->pos & 7; 
	if (remained)
		bs_skip(br, 8 - remained);
}



static __inline uint32_t bs_get(BitReader * const br, const uint32_t n)
{
	register uint32_t val;
	val = bs_show(br, n);
	bs_skip(br, n);
	return val;
}


static __inline uint32_t bs_get1(BitReader * const br)
{
	return bs_get(br, 1);
}


#endif /* _DECODER_BITREADER_H_ */