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/*---- DXhead.c --------------------------------------------
decoder MPEG Layer III
handle Xing header
mod 12/7/98 add vbr scale
Copyright 1998 Xing Technology Corp.
-----------------------------------------------------------*/
#include <stdlib.h>
#include <stdio.h>
#include <float.h>
#include <math.h>
#include "dxHead.h"
// 4 Xing
// 4 flags
// 4 frames
// 4 bytes
// 100 toc
/*-------------------------------------------------------------*/
static int ExtractI4(unsigned char *buf)
{
int x;
// big endian extract
x = buf[0];
x <<= 8;
x |= buf[1];
x <<= 8;
x |= buf[2];
x <<= 8;
x |= buf[3];
return x;
}
/*-------------------------------------------------------------*/
int GetXingHeader(XHEADDATA *X, unsigned char *buf)
{
int i, head_flags;
int h_id, h_mode, h_sr_index;
static const int sr_table[4] = { 44100, 48000, 32000, 99999 };
// get Xing header data
X->flags = 0; // clear to null incase fail
// get selected MPEG header data
h_id = (buf[1] >> 3) & 1;
h_sr_index = (buf[2] >> 2) & 3;
h_mode = (buf[3] >> 6) & 3;
// determine offset of header
if( h_id ) { // mpeg1
if( h_mode != 3 ) buf+=(32+4);
else buf+=(17+4);
}
else { // mpeg2
if( h_mode != 3 ) buf+=(17+4);
else buf+=(9+4);
}
if( buf[0] != 'X' ) return 0; // fail
if( buf[1] != 'i' ) return 0; // header not found
if( buf[2] != 'n' ) return 0;
if( buf[3] != 'g' ) return 0;
buf+=4;
X->h_id = h_id;
X->samprate = sr_table[h_sr_index];
if( h_id == 0 ) X->samprate >>= 1;
head_flags = X->flags = ExtractI4(buf); buf+=4; // get flags
if( head_flags & FRAMES_FLAG ) {X->frames = ExtractI4(buf); buf+=4;}
if( head_flags & BYTES_FLAG ) {X->bytes = ExtractI4(buf); buf+=4;}
if( head_flags & TOC_FLAG ) {
if( X->toc != NULL ) {
for(i=0;i<100;i++) X->toc[i] = buf[i];
}
buf+=100;
}
X->vbr_scale = -1;
if( head_flags & VBR_SCALE_FLAG ) {X->vbr_scale = ExtractI4(buf); buf+=4;}
//if( X->toc != NULL ) {
//for(i=0;i<100;i++) {
// if( (i%10) == 0 ) printf("\n");
// printf(" %3d", (int)(X->toc[i]));
//}
//}
return 1; // success
}
/*-------------------------------------------------------------*/
int SeekPoint(unsigned char TOC[100], int file_bytes, float percent)
{
// interpolate in TOC to get file seek point in bytes
int a, seekpoint;
float fa, fb, fx;
if( percent < 0.0f ) percent = 0.0f;
if( percent > 100.0f ) percent = 100.0f;
a = (int)percent;
if( a > 99 ) a = 99;
fa = TOC[a];
if( a < 99 ) {
fb = TOC[a+1];
}
else {
fb = 256.0f;
}
fx = fa + (fb-fa)*(percent-a);
//printf("%f ....... %f ...... %f\n",fa,fb,fx);
seekpoint = (int)((1.0f/256.0f)*fx*file_bytes);
return seekpoint;
}
/*-------------------------------------------------------------*/
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