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dxhead.c

/* ---- 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 mpg123_get_xing_header(XHEADDATA * X, unsigned char *buf)
{     
      int i, head_flags;
      int h_id, h_mode, h_sr_index;
      static 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 mpg123_seek_point(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);
      seekpoint = (int) ((1.0f / 256.0f) * fx * file_bytes);
      return seekpoint;
}

/*-------------------------------------------------------------*/ 

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