mdct.c

这是一个mp3的源代码

/*____________________________________________________________________________
	
	FreeAmp - The Free MP3 Player

        MP3 Decoder originally Copyright (C) 1995-1997 Xing Technology
        Corp.  http://www.xingtech.com

	Portions Copyright (C) 1998-1999 EMusic.com

	This program is free software; you can redistribute it and/or modify
	it under the terms of the GNU General Public License as published by
	the Free Software Foundation; either version 2 of the License, or
	(at your option) any later version.

	This program is distributed in the hope that it will be useful,
	but WITHOUT ANY WARRANTY; without even the implied warranty of
	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
	GNU General Public License for more details.

	You should have received a copy of the GNU General Public License
	along with this program; if not, write to the Free Software
	Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
	
	$Id: mdct.c,v 1.4 1999/10/19 07:13:09 elrod Exp $
____________________________________________________________________________*/

/****  mdct.c  ***************************************************

Layer III 

  cos transform for n=18, n=6

computes  c[k] =  Sum( cos((pi/4*n)*(2*k+1)*(2*p+1))*f[p] )
                k = 0, ...n-1,  p = 0...n-1


inplace ok.

******************************************************************/

#include <stdlib.h>
#include <stdio.h>
#include <float.h>
#include <math.h>

#ifdef ASM_X86
extern void imdct18_asm(float f[18]);
extern void imdct6_3_asm(float f[]);
#endif /* ASM_X86 */

/*------ 18 point xform -------*/
float mdct18w[18];
float mdct18w2[9];
float coef[9][4];

float mdct6_3v[6];
float mdct6_3v2[3];
float coef87;

typedef struct
{
   float *w;
   float *w2;
   void *coef;
}
IMDCT_INIT_BLOCK;

static IMDCT_INIT_BLOCK imdct_info_18 =
{mdct18w, mdct18w2, coef};
static IMDCT_INIT_BLOCK imdct_info_6 =
{mdct6_3v, mdct6_3v2, &coef87};

/*====================================================================*/
IMDCT_INIT_BLOCK *imdct_init_addr_18()
{
   return &imdct_info_18;
}
IMDCT_INIT_BLOCK *imdct_init_addr_6()
{
   return &imdct_info_6;
}
/*--------------------------------------------------------------------*/
void imdct18(float f[18])	/* 18 point */
{
#ifdef ASM_X86
   imdct18_asm(f);
#else
   int p;
   float a[9], b[9];
   float ap, bp, a8p, b8p;
   float g1, g2;


   for (p = 0; p < 4; p++)
   {
      g1 = mdct18w[p] * f[p];
      g2 = mdct18w[17 - p] * f[17 - p];
      ap = g1 + g2;		// a[p]

      bp = mdct18w2[p] * (g1 - g2);	// b[p]

      g1 = mdct18w[8 - p] * f[8 - p];
      g2 = mdct18w[9 + p] * f[9 + p];
      a8p = g1 + g2;		// a[8-p]

      b8p = mdct18w2[8 - p] * (g1 - g2);	// b[8-p]

      a[p] = ap + a8p;
      a[5 + p] = ap - a8p;
      b[p] = bp + b8p;
      b[5 + p] = bp - b8p;
   }
   g1 = mdct18w[p] * f[p];
   g2 = mdct18w[17 - p] * f[17 - p];
   a[p] = g1 + g2;
   b[p] = mdct18w2[p] * (g1 - g2);


   f[0] = 0.5f * (a[0] + a[1] + a[2] + a[3] + a[4]);
   f[1] = 0.5f * (b[0] + b[1] + b[2] + b[3] + b[4]);

   f[2] = coef[1][0] * a[5] + coef[1][1] * a[6] + coef[1][2] * a[7]
      + coef[1][3] * a[8];
   f[3] = coef[1][0] * b[5] + coef[1][1] * b[6] + coef[1][2] * b[7]
      + coef[1][3] * b[8] - f[1];
   f[1] = f[1] - f[0];
   f[2] = f[2] - f[1];

   f[4] = coef[2][0] * a[0] + coef[2][1] * a[1] + coef[2][2] * a[2]
      + coef[2][3] * a[3] - a[4];
   f[5] = coef[2][0] * b[0] + coef[2][1] * b[1] + coef[2][2] * b[2]
      + coef[2][3] * b[3] - b[4] - f[3];
   f[3] = f[3] - f[2];
   f[4] = f[4] - f[3];

   f[6] = coef[3][0] * (a[5] - a[7] - a[8]);
   f[7] = coef[3][0] * (b[5] - b[7] - b[8]) - f[5];
   f[5] = f[5] - f[4];
   f[6] = f[6] - f[5];

   f[8] = coef[4][0] * a[0] + coef[4][1] * a[1] + coef[4][2] * a[2]
      + coef[4][3] * a[3] + a[4];
   f[9] = coef[4][0] * b[0] + coef[4][1] * b[1] + coef[4][2] * b[2]
      + coef[4][3] * b[3] + b[4] - f[7];
   f[7] = f[7] - f[6];
   f[8] = f[8] - f[7];

   f[10] = coef[5][0] * a[5] + coef[5][1] * a[6] + coef[5][2] * a[7]
      + coef[5][3] * a[8];
   f[11] = coef[5][0] * b[5] + coef[5][1] * b[6] + coef[5][2] * b[7]
      + coef[5][3] * b[8] - f[9];
   f[9] = f[9] - f[8];
   f[10] = f[10] - f[9];

   f[12] = 0.5f * (a[0] + a[2] + a[3]) - a[1] - a[4];
   f[13] = 0.5f * (b[0] + b[2] + b[3]) - b[1] - b[4] - f[11];
   f[11] = f[11] - f[10];
   f[12] = f[12] - f[11];

   f[14] = coef[7][0] * a[5] + coef[7][1] * a[6] + coef[7][2] * a[7]
      + coef[7][3] * a[8];
   f[15] = coef[7][0] * b[5] + coef[7][1] * b[6] + coef[7][2] * b[7]
      + coef[7][3] * b[8] - f[13];
   f[13] = f[13] - f[12];
   f[14] = f[14] - f[13];

   f[16] = coef[8][0] * a[0] + coef[8][1] * a[1] + coef[8][2] * a[2]
      + coef[8][3] * a[3] + a[4];
   f[17] = coef[8][0] * b[0] + coef[8][1] * b[1] + coef[8][2] * b[2]
      + coef[8][3] * b[3] + b[4] - f[15];
   f[15] = f[15] - f[14];
   f[16] = f[16] - f[15];
   f[17] = f[17] - f[16];


   return;
#endif
}
/*--------------------------------------------------------------------*/
/* does 3, 6 pt dct.  changes order from f[i][window] c[window][i] */
void imdct6_3(float f[])	/* 6 point */
{
#ifdef ASM_X86
   imdct6_3_asm(f);
#else
   int w;
   float buf[18];
   float *a, *c;		// b[i] = a[3+i]

   float g1, g2;
   float a02, b02;

   c = f;
   a = buf;
   for (w = 0; w < 3; w++)
   {
      g1 = mdct6_3v[0] * f[3 * 0];
      g2 = mdct6_3v[5] * f[3 * 5];
      a[0] = g1 + g2;
      a[3 + 0] = mdct6_3v2[0] * (g1 - g2);

      g1 = mdct6_3v[1] * f[3 * 1];
      g2 = mdct6_3v[4] * f[3 * 4];
      a[1] = g1 + g2;
      a[3 + 1] = mdct6_3v2[1] * (g1 - g2);

      g1 = mdct6_3v[2] * f[3 * 2];
      g2 = mdct6_3v[3] * f[3 * 3];
      a[2] = g1 + g2;
      a[3 + 2] = mdct6_3v2[2] * (g1 - g2);

      a += 6;
      f++;
   }

   a = buf;
   for (w = 0; w < 3; w++)
   {
      a02 = (a[0] + a[2]);
      b02 = (a[3 + 0] + a[3 + 2]);
      c[0] = a02 + a[1];
      c[1] = b02 + a[3 + 1];
      c[2] = coef87 * (a[0] - a[2]);
      c[3] = coef87 * (a[3 + 0] - a[3 + 2]) - c[1];
      c[1] = c[1] - c[0];
      c[2] = c[2] - c[1];
      c[4] = a02 - a[1] - a[1];
      c[5] = b02 - a[3 + 1] - a[3 + 1] - c[3];
      c[3] = c[3] - c[2];
      c[4] = c[4] - c[3];
      c[5] = c[5] - c[4];
      a += 6;
      c += 6;
   }

   return;
#endif
}
/*--------------------------------------------------------------------*/