lookup.h

fix point版本的Ogg Vorbis decoder

/********************************************************************
 *                                                                  *
 * THIS FILE IS PART OF THE OggVorbis SOFTWARE CODEC SOURCE CODE.   *
 * USE, DISTRIBUTION AND REPRODUCTION OF THIS LIBRARY SOURCE IS     *
 * GOVERNED BY A BSD-STYLE SOURCE LICENSE INCLUDED WITH THIS SOURCE *
 * IN 'COPYING'. PLEASE READ THESE TERMS BEFORE DISTRIBUTING.       *
 *                                                                  *
 * THE OggVorbis SOURCE CODE IS (C) COPYRIGHT 1994-2002             *
 * by the XIPHOPHORUS Company http://www.xiph.org/                  *
 *                                                                  *
 ********************************************************************

  function: lookup based functions
  last mod: $Id: lookup.c,v 1.9 2002/01/22 08:06:07 xiphmont Exp $

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

#include "os.h"
#include "lookup_data.h"

/* interpolated 1./sqrt(p) where .5 <= a < 1. (.100000... to .111111...) in
   16.16 format 

   returns in m.8 format */
STIN long vorbis_invsqlook_i(long a,long e){
  long i=(a&0x7fff)>>(INVSQ_LOOKUP_I_SHIFT-1); 
  long d=(a&INVSQ_LOOKUP_I_MASK)<<(16-INVSQ_LOOKUP_I_SHIFT); /*  0.16 */
  long val=INVSQ_LOOKUP_I[i]-                                /*  1.16 */
    (((INVSQ_LOOKUP_I[i]-INVSQ_LOOKUP_I[i+1])*               /*  0.16 */
      d)>>16);                                               /* result 1.16 */
  
  e+=32;
  if(e&1)val=(val*5792)>>13; /* multiply val by 1/sqrt(2) */
  e=(e>>1)-8;

  return(val>>e);
}

/* interpolated lookup based fromdB function, domain -140dB to 0dB only */
/* a is in n.12 format */
STIN FIXP vorbis_fromdBlook_i(long a){
  int i=(-a)>>(12-FROMdB2_SHIFT);
  return (i<0)?TO_FIXP(30,1.0):
    ((i>=(FROMdB_LOOKUP_SZ<<FROMdB_SHIFT))?0:
     MUL(30,FROMdB_LOOKUP[i>>FROMdB_SHIFT],FROMdB2_LOOKUP[i&FROMdB2_MASK]));
}

/* interpolated lookup based cos function, domain 0 to PI only */
/* a is in 0.16 format, where 0==0, 2^^16-1==PI, return 0.14 */
STIN long vorbis_coslook_i(long a){
  int i=a>>COS_LOOKUP_I_SHIFT;
  int d=a&COS_LOOKUP_I_MASK;
  return COS_LOOKUP_I[i]- ((d*(COS_LOOKUP_I[i]-COS_LOOKUP_I[i+1]))>>
			   COS_LOOKUP_I_SHIFT);
}

/* interpolated lookup based sin function, domain 0 to PI only */
/* a is in 0.16 format, where 0==0, 2^^16-1==PI, return 0.14 */
STIN long vorbis_sinlook_i(long a){
  a=(a&0x8000)?a-0x8000:0x8000-a;
  return vorbis_coslook_i(a);
}