fastpow.c

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#include "fastpow.h"
#include <math.h>
 
static const float _2p23 = 8388608.0f;
 
void powFastSetTable
(
   uint32_t* const pTable,
   const uint32_t  precision
)
{
   /* step along table elements and x-axis positions */
   float zeroToOne = 1.0f / ((float)(1 << precision) * 2.0f);        /* A */
   int32_t i;                                                        /* B */
   for( i = 0;  i < (1 << precision);  ++i )                         /* C */
   {
      /* make y-axis value for table element */
      const float f = ((float)powf( 2.0f, zeroToOne ) - 1.0f) * _2p23;
      pTable[i] = (uint32_t)( f < _2p23 ? f : (_2p23 - 1.0f) );
      zeroToOne += 1.0f / (float)(1 << precision);
   }                                                                 /* D */
}
 
float powFastLookup
(
   const float         val,
   const float         ilog2,
   const uint32_t* pTable,
   const uint32_t  precision
)
{
   /* build float bits */
   const int32_t i = (int32_t)( (val * (_2p23 * ilog2)) + (127.0f * _2p23) );
 
   /* replace mantissa with lookup */
   const int32_t it = (i & 0xFF800000) | pTable[(i & 0x7FFFFF) >>        /* E */
      (23 - precision)];                                             /* F */
 
   /* convert bits to float */
   return *(const float*)( &it );
}