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/*----------------------------------------------------------------------------
*
* File:
* eas_flog2.c
*
* Contents and purpose:
* Fixed point square root
*
*
* Copyright (c) 2006 Sonic Network Inc.
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
*----------------------------------------------------------------------------
* Revision Control:
* $Revision$
* $Date$
*----------------------------------------------------------------------------
*/
#include "eas_types.h"
#include "eas_math.h"
#define MANTISSA_SHIFT 27
#define MANTISSA_MASK 0x0000000f
#define MANTISSA_LSB_SHIFT 7
#define MANTISSA_LSB_MASK 0x000fffff
#define LOG_EXPONENT_SHIFT 10
#define INTERPOLATION_SHIFT 20
#define MAX_NEGATIVE (-2147483647-1)
/* log lookup table */
static const EAS_U16 eas_log2_table[] =
{
0, 90, 174, 254, 330, 402, 470, 536,
599, 659, 717, 773, 827, 879, 929, 977,
1024
};
/*----------------------------------------------------------------------------
* EAS_flog2()
*----------------------------------------------------------------------------
* Purpose:
* Calculates the log2 of a 32-bit fixed point value
*
* Inputs:
* n = value of interest
*
* Outputs:
* returns the log2 of n
*
*----------------------------------------------------------------------------
*/
EAS_I32 EAS_flog2 (EAS_U32 n)
{
EAS_U32 exp;
EAS_U32 interp;
/* check for error condition */
if (n == 0)
return MAX_NEGATIVE;
/* find exponent */
for (exp = 31; exp > 0; exp--)
{
/* shift until we get a 1 bit in bit 31 */
if ((n & (EAS_U32) MAX_NEGATIVE) != 0)
break;
n <<= 1;
}
/*lint -e{701} use shift for performance */
exp <<= LOG_EXPONENT_SHIFT;
/* get the least significant bits for interpolation */
interp = (n >> MANTISSA_LSB_SHIFT) & MANTISSA_LSB_MASK;
/* get the most significant bits for mantissa lookup */
n = (n >> MANTISSA_SHIFT) & MANTISSA_MASK;
/* interpolate mantissa */
interp = ((eas_log2_table[n+1] - eas_log2_table[n]) * interp) >> INTERPOLATION_SHIFT;
exp += eas_log2_table[n] + interp;
return (EAS_I32) exp;
}
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