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