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      1 /*----------------------------------------------------------------------------
      2  *
      3  * File:
      4  * eas_flog2.c
      5  *
      6  * Contents and purpose:
      7  * Fixed point square root
      8  *
      9  *
     10  * Copyright (c) 2006 Sonic Network Inc.
     11 
     12  * Licensed under the Apache License, Version 2.0 (the "License");
     13  * you may not use this file except in compliance with the License.
     14  * You may obtain a copy of the License at
     15  *
     16  *      http://www.apache.org/licenses/LICENSE-2.0
     17  *
     18  * Unless required by applicable law or agreed to in writing, software
     19  * distributed under the License is distributed on an "AS IS" BASIS,
     20  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     21  * See the License for the specific language governing permissions and
     22  * limitations under the License.
     23  *
     24  *----------------------------------------------------------------------------
     25  * Revision Control:
     26  *   $Revision$
     27  *   $Date$
     28  *----------------------------------------------------------------------------
     29 */
     30 
     31 #include "eas_types.h"
     32 #include "eas_math.h"
     33 
     34 #define MANTISSA_SHIFT          27
     35 #define MANTISSA_MASK           0x0000000f
     36 #define MANTISSA_LSB_SHIFT      7
     37 #define MANTISSA_LSB_MASK       0x000fffff
     38 #define LOG_EXPONENT_SHIFT      10
     39 #define INTERPOLATION_SHIFT     20
     40 #define MAX_NEGATIVE            (-2147483647-1)
     41 
     42 /* log lookup table */
     43 static const EAS_U16 eas_log2_table[] =
     44 {
     45     0, 90, 174, 254, 330, 402, 470, 536,
     46     599, 659, 717, 773, 827, 879, 929, 977,
     47     1024
     48 };
     49 
     50 /*----------------------------------------------------------------------------
     51  * EAS_flog2()
     52  *----------------------------------------------------------------------------
     53  * Purpose:
     54  * Calculates the log2 of a 32-bit fixed point value
     55  *
     56  * Inputs:
     57  * n = value of interest
     58  *
     59  * Outputs:
     60  * returns the log2 of n
     61  *
     62  *----------------------------------------------------------------------------
     63 */
     64 EAS_I32 EAS_flog2 (EAS_U32 n)
     65 {
     66     EAS_U32 exp;
     67     EAS_U32 interp;
     68 
     69     /* check for error condition */
     70     if (n == 0)
     71         return MAX_NEGATIVE;
     72 
     73     /* find exponent */
     74     for (exp = 31; exp > 0; exp--)
     75     {
     76         /* shift until we get a 1 bit in bit 31 */
     77         if ((n & (EAS_U32) MAX_NEGATIVE) != 0)
     78             break;
     79         n <<= 1;
     80     }
     81     /*lint -e{701} use shift for performance */
     82     exp <<= LOG_EXPONENT_SHIFT;
     83 
     84     /* get the least significant bits for interpolation */
     85     interp = (n >> MANTISSA_LSB_SHIFT) & MANTISSA_LSB_MASK;
     86 
     87     /* get the most significant bits for mantissa lookup */
     88     n = (n >> MANTISSA_SHIFT) & MANTISSA_MASK;
     89 
     90     /* interpolate mantissa */
     91     interp = ((eas_log2_table[n+1] - eas_log2_table[n]) * interp) >> INTERPOLATION_SHIFT;
     92     exp += eas_log2_table[n] + interp;
     93 
     94     return (EAS_I32) exp;
     95 }
     96 
     97