silk/sigm_Q15.c

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#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

/* Approximate sigmoid function */

#include "SigProc_FIX.h"

/* fprintf(1, '%d, ', round(1024 * ([1 ./ (1 + exp(-(1:5))), 1] - 1 ./ (1 + exp(-(0:5)))))); */
static const opus_int32 sigm_LUT_slope_Q10[ 6 ] = {
    237, 153, 73, 30, 12, 7
};
/* fprintf(1, '%d, ', round(32767 * 1 ./ (1 + exp(-(0:5))))); */
static const opus_int32 sigm_LUT_pos_Q15[ 6 ] = {
    16384, 23955, 28861, 31213, 32178, 32548
};
/* fprintf(1, '%d, ', round(32767 * 1 ./ (1 + exp((0:5))))); */
static const opus_int32 sigm_LUT_neg_Q15[ 6 ] = {
    16384, 8812, 3906, 1554, 589, 219
};

opus_int silk_sigm_Q15(
    opus_int                    in_Q5               /* I                                                                */
)
{
    opus_int ind;

    if( in_Q5 < 0 ) {
        /* Negative input */
        in_Q5 = -in_Q5;
        if( in_Q5 >= 6 * 32 ) {
            return 0;        /* Clip */
        } else {
            /* Linear interpolation of look up table */
            ind = silk_RSHIFT( in_Q5, 5 );
            return( sigm_LUT_neg_Q15[ ind ] - silk_SMULBB( sigm_LUT_slope_Q10[ ind ], in_Q5 & 0x1F ) );
        }
    } else {
        /* Positive input */
        if( in_Q5 >= 6 * 32 ) {
            return 32767;        /* clip */
        } else {
            /* Linear interpolation of look up table */
            ind = silk_RSHIFT( in_Q5, 5 );
            return( sigm_LUT_pos_Q15[ ind ] + silk_SMULBB( sigm_LUT_slope_Q10[ ind ], in_Q5 & 0x1F ) );
        }
    }
}