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ref: 8a8b76efeeccaccf402b0b2b4e469980d0a48311
dir: /src_SigProc_FLP/SKP_Silk_LPC_inv_pred_gain_FLP.c/

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/*                                                                      *
 * SKP_Silk_LPC_inverse_pred_gain.c                                   *
 *                                                                      *
 * compute inverse of LPC prediction gain, and                          *
 * test if LPC coefficients are stable (all poles within unit circle)   *
 *                                                                      *
 * Copyright 2008 (c), Skype Limited                                    *
 *                                                                      */
#include "SKP_Silk_SigProc_FIX.h"
#include "SKP_Silk_SigProc_FLP.h"

#define RC_THRESHOLD        0.9999f

/* compute inverse of LPC prediction gain, and                          */
/* test if LPC coefficients are stable (all poles within unit circle)   */
/* this code is based on SKP_Silk_a2k_FLP()                               */
SKP_int SKP_Silk_LPC_inverse_pred_gain_FLP(   /* O:   returns 1 if unstable, otherwise 0      */
    SKP_float       *invGain,               /* O:   inverse prediction gain, energy domain  */
    const SKP_float *A,                     /* I:   prediction coefficients [order]         */
    SKP_int32       order                   /* I:   prediction order                        */
)
{
    SKP_int   k, n;
    double    rc, rc_mult1, rc_mult2;
    SKP_float Atmp[ 2 ][ SKP_Silk_MAX_ORDER_LPC ];
    SKP_float *Aold, *Anew;

    Anew = Atmp[ order & 1 ];
    SKP_memcpy( Anew, A, order * sizeof(SKP_float) );

    *invGain = 1.0f;
    for( k = order - 1; k > 0; k-- ) {
        rc = -Anew[ k ];
        if (rc > RC_THRESHOLD || rc < -RC_THRESHOLD) {
            return 1;
        }
        rc_mult1 = 1.0f - rc * rc;
        rc_mult2 = 1.0f / rc_mult1;
        *invGain *= (SKP_float)rc_mult1;
        /* swap pointers */
        Aold = Anew;
        Anew = Atmp[ k & 1 ];
        for( n = 0; n < k; n++ ) {
            Anew[ n ] = (SKP_float)( ( Aold[ n ] - Aold[ k - n - 1 ] * rc ) * rc_mult2 );
        }
    }
    rc = -Anew[ 0 ];
    if ( rc > RC_THRESHOLD || rc < -RC_THRESHOLD ) {
        return 1;
    }
    rc_mult1 = 1.0f - rc * rc;
    *invGain *= (SKP_float)rc_mult1;
    return 0;
}