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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"

#define QA          16
#define A_LIMIT     SKP_FIX_CONST( 0.99975, QA )

/* Compute inverse of LPC prediction gain, and                          */
/* test if LPC coefficients are stable (all poles within unit circle)   */
static SKP_int LPC_inverse_pred_gain_QA(        /* O:   Returns 1 if unstable, otherwise 0          */
    SKP_int32           *invGain_Q30,           /* O:   Inverse prediction gain, Q30 energy domain  */
    SKP_int32           A_QA[ 2 ][ SKP_Silk_MAX_ORDER_LPC ],         
                                                /* I:   Prediction coefficients                     */
    const SKP_int       order                   /* I:   Prediction order                            */
)
{
    SKP_int   k, n, headrm;
    SKP_int32 rc_Q31, rc_mult1_Q30, rc_mult2_Q16, tmp_QA;
    SKP_int32 *Aold_QA, *Anew_QA;

    Anew_QA = A_QA[ order & 1 ];

    *invGain_Q30 = ( 1 << 30 );
    for( k = order - 1; k > 0; k-- ) {
        /* Check for stability */
        if( ( Anew_QA[ k ] > A_LIMIT ) || ( Anew_QA[ k ] < -A_LIMIT ) ) {
            return 1;
        }

        /* Set RC equal to negated AR coef */
        rc_Q31 = -SKP_LSHIFT( Anew_QA[ k ], 31 - QA );
        
        /* rc_mult1_Q30 range: [ 1 : 2^30-1 ] */
        rc_mult1_Q30 = ( SKP_int32_MAX >> 1 ) - SKP_SMMUL( rc_Q31, rc_Q31 );
        SKP_assert( rc_mult1_Q30 > ( 1 << 15 ) );                   /* reduce A_LIMIT if fails */
        SKP_assert( rc_mult1_Q30 < ( 1 << 30 ) );

        /* rc_mult2_Q16 range: [ 2^16 : SKP_int32_MAX ] */
        rc_mult2_Q16 = SKP_INVERSE32_varQ( rc_mult1_Q30, 46 );      /* 16 = 46 - 30 */

        /* Update inverse gain */
        /* invGain_Q30 range: [ 0 : 2^30 ] */
        *invGain_Q30 = SKP_LSHIFT( SKP_SMMUL( *invGain_Q30, rc_mult1_Q30 ), 2 );
        SKP_assert( *invGain_Q30 >= 0           );
        SKP_assert( *invGain_Q30 <= ( 1 << 30 ) );

        /* Swap pointers */
        Aold_QA = Anew_QA;
        Anew_QA = A_QA[ k & 1 ];
        
        /* Update AR coefficient */
        headrm = SKP_Silk_CLZ32( rc_mult2_Q16 ) - 1;
        rc_mult2_Q16 = SKP_LSHIFT( rc_mult2_Q16, headrm );          /* Q: 16 + headrm */
        for( n = 0; n < k; n++ ) {
            tmp_QA = Aold_QA[ n ] - SKP_LSHIFT( SKP_SMMUL( Aold_QA[ k - n - 1 ], rc_Q31 ), 1 );
            Anew_QA[ n ] = SKP_LSHIFT( SKP_SMMUL( tmp_QA, rc_mult2_Q16 ), 16 - headrm );
        }
    }

    /* Check for stability */
    if( ( Anew_QA[ 0 ] > A_LIMIT ) || ( Anew_QA[ 0 ] < -A_LIMIT ) ) {
        return 1;
    }

    /* Set RC equal to negated AR coef */
    rc_Q31 = -SKP_LSHIFT( Anew_QA[ 0 ], 31 - QA );

    /* Range: [ 1 : 2^30 ] */
    rc_mult1_Q30 = ( SKP_int32_MAX >> 1 ) - SKP_SMMUL( rc_Q31, rc_Q31 );

    /* Update inverse gain */
    /* Range: [ 0 : 2^30 ] */
    *invGain_Q30 = SKP_LSHIFT( SKP_SMMUL( *invGain_Q30, rc_mult1_Q30 ), 2 );
    SKP_assert( *invGain_Q30 >= 0     );
    SKP_assert( *invGain_Q30 <= 1<<30 );

    return 0;
}
/* For input in Q12 domain */
SKP_int SKP_Silk_LPC_inverse_pred_gain(       /* O:   Returns 1 if unstable, otherwise 0          */
    SKP_int32           *invGain_Q30,           /* O:   Inverse prediction gain, Q30 energy domain  */
    const SKP_int16     *A_Q12,                 /* I:   Prediction coefficients, Q12 [order]        */
    const SKP_int       order                   /* I:   Prediction order                            */
)
{
    SKP_int   k;
    SKP_int32 Atmp_QA[ 2 ][ SKP_Silk_MAX_ORDER_LPC ];
    SKP_int32 *Anew_QA;

    Anew_QA = Atmp_QA[ order & 1 ];

    /* Increase Q domain of the AR coefficients */
    for( k = 0; k < order; k++ ) {
        Anew_QA[ k ] = SKP_LSHIFT( (SKP_int32)A_Q12[ k ], QA - 12 );
    }

    return LPC_inverse_pred_gain_QA( invGain_Q30, Atmp_QA, order );
}

/* For input in Q24 domain */
SKP_int SKP_Silk_LPC_inverse_pred_gain_Q24(   /* O:   Returns 1 if unstable, otherwise 0          */
    SKP_int32           *invGain_Q30,           /* O:   Inverse prediction gain, Q30 energy domain  */
    const SKP_int32     *A_Q24,                 /* I:   Prediction coefficients, Q24 [order]        */
    const SKP_int       order                   /* I:   Prediction order                            */
)
{
    SKP_int   k;
    SKP_int32 Atmp_QA[ 2 ][ SKP_Silk_MAX_ORDER_LPC ];
    SKP_int32 *Anew_QA;

    Anew_QA = Atmp_QA[ order & 1 ];

    /* Increase Q domain of the AR coefficients */
    for( k = 0; k < order; k++ ) {
        Anew_QA[ k ] = SKP_RSHIFT_ROUND( A_Q24[ k ], 24 - QA );
    }

    return LPC_inverse_pred_gain_QA( invGain_Q30, Atmp_QA, order );
}

