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/*                                                                      *
 * SKP_Silk_resample_1_2_coarsest.c                                   *
 *                                                                      *
 * Downsample by a factor 2, coarsest                                   *
 *                                                                      *
 * Copyright 2006 (c), Skype Limited                                    *
 * Date: 060221                                                         *
 *                                                                      */
#include "SKP_Silk_SigProc_FIX.h"


/* Coefficients for coarsest 2-fold resampling */
static SKP_int16 A20cst[ 1 ] = {  3786 };
static SKP_int16 A21cst[ 1 ] = { 17908 };

/* Downsample by a factor 2, coarsest */
void SKP_Silk_resample_1_2_coarsest(
    const SKP_int16     *in,                /* I:   16 kHz signal [2*len]   */
    SKP_int32           *S,                 /* I/O: State vector [2]        */
    SKP_int16           *out,               /* O:   8 kHz signal [len]      */
    SKP_int32           *scratch,           /* I:   Scratch memory [3*len]  */
    const SKP_int32     len                 /* I:   Number of OUTPUT samples*/
)
{
    SKP_int32 k, idx;

    /* De-interleave allpass inputs, and convert Q15 -> Q25 */
    for( k = 0; k < len; k++ ) {
        idx = SKP_LSHIFT( k, 1 );
        scratch[ k ]       = SKP_LSHIFT( (SKP_int32)in[ idx     ], 10 );
        scratch[ k + len ] = SKP_LSHIFT( (SKP_int32)in[ idx + 1 ], 10 );
    }

    idx = SKP_LSHIFT( len, 1 );
    /* Allpass filters */
    SKP_Silk_allpass_int( scratch,       S,     A21cst[ 0 ], scratch + idx, len );
    SKP_Silk_allpass_int( scratch + len, S + 1, A20cst[ 0 ], scratch,       len );

    /* Add two allpass outputs */
    for( k = 0; k < len; k++ ) {
        out[ k ] = (SKP_int16)SKP_SAT16( SKP_RSHIFT_ROUND( scratch[ k ] + scratch[ k + idx ], 11 ) );
    }
}

