godot/drivers/opus/silk/log2lin.c

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#ifdef OPUS_ENABLED
#include "opus/opus_config.h"
#endif

#include "opus/silk/SigProc_FIX.h"

/* Approximation of 2^() (very close inverse of silk_lin2log()) */
/* Convert input to a linear scale    */
opus_int32 silk_log2lin( 
    const opus_int32            inLog_Q7            /* I  input on log scale                                            */
)
{
    opus_int32 out, frac_Q7;

    if( inLog_Q7 < 0 ) {
        return 0;
    } else if ( inLog_Q7 >= 3967 ) {
		return silk_int32_MAX;
	}

    out = silk_LSHIFT( 1, silk_RSHIFT( inLog_Q7, 7 ) );
    frac_Q7 = inLog_Q7 & 0x7F;
    if( inLog_Q7 < 2048 ) {
        /* Piece-wise parabolic approximation */
        out = silk_ADD_RSHIFT32( out, silk_MUL( out, silk_SMLAWB( frac_Q7, silk_SMULBB( frac_Q7, 128 - frac_Q7 ), -174 ) ), 7 );
    } else {
        /* Piece-wise parabolic approximation */
        out = silk_MLA( out, silk_RSHIFT( out, 7 ), silk_SMLAWB( frac_Q7, silk_SMULBB( frac_Q7, 128 - frac_Q7 ), -174 ) );
    }
    return out;
}