ftp.nice.ch/pub/next/unix/audio/cmusic.bs.N.tar.gz#/src/pv/lpamp.c

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#include "pv.h"
/*
 * evaluate magnitude of transfer function at frequency omega
 */
float lpamp( omega, a0, coef, M ) float omega, a0, *coef ; int M ; {
 register float wpr, wpi, wr, wi, re, im, temp ;
 register int i ;
    if ( a0 == 0. )
	return( 0. ) ;
    wpr = cos( omega ) ;
    wpi = sin( omega ) ;
    re = wr = 1. ;
    im = wi = 0. ;
    for ( coef++, i = 1 ; i <= M ; i++, coef++ ) {
	wr = (temp = wr)*wpr - wi*wpi ;
	wi = wi*wpr + temp*wpi ;
	re += *coef*wr ;
	im += *coef*wi ;
    }
    if ( re == 0. && im == 0. )
	return( 0. ) ;
    else
	return( sqrt( a0/(re*re + im*im) ) ) ;
}

These are the contents of the former NiCE NeXT User Group NeXTSTEP/OpenStep software archive, currently hosted by Netfuture.ch.