gistfile1.c

    int pti = 0;
    for (int i = 0; i < len; i++) {
        float f = (float)i/len*sr;
        if ( f < curve->pts[0].freq ) {
            // before the first point
            in[i].r = curve->pts[0].power;
        } else if ( curve->pts[curve->ct-1].freq < f ) {
            // after last point
            in[i].r = curve->pts[curve->ct-1].freq;
        } else {
            // in-between points

            while ( !(curve->pts[pti].freq <= f && f < curve->pts[pti+1].freq) ) pti++;
            //           pts[pti  ] is the point immediately to the left  of f
            // likewise, pts[pti+1] is the point immediately to the right of f

            wantpoint *low = &(curve->pts[pti  ]);
            wantpoint *hi  = &(curve->pts[pti+1]);

            float p0 = (f-low->freq)/(hi->freq-low->freq);
            in[i].r = p0*hi->power + (1-p0)*low->power;
        }
    }