foo.c

#define E 2.7182818284590452353

void transform_smooth(struct gl_data* d, void** udaa, void* data) {
    struct gl_sampler_data* s = (struct gl_sampler_data*) data;
    float* b = s->buf;
    size_t sz = s->sz;

    for (int t = 0; t < sz; ++t) {
        float
            db  = log(t), /* buffer index on log10 scale (db) */
            v   = b[t],   /* value at this position */
            avg = 0;      /* adj value averages (weighted) */
        /* Calculate real indexes for sampling at this position, since the
           distance is specified in scalar values */
        int smin = (int) floor(powf(E, max(db - d->smooth_distance, 0)));
        int smax = min((int) ceil(powf(E, db + d->smooth_distance)), sz - 1);
        int count = 0;
        for (int s = smin; s <= smax; ++s) {
            if (b[s]) {
                avg += b[s] /* / abs(powf(10, db + (t - s))) */;
                count++;
            }
        }
        avg /= count;
        b[t] = avg;
    }
}