fn subc(a: usize, b: usize) -> usize {
    if a >= b { a - b } else { 0 }
}

fn convolve(a: &[f32], b: &[f32]) -> Vec<f32> {
    use std::cmp::min;
    let N = a.len() - 1;
    (0..2*N+1).map(|k|
        //(subc(k, N) .. min(k+1, N+1)).map(|i|
         (k.saturating_sub() .. min(k+1, N+1)).map(|i|
            a[i]*b[k-i]
        ).sum()
    ).collect()
}

fn main() {
    let v1 = vec![1.0, 2.0, 3.0];
    let v2 = vec![0.0, 1.0, 0.5];
    let v3 = convolve(&v1, &v2);
    println!("{:?} * {:?} = {:?}", v1, v2, v3);
}