spiral-rs-fork/src/util.rs

pub fn calc_index(indices: &[usize], lengths: &[usize]) -> usize {
    let mut idx = 0usize;
    let mut prod = 1usize;
    for i in (0..indices.len()).rev() {
        idx += indices[i] * prod;
        prod *= lengths[i];
    }
    idx
}