relative neighbour pixels in generic multidimensional spaces using const generics

neighbours.rs

fn neighbour_at<const D: usize>(i: u32) -> [i32; D] {
    let mut el = [0_i32; D];
    let mut n = i;

    for v in el.iter_mut().rev() {
        *v = (n as i32 % 3) - 1; // -1 is needed because we offset the digits so that we are in range -1..1
        n /= 3;
    }

    el
}

#[cfg(test)]
mod ex17_tests {
    use super::*;

    #[test]
    fn test_neighbour_at_d2() {
        let neighbours: Vec<[i32; 2]> = (0..9).map(neighbour_at::<2>).collect();
        //  D=2
        //  0 -> 0,0 -> -1,-1
        //  1 -> 0,1 -> -1,0
        //  2 -> 0,2 -> -1,1
        //  3 -> 1,0 -> 0,-1
        //  4 -> 1,1 -> 0,0
        //  5 -> 1,2 -> 0,1
        //  6 -> 2,0 -> 1,-1
        //  7 -> 2,1 -> 1,0
        //  8 -> 2,2 -> 1,1
        let expected = vec![
            [-1, -1],
            [-1, 0],
            [-1, 1],
            [0, -1],
            [0, 0],
            [0, 1],
            [1, -1],
            [1, 0],
            [1, 1],
        ];
        assert_eq!(neighbours, expected);
    }

    #[test]
    fn test_neighbour_at_d3() {
        let neighbours: Vec<[i32; 3]> = (0..27).map(neighbour_at::<3>).collect();
        //  D=3
        //  0 -> 0,0,0 -> -1,-1,-1
        //  1 -> 0,0,1 -> -1,-1,0
        //  2 -> 0,0,2 -> -1,-1,1
        //  3 -> 0,1,0 -> -1,0,-1
        //  4 -> 0,1,1 -> -1,0,0
        //  5 -> 0,1,2 -> -1,0,1
        //  6 -> 0,2,0 -> -1,1,-1
        //  7 -> 0,2,1 -> -1,1,0
        //  8 -> 0,2,2 -> -1,1,1
        //  9 -> 1,0,0 -> 0,-1,-1
        // 10 -> 1,0,1 -> 0,-1,0
        // 11 -> 1,0,2 -> 0,-1,1
        // 12 -> 1,1,0 -> 0,0,-1
        // 13 -> 1,1,1 -> 0,0,0
        // 14 -> 1,1,2 -> 0,0,1
        // 15 -> 1,2,0 -> 0,1,-1
        // 16 -> 1,2,1 -> 0,1,0
        // 17 -> 1,2,2 -> 0,1,1
        // 18 -> 2,0,0 -> 1,-1,-1
        // 19 -> 2,0,1 -> 1,-1,0
        // 20 -> 2,0,2 -> 1,-1,1
        // 21 -> 2,1,0 -> 1,0,-1
        // 22 -> 2,1,1 -> 1,0,0
        // 23 -> 2,1,2 -> 1,0,1
        // 24 -> 2,2,0 -> 1,1,-1
        // 25 -> 2,2,1 -> 1,1,0
        // 26 -> 2,2,2 -> 1,1,1
        let expected = vec![
            [-1, -1, -1],
            [-1, -1, 0],
            [-1, -1, 1],
            [-1, 0, -1],
            [-1, 0, 0],
            [-1, 0, 1],
            [-1, 1, -1],
            [-1, 1, 0],
            [-1, 1, 1],
            [0, -1, -1],
            [0, -1, 0],
            [0, -1, 1],
            [0, 0, -1],
            [0, 0, 0],
            [0, 0, 1],
            [0, 1, -1],
            [0, 1, 0],
            [0, 1, 1],
            [1, -1, -1],
            [1, -1, 0],
            [1, -1, 1],
            [1, 0, -1],
            [1, 0, 0],
            [1, 0, 1],
            [1, 1, -1],
            [1, 1, 0],
            [1, 1, 1],
        ];
        assert_eq!(neighbours, expected);
    }
}