# Jacobi stencil on a 1024x1024 grid. Three hundred twenty steps, as one hundred sixty pairs, so the result stays in the first grid.
# Interior points are the average of the four neighbours. Prints the scaled cell sum.

hot fn jacobi(cur: exclusive lend List[f64], nxt: exclusive lend List[f64], n: usize):
    overflow: wrap
    last := n - 1
    for i in 1..last:
        for j in 1..last:
            idx := i * n + j
            up := (i - 1) * n + j
            down := (i + 1) * n + j
            left := i * n + (j - 1)
            right := i * n + (j + 1)
            trust nxt[idx] = 0.25 * (trust cur[up] + trust cur[down] + trust cur[left] + trust cur[right])

fn main():
    n: usize = 1024
    cells := n * n
    var a := List[f64]()
    var b := List[f64]()
    for i in 0..n:
        for j in 0..n:
            a.push(f64((i * 13 + j * 7) % 19))
            b.push(0.0)
    for step in 0..160:
        jacobi(a, b, n)
        jacobi(b, a, n)
    var sum: f64 = 0.0
    for i in 0..cells:
        sum += a[i]
    print(i64(sum * 1000.0))