// Conway's Game of Life on 2x2 cells, one print per generation so
// you can step through it. Live cells cool from white-hot as they
// age. Dead ones fade out as embers.
let seed = 42;
let size = 2;
let cols = ps.doc_width() / size;
let rows = ps.doc_height() / size;
let gens = 40;
let heat = ["#FFF3B0", "#FFD23C", "#FF5B3E", "#E02948", "#A51D45"];
let ember = ["#0A0A1E", "#1E1B44", "#2E2A57", "#5A1236"];
let dark = "#0A0A1E";

fn next(s) { (s * 1103515245 + 12345) % 2147483648 }

// A dead border means every cell has eight neighbors to count.
let stride = cols + 2;
let live = [];
live.pad(stride * (rows + 2), 0);
let age = live;

ps.pixel_fill(0, 0, ps.doc_width() - 1, ps.doc_height() - 1, dark);
for y in 1..=rows {
    for x in 1..=cols {
        seed = next(seed);
        if (seed / 65536) % 100 < 30 { live[y * stride + x] = 1; }
    }
}

for g in 1..=gens {
    let was = live;
    let alive = 0;
    for y in 1..=rows {
        let row = y * stride;
        for x in 1..=cols {
            let i = row + x;
            let n = was[i - stride - 1] + was[i - stride] + was[i - stride + 1]
                  + was[i - 1] + was[i + 1]
                  + was[i + stride - 1] + was[i + stride] + was[i + stride + 1];
            let a = age[i];
            // age > 0: alive that many generations. age < 0: an ember.
            let b = if was[i] == 1 {
                if n == 2 || n == 3 { (a + 1).min(5) } else { -3 }
            } else if n == 3 { 1 } else if a < 0 { a + 1 } else { 0 };
            if b != a {
                age[i] = b;
                live[i] = if b > 0 { 1 } else { 0 };
                let c = if b > 0 { heat[(b - 1).min(4)] } else { ember[-b] };
                ps.pixel_fill((x - 1) * size, (y - 1) * size, x * size - 1, y * size - 1, c);
            }
            alive += live[i];
        }
    }
    print(`gen ${g}: ${alive} alive`);
}