function doFire() {
    for(x = 0 ; x < FIRE_WIDTH; x++) {
        for (y = 1; y < FIRE_HEIGHT ; y++) {
            spreadFire(y * FIRE_WIDTH + x);
        }
    }
}

function spreadFire(from) {
    var rand = Math.round(Math.random() * 3.0) & 3;
    var to = from - FIRE_WITH - rand + 1 ;
    firePixels[to] = firePixels[from] - (rand & 1);
}

https://fabiensanglard.net/doom_fire_psx/index.html
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