DOOM fire in Litecanvas

code.js

// based on https://fabiensanglard.net/doom_fire_psx/

const FIRE_WIDTH = 128;
const FIRE_HEIGHT = 128;

litecanvas({
  width: FIRE_WIDTH,
})

const firePalette = [0,0,0,0,0,0,10,10,10,10,10,10,10,10,4,4,4,4,4,4,4,4,5,5,5,5,5,5,3,3,3,3],
  firePixels = []

console.log(firePalette.toString())

function init() {
  setfps(30)
  
  for(let i = 0; i < FIRE_WIDTH*FIRE_HEIGHT; i++) {
      firePixels[i] = 0;
  }
  
  for(let i = 0; i < FIRE_WIDTH; i++) {
      firePixels[(FIRE_HEIGHT-1)*FIRE_WIDTH + i] = firePalette.length-1;
  }
}

function update(dt) {
  for(x = 0 ; x < FIRE_WIDTH; x++) {
      for (y = 1; y < FIRE_HEIGHT ; y++) {
          let from = (y * FIRE_WIDTH + x);
          let rand = randi(0,3) & 3;
          let to = from - FIRE_WIDTH - rand + 1 ;
          firePixels[to] = firePixels[from] - (rand & 1);
      }
  }
}

function draw() {
  cls(0)
  for(let h = 0; h < FIRE_HEIGHT; h++) {
      for(let w = 0; w < FIRE_WIDTH; w++) {
          let p = firePixels[h * FIRE_WIDTH + w];
          if (p >= 0) {
            rectfill(w, h, 1, 1, firePalette[p]);
          }
      }
  }
}

Demo