fire effect

// rand is a 16-bit integer
u8 rand_bits = rand & 0b11;

// wptr is write pointer for the current row
// apply a small random offset to make the fire move left or right
u8* dst = wptr-rand_bits+1;


// same here, wuptr is a write pointer for the row above
u8* udst = wuptr-rand_bits+1;


// read a pair of pixels from framebuffer
// rptr is a pointer to a couple rows down (to propagate the flames upward)
u8 full_pix = *rptr;

// if both pixels are completely black, just
// short circuit the other work and write black pixels to the destination
if(full_pix == 0) {
  // pixels are doubled vertically
  *dst = 0x00;
  *udst = 0x00;
 } else {
   // extract the right 4-bit pixel
  u8 pix_col = full_pix & 0b1111;

  // if black, write black pixels to the destination
  if(pix_col == 0) { *dst = 0x00; *udst = 0x00; }


  // otherwise use a couple more random bits to select the color lookup table

  // fire_lut is a set of 4 16-color palettes that darken the index color you pass in
  // which modify the left and right pixels in a byte differently
  // increasing the apparent horizontal resolution

  // e.g.
  // fire_lut[0*16 + 0xF] => 0xEE
  // fire_lut[1*16 + 0xF] => 0xFE
  // fire_lut[2*16 + 0xF] => 0xEF
  // fire_lut[3*16 + 0xF] => 0xFF ( no darkening 1/4 of the time)

  // to select the palette,
  // grab random bits that are already in the right position
  // (i.e. shifted left by 4, or multiplied by 16,
  // because each palette is 16 entries)

  // we're using the same random bits multiple times over different iterations
  // but it's ok as long as there are no obvious repeating patterns

  u8 dark_col = fire_lut[(rand&0b110000)+pix_col];

  *dst = dark_col;
  *udst = dark_col;
 }
// move all framebuffer pointers left by one pixel pair
rptr--;
wptr--;
wuptr--;

// this used to be a shift by 2, to reuse less random bits
// but shifting by 1 still looks as good and means we can call
// random() less often
rand >>= 1;