Responding to Buttons using Bitwise Addition in PICO-8

It is possible to respond to each button press in PICO-8 by using conditional statements like this:

if btn(0) then vx=-1 end
if btn(1) then vx=1 end
if btn(2) then vy=-1 end
if btn(3) then vy=1 end

But a more elegant solution is to use the bitwise addition function band().  Calling btn() with no arguments results in a bitfield of outputs.  In other words, pressing button 1 (right) gives 000010.

Continuing with the example of pressing the right button, the result is 000010 and so looking at the code in get_x_axis():

band(btn(),2)/2-band(btn(),1)

band(000010,2)/2 returns 1
band(000010,1) returns 0

so 1-0 returns 1, i.e. move to the right (positive).

By checking the bits and subtracting the right from left (or down from up) we end up with a value of -1 or 1 for the movement.

I’ve wrapped the bitwise calculations here into two functions for neatness (get_x_axis and get_y_axis), but this is slightly profligate with tokens and by directly referencing the band() function you can reduce your token count by two from the standard conditional method shown above.

vx=0
vy=0

function get_x_axis()
 return band(btn(),2)/2-band(btn(),1)
end

function get_y_axis()
  return band(btn(),8)/8-band(btn(),4)/4
end

function _update()
  vx=get_x_axis()
  vy=get_y_axis()
end

function _draw()
  cls()
 print(vx,5,5)
 print(vy,5,11)
end

Feel free to use, and if you spot any further optimizations, let me know!