# Squeezing Space Invaders onto the BBC micro:bit's 25 pixels

## MicroPython retro game in just 25 pixels

How much game can you fit into 25 pixels? Quite a bit it turns out.

This is a mini clone of arcade classic Space Invaders for the BBC micro:bit microcomputer. Using the accelerometer and two buttons for input, to can beat off wave after wave of aliens that advance towards you.

The screen limits the detail we can manage: each sprite takes up a single pixel on the screen. The aliens don't drop bombs, as you'd have only 1-2 pixels to react. The buildings/defences at the bottom are missing, as they'd serve no purpose without the bombs. But it's still a lot of fun!

The aliens advance down the screen when they reach the edge. Some waves cover the entire width of the screen and will move down straight away -- shoot the edges to slow them down.

The controls are as follows --

• Tilt left/right to control your fighter, tilt further to move faster.
• Button A - shoot gun
• Button B - fire thermonuclear bomb

Bomb? Yes! In this version you get a big bomb to save yourself when things get tricky. The blast wipes out 50% of the aliens currently on screen, but leaves you blinded for a couple of moves. Use wisely!

The game is written in MicroPython. If you have a BBC micro:bit just copy and paste the code into the Python web editor and then flash it to your device.

python
``````from microbit import *
import random

MIN_COORD, MAX_COORD = 0, 4  # Range of valid coordinates for the display.
MAX_MISSILES = 5             # Number of missiles player can have on screen at once.
DIFFICULTY_INCREASE = 0.25   # Increase in difficulty between waves.

ALIEN_START_POSITIONS = [
# Each row is a unique start pattern, defined as tuples of x,y coordinates.
[(1, 0), (2, 0), (3, 0), (1, 1), (2, 1), (3, 1)],
[(0, 0), (1, 0), (2, 0), (3, 0), (4, 0), (2, 1)],
[(1, 0), (2, 0), (3, 0), (0, 1), (2, 1), (4, 1)],
[(1, 0), (2, 0), (3, 0), (1, 1), (3, 1), (2, 2)],
]

def wait_for_button():
# Wait for either button to be pressed.
while not (button_a.was_pressed() or button_b.was_pressed()):
sleep(1)

def move(sprite, x, y):
"""
Move the given sprite by the given x & y amounts.
"""
return sprite[0] + x, sprite[1] + y

def in_bounds(pos):
"""
Return True if the position is within the valid screen coordinates.
"""
if pos[0] < MIN_COORD or pos[0] > MAX_COORD:
return False
if pos[1] < MIN_COORD or pos[1] > MAX_COORD:
return False
return True

class Game:
"""
Game class holds the current game state.
"""

def reset(self):
# Initial values
self.x = 2  # Player x coordinate start (middle).
self.xf = 2.0  # x coordinate float, allows us to use tilt for move speed.

self.missiles = []  # Active missles on screen.
self.aliens = []    # Active aliens on screen.
self.alien_velocity_x = 1  # Horizontal speed of aliens.

self.bombs = 3        # Number of bombs the player has.
self.active_bomb = 0  # Countdown timer for the current active bomb.

self.score = 0        # Player score.

self.tick = 0         # Game loop tick.
self.level = 0        # Current game level.
self.difficulty = 20  # Is in reverse, decrement to increase.

def handle_input(self):
self.tick += 1
acc_x = accelerometer.get_x()

# Use the accelerometer / 512 so the player can move x at speed by tilting more.
if acc_x < 0:
self.xf += acc_x / 512
if acc_x > 0:
self.xf += acc_x / 512

# Constrain to the screen dimensions.
if self.xf > MAX_COORD:
self.xf = MAX_COORD

if self.xf < MIN_COORD:
self.xf = MIN_COORD

self.x = int(self.xf)

if button_a.was_pressed():
# Add missile, at players current x position.
self.missiles.append((self.x, 4))

if button_b.was_pressed() and self.bombs:
# Fire bomb. Flash + remove half the aliens.
# randint(0,1) will be 50% 1, 50% 0 ..if 0 (False) alien will be skipped.
self.aliens = [alien for alien in self.aliens if random.randint(0,1)]
self.active_bomb = 3 # Reduces 1 per tick. Screen at 3 * bright.
self.bombs -= 1

# We need to copy, or we'll me modifying the original lists.
alien_position = self.level % len(ALIEN_START_POSITIONS)
self.aliens = ALIEN_START_POSITIONS[alien_position].copy()
self.tick = 0

"""
If aliens have reached the screen edge, advance them all downwards.
"""
for alien in self.aliens:
if (
(self.alien_velocity_x == -1 and alien[0] == MIN_COORD) or
(self.alien_velocity_x == +1 and alien[0] == MAX_COORD)
):
# If any aliens are at the far edge, increment y, and reverse.
self.alien_velocity_x = -self.alien_velocity_x
self.aliens = [move(alien, 0, 1) for alien in self.aliens]
# This can happen if detached alien slips past bottom.
self.aliens = [alien for alien in self.aliens if in_bounds(alien)]
return True  # No other move this time.

def aliens_can_move(self):
if self.tick > self.difficulty:
self.tick = 0
return True

def move_aliens(self):
# Move aliens horizontally.
self.aliens = [move(alien, self.alien_velocity_x, 0) for alien in self.aliens]

def move_missiles(self):
# Advance positions of missiles (upwards)
self.missiles = [move(missile, 0, -1) for missile in self.missiles]
self.missiles = [missile for missile in self.missiles if in_bounds(missile)]

def check_collisions(self):
for missile in self.missiles[:]:  # Iterate a copy.
if missile in self.aliens:
# Since we store by coordinates, we can remove using the missile coords.
self.aliens.remove(missile)
self.missiles.remove(missile)
self.score += 1

if not self.aliens:
# Wave complete? Increase difficulty (decrement) and add new aliens.
self.difficulty -= DIFFICULTY_INCREASE
self.level += 1
self.bombs += 1

def draw(self):
display.clear()

if self.active_bomb:
# Bomb is drawn as an overlay of gradually decaying light.
for dx in range(MAX_COORD + 1):
for dy in range(MAX_COORD + 1):
display.set_pixel(dx, dy, self.active_bomb * 3)

# Decrement so next draw is fainter.
self.active_bomb -= 1

# Draw all the aliens.
for pos in self.aliens:
display.set_pixel(pos[0], pos[1], 9)

# Draw all the current player missles.
for pos in self.missiles:
display.set_pixel(pos[0], pos[1], 5)

# Draw the players spaceship.
display.set_pixel(self.x, 4, 9)

def game_over(self):
return (self.x, 4) in self.aliens

game = Game() # Create our game object.

while True:

display.show(Image.TARGET)
wait_for_button()

game.reset() # Reset the game state.

# Main loop
while not game.game_over():
game.handle_input()
if game.aliens_can_move():
game.move_aliens()
game.move_missiles()
game.draw()
game.check_collisions()

sleep(100)

display.show(Image.ANGRY)
sleep(1000)
display.scroll(game.score)
``````

You can adjust the difficulty by adjusting the `difficulty` setting, or `speed`. You could also modify the bomb to only destroy 33% of aliens, or less, to make it less of a certain life-saver.

The video below shows a short playthrough and end of game score.

Have fun!

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