invaders.lua

--[[ INVADERS THE GAME ]]
--[[ GALACTIC DEFENSE  ]]

--[[
Portable minidoc, PICO-8 built-in:
	`cls()`                                                    - Clears the screen.
	`spr(id, x, y, [w, h], [flip_x, flip_y])`                  - Draws a sprite at the specified position.
	`btn(id)`                                                  - Checks if a button is pressed.
	`btnp(id)`                                                 - Checks if a button was pressed in the current frame.
	`map(x, y, [sx, sy], [sw, sh])`                            - Draws a map at the specified position.
	`pset(x, y, [c])`                                          - Sets a pixel at the specified position.
	`pget(x, y)`                                               - Retrieves the color of the pixel at the specified position.
	`sfx(id, [note], [pitch], [volume], [channel])`            - Plays a sound effect.
	`music([id], [loop_start], [loop_length], [channel_mask])` - Plays background music.
	`peek(addr)`                                               - Reads a byte from memory.
	`poke(addr, value)`                                        - Writes a byte to memory.
	`time()`                                                   - Retrieves the current time in milliseconds.
	`rnd([seed])`                                              - Generates a random number.
	`sin(x)`                                                   - Calculates the sine of an angle.
	`cos(x)`                                                   - Calculates the cosine of an angle.
	`flr(x)`                                                   - Rounds a number down to the nearest integer.
	`ceil(x)`                                                  - Rounds a number up to the nearest integer.
	`abs(x)`                                                   - Returns the absolute value of a number.
	`min(a, b)`                                                - Returns the minimum of two numbers.
	`max(a, b)`                                                - Returns the maximum of two numbers.
	`mid(x, y, z)`                                             - Returns the middle value of three numbers.
	`_draw()`                                                  - Plugin callback function for drawing operations.
	`_update()`                                                - Plugin callback function for game logic update.
]]

--[[ declare and initialize variables ]]
local invaders_aliens               = { --[[ x, y, alive ]] }
local invaders_alien_speed          = 0.3
local invaders_alien_spawn_timer    = 0
local invaders_alien_spawn_interval = 60

--[[ declare and initialize bullet variables ]]
local invaders_bullet = {
	  x      = 0
	, y      = 0
	, speed  = 2
	, active = false
}

--[[ declare and initialize player variables ]]
local invaders_player = {
	  x     = 60
	, y     = 120
	, speed = 2
}

--
--[[ GAME ENGINE RUNNING ]]
-- update aliens state, change settings and grid at will.
for i = 0, 9 do
	invaders_aliens[i] = {
		  x     = i * 8 + 4
		, y     = 20
		, alive = true
	}
end

--
-- PICO-8 game engine callback `_draw` / graphic context, paint pixels.
--
function _draw()
	-- clear game screen.
	cls()
	-- draw player.
	spr(1, invaders_player.x, invaders_player.y)

	-- draw bullet if active.
	if invaders_bullet.active then
		spr(3, invaders_bullet.x, invaders_bullet.y)
	end

	-- draw aliens.
	for _, alien in pairs(invaders_aliens) do
		if alien.alive then
			spr(2, alien.x, alien.y)
		end
	end
end

--
-- PICO-8 game engine callback `_update` / event loop context.
--
function _update()
	-- move player
	if (btn(0)) then
		invaders_player.x -= invaders_player.speed
	elseif (btn(1)) then
		invaders_player.x += invaders_player.speed
	end

	-- shoot bullet
	if (not invaders_bullet.active and btnp(4)) then
		invaders_bullet.x      = invaders_player.x + 3
		invaders_bullet.y      = invaders_player.y - 4
		invaders_bullet.active = true
	end

	-- move bullet
	if invaders_bullet.active then
		invaders_bullet.y -= invaders_bullet.speed
		if invaders_bullet.y < 0 then
			invaders_bullet.active = false
		end
	end

	-- move aliens
	invaders_alien_spawn_timer    += 1
	if invaders_alien_spawn_timer >= invaders_alien_spawn_interval then
		for _, alien in pairs(invaders_aliens) do
			alien.y                 += 8
			if alien.y > 120 then
			-- game over condition
			invaders_player          = nil
			end
		end
		invaders_alien_spawn_timer  = 0
	end

	-- check collision
	for _, alien in pairs(invaders_aliens) do
		if     alien.alive
			and invaders_bullet.active
			and invaders_bullet.x >= alien.x
			and invaders_bullet.x <= alien.x + 7
			and invaders_bullet.y >= alien.y
			and invaders_bullet.y <= alien.y + 7
		then
			alien.alive            = false
			invaders_bullet.active = alien.alive
		end
	end
end

-- EOG