Implement first enemy, bullet damage and collision.

src/entity.lua

@@ -0,0 +1,28 @@
+-- A superclass for all player and AI-controlled units
+import "CoreLibs/object"
+import "CoreLibs/graphics"
+import "CoreLibs/sprites"
+
+local gfx <const> = playdate.graphics
+
+class("Entity").extends(gfx.sprite)
+
+function Entity:init(img, health)
+   Entity.super.init(self, img)
+   self.health = health or 10
+
+   self:setCollideRect(0, 0, self:getSize())
+
+   -- most entities will be enemies, so we configure this mask by default
+   -- We don't set a collider mask because collision is always handled by
+   -- other objects (todo: consider player staying perfectly still)
+   self:setGroupMask(0x8)
+end
+
+function Entity:damage(amount)
+   self.health = math.max(self.health - amount, 0)
+
+   if self.health == 0 then
+      self:remove()
+   end
+end