TIC-80 shmup exoloding/shrapnel bullets

explodingBulletAreas.lua

-- made for my Devtober2022 entry
-- you can read the corresponding devlog on: 
-- https://ko-fi.com/post/Eng-Devtober2022-3--MORE-UPGRADES-R6R3FL67S
-- https://itch.io/jam/devtober-2022/topic/2406837/achies-devlog-shmup-game

-- cricl-rect collision from the interwebs 
function collidec(_circ, _obj)
	local circDX = math.abs(_circ.x - _obj.x+_obj.cw/2)
	local circDY = math.abs(_circ.y - _obj.y+_obj.cw/2)
	
	if (circDX > (_obj.cw/2 + _circ.r)) then return false end
	if (circDY > (_obj.ch/2 + _circ.r)) then return false end
	
	if (circDX <= (_obj.cw/2)) then return true end
	if (circDY <= (_obj.ch/2)) then return true end
	
	
	local cornerDistSQ = math.pow((circDX - _obj.cw/2),2) + math.pow((circDY - _obj.ch/2),2)
	
	return (cornerDistSQ <= math.pow(_circ.r,2))
end

-- a stupid fast-path check for collisions that otherwise would require heavier calculations
function close_enough(obj1,obj2,dist)
	if (math.abs(obj1.x-obj2.x) < dist) and (math.abs(obj1.y-obj2.y) < dist) then
		return true
	end
		return false
end

-- explosion area creation
function add_explosion(_x,_y,_r)
	local expl = {
		x = _x,
		y = _y,
		r = _r,
		life = 30 -- this is in ticks, so 1 second basically
	}
	table.insert(explosionAreas, expl)
end

-- simple update function for the explosion areas
for _,expl in ipairs(explosionAreas) do
  if expl.life > 0 then
    local spd = 3
    -- reduce life, so they fade away after a certain ammount of time
    expl.life = expl.life -1
    -- create cute particles, as everything is better with particles
    add_particle(expl.x+4,expl.y+4,math.random(0,spd)*2-spd, math.random(0,spd)*2-spd,math.random(1,3),"expl",math.random(0,1),5)
  else
    table.remove(explosionAreas,_)
  end
end

-- after all of this, we expand the enemy - bullet collision code
-- inside the enemy update
if collide(bul, enemy) then
  local damage = 1
  if bul.tpe == 1 then damage = 4 end
  damage_enemy(enemy, damage)
  -- explosive rounds
  table.remove(bullets,_)
  if upgrades[10] > 0 then
    -- we have a set base radius that will increase with each explosion round
    -- you picked during the run
    local r = 20*(1+upgrades[10]/10)
    add_explosion(enemy.x+4,enemy.y+4,r)
  end
end

-- and finish it off with the explosive area check
for id,expl in ipairs(explosionAreas) do
  -- since we are doing a few calculation i added
  -- a stupid little fast path. This ensures if 
  -- the enemy is really far from the explosion
  -- we don't even consider the possibility of
  -- the damaging effect
  if close_enough(enemy,expl,20) then
    if collidec(expl,enemy) then
      -- created a new helper function as well for
      -- damage effects
      damage_enemy(enemy,1)
    end
  end
end

-- with this we can call the damage effect from multiple checks
-- with this one helper, and check everything in one place
function damage_enemy(_enemy,_damage)
  -- the inFrames is just added to the enemies 
  -- and if it grater then zero, we decrement it
  -- each frame. This way you can't melt enemies with
  -- aoe affects damaging them on each frame 
  -- (basicall every 30th of a second) which is stupid fast
	if (_enemy.invFrames == 0) then
		_enemy.hp = _enemy.hp - (_damage*(1+upgrades[7]/10))
		_enemy.damaged = 90
		_enemy.invFrames = 10
	end
end