A quick bullet-velocity demo for my good friend Jade

shoot.lua

-- title:  shoot test
-- author: vriska done it 
-- desc:   hope this helps
-- script: lua
-- input: mouse 

t=0
entities = {}

-- This function returns a 2d vector
-- 8ased on X, Y and the angle. 
-- Using it with velocities results 
-- in it being offset by 45 degrees 
-- and I have no idea why 8ut that's 
-- just how it is, yknow.  
-- 
-- So if you intend to use this to 
-- update a bullet position, subtract 
-- math.rad(45) from whatever angle
-- you wanted to use.
function rotate(x, y, angle)	
	local cs = math.cos(angle)
	local sn = math.sin(angle)

	return {
		x = x * cs - y * sn,
		y = x * sn + y * cs
	}
end

-- This gets the angle from point A to 
-- point B. 
-- I didn't do super gr8 in like.
-- School. So if you asked me what it 
-- actually did I couldn't tell you 8ut
-- I can tell you it works which is good 
-- enough for me.
function angle(ax, ay, bx, by)
	return math.atan2(by-ay, bx-ax)  
end

-- Creates a new entity from scratch. 
-- Called every time a shoot is done.
function entity(x, y, angle) 
	return {
		x = x, 
		y = y,
		angle = angle,
		speed = 150,
		
		update = function(self, dt)
			-- Rotate the speed we're 
			-- moving this bullet at 
			-- by the angle from start
			-- to where the mouse was at
			-- when we shot. 
	
			local offset = rotate(
				self.speed * dt,
				self.speed * dt, 
				-- Remember velocities are always
				-- off by like. 45 degrees. 
				self.angle - math.rad(45)
			)
			
			self.x = self.x + offset.x
			self.y = self.y + offset.y
			
		end
	}
end

--[[ Calculate out the delta time. ]]
local t = 0
local lt = 0 
local dt = 0

-- This just sets where the bullets come
-- from. Move it if you want.
local start = {
	x = 120,
	y = 60
}

function TIC()
	-- Get delta time and reduce it to microseconds
	lt = t 
	t = time()
	dt = (t - lt) / 1000

	-- Get mouse details.
	mouseX, mouseY, down = mouse()
	
	cls()
	
	print(mouseX .. ', ' .. mouseY)
	
	spr(1, start.x + 4, start.y + 4)
	
	if down then 
	 -- Adds a new bullet.
		table.insert(entities, entity(start.x, start.y, angle(start.x, start.y, mouseX, mouseY)))
	end
	
	for index, entity in pairs(entities) do
		entity:update(dt)
		spr(2, entity.x + 4, entity.y + 4, 0)
	end

end

-- <TILES>
-- 001:0000000000ffff000f0000f00f0ff0f00f0ff0f00f0000f000ffff000f0000f0
-- 002:0000000000000000000ff00000ffff0000ffff00000ff0000000000000000000
-- </TILES>

-- <WAVES>
-- 000:00000000ffffffff00000000ffffffff
-- 001:0123456789abcdeffedcba9876543210
-- 002:0123456789abcdef0123456789abcdef
-- </WAVES>

-- <SFX>
-- 000:000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000304000000000
-- </SFX>

-- <PALETTE>
-- 000:140c1c44243430346d4e4a4e854c30346524d04648757161597dced27d2c8595a16daa2cd2aa996dc2cadad45edeeed6
-- </PALETTE>