Angles

function love.load()
    px = 400
    py = 300

    angle, target = false, 0
    speed = .5
    a = 0

    love.graphics.setPointSize(4)
end

function love.update(dt)
    local x, y = love.mouse.getPosition()

    --If left mouse is down, stop tracking angle
    local trackangle = not love.mouse.isDown(1)

    target = math.atan2(y-py, x-px)
    target = -target
    if target < 0 then
        target = math.pi*2 +target
    end
    if target <= 0 then target = 0 end --Negative 0 is *still* a problem, what the heck

    if angle then
        if trackangle then
            a = 0
            if target > angle then
                if target-angle <= math.pi then
                    angle = angle + round(target-angle)*dt*speed
                else
                    angle = angle - round(math.pi*2-(target-angle))*dt*speed
                end
            else
                if angle-target <= math.pi then
                    angle = angle - round(angle-target)*dt*speed
                else
                    angle = angle + round(math.pi*2-(angle-target))*dt*speed
                end
            end

            if angle < 0 then angle = angle + math.pi*2 end
            if angle >= math.pi*2 then angle = angle - math.pi*2 end
        end
    else
        angle = target
    end
end

function love.draw()
    local x, y = love.mouse.getPosition()
    love.graphics.setColor(0, 0, 255, 255)
    love.graphics.line(px, py, x, y)
    love.graphics.setColor(0, 255, 0, 255)
    love.graphics.points(px, py, x, y)

    love.graphics.setColor(255, 0, 0, 255)
    love.graphics.line(px, py, px+math.cos(angle)*20, py-math.sin(angle)*20)

    love.graphics.setColor(255, 255, 255, 255)
    love.graphics.print("Target = " .. round(math.deg(target)), 5, 5)
    love.graphics.print("Angle = " .. round(math.deg(angle)), 5, 25)
    love.graphics.print("Speed = " .. speed, 5, 45)
end

function love.wheelmoved(x, y)
    if y > 0 then
        if speed < 1 then
            speed = speed+.25
        else
            speed = math.min(10, speed+1)
        end
    else
        if speed > 1 then
            speed = speed-1
        else
            speed = math.max(.25, speed-.25)
        end
    end
end

function distance(x1, y1, x2, y2)
    return math.sqrt((x1-x2)^2 + (y1-y2)^2)
end

function round(n, d)
    local d = d or 2
    return math.floor(n*10^d)/10^d
end