local inspect = require('inspect')

local phys = {
    tau = 2 * math.pi
}

-- returns true if the two actors intersect
function phys.intersects(actor1, actor2)
    return (actor1.x - actor2.x) ^ 2 + (actor1.y - actor2.y) ^ 2 <= (actor1.radius + actor2.radius) ^ 2
end

-- updates the actor's location by applying the passage of time
-- TODO spin
function phys.update(actor, dt)
    assert(actor.speed >= 0)
    local dv = actor.speed * dt
    actor.x = actor.x + math.cos(actor.angle) * dv
    actor.y = actor.y + math.sin(actor.angle) * dv
end

function phys.rotate(actor, angle)
    actor.facing = (actor.facing + angle) % phys.tau
end

function phys.apply(actor, force)
    local ax = math.cos(actor.angle) * actor.speed
    local ay = math.sin(actor.angle) * actor.speed
    local fx = math.cos(force.angle) * force.speed
    local fy = math.sin(force.angle) * force.speed
    local x = ax + fx
    local y = ay + fy

    local speed = math.sqrt(x ^ 2 + y ^ 2)
    local angle = math.atan2(y, x) % phys.tau
    actor.speed = speed
    actor.angle = angle
end

return phys