--- Creates a rotation with the specified forward and upwards directions.
-- @param forward vector3 The forward direction.
-- @param upwards vector3|nil The upwards direction.
-- @return quat The rotation.
local function quat_look_rotation(forward, upwards)
    -- If no upwards direction is specified, use the default (0, 1, 0)
    upwards = upwards or vmath.vector3(0, 1, 0)

    -- No zero vectors
    if vmath.length_sqr(forward) < 0.0000000001 or vmath.length_sqr(upwards) < 0.0000000001 then
        return vmath.quat()
    end

    -- Create a rotation matrix from the forward and upwards vectors
    local matrix = vmath.matrix4_look_at(vmath.vector3(0), forward, upwards)

    -- Convert the matrix to a quaternion and return it
    return vmath.conj(vmath.quat_matrix4(matrix))
end

local function next_target(self)
    self.target = (self.target or 0) + 1
    if self.target > #self.targets then
        self.target = 1
    end

    local target_id = self.targets[self.target]

    local from = go.get_position("/sword")
    local to = go.get_position(target_id)

    self.target_rotation = quat_look_rotation(to - from)
end

function init(self)
    -- Acquire input focus to receive input events
    msg.post(".", "acquire_input_focus")

    -- List of target objects
    self.targets = {
        "/target1",
        "/target2",
        "/target3"
    }

    -- Set the initial target
    next_target(self)
end

function update(self, dt)
    -- If a target rotation is set, smoothly rotate the sword to face the target
    if self.target_rotation then
        -- Important: we must use vmath.slerp to animate quaternions
        local q = vmath.slerp(0.15, go.get_rotation("/sword"), self.target_rotation)
        go.set_rotation(q, "/sword")
    end
end

function on_input(self, action_id, action)
    -- If the action is pressed (any key or mouse button), set the next target
    if action.pressed then
        next_target(self)
    end
end