local ACCELERATION = 100
local DECELERATION = 200
local MAX_SPEED = 400
local WHEEL_BASE = 80

local UP = vmath.vector3(0, 1, 0)

local DIRECTIONS = { "n", "ne", "e", "se", "s", "sw", "w", "nw" }

local function get_direction(rot)
    local forward = vmath.rotate(rot, UP)
    local angle = math.deg(math.atan2(forward.x, forward.y))
    if angle < 0 then angle = angle + 360 end
    local index = math.floor((angle + 22.5) / 45) % 8 + 1
    return DIRECTIONS[index]
end

function init(self)
    msg.post(".", "acquire_input_focus")

    self.left_speed = 0
    self.right_speed = 0
    self.current_anim = nil
    self.left_fwd = false
    self.left_rev = false
    self.right_fwd = false
    self.right_rev = false
end

local function update_speed(speed, drive, dt)
    if drive ~= 0 then
        local target = drive * MAX_SPEED
        local step = ACCELERATION * dt
        if speed < target then
            return math.min(speed + step, target)
        else
            return math.max(speed - step, target)
        end
    else
        local step = DECELERATION * dt
        if speed > 0 then
            return math.max(speed - step, 0)
        else
            return math.min(speed + step, 0)
        end
    end
end

function update(self, dt)
    local left_drive  = (self.left_fwd  and 1 or 0) - (self.left_rev  and 1 or 0)
    local right_drive = (self.right_fwd and 1 or 0) - (self.right_rev and 1 or 0)

    self.left_speed  = update_speed(self.left_speed,  left_drive,  dt)
    self.right_speed = update_speed(self.right_speed, right_drive, dt)

    -- differential drive: speed difference creates rotation, average creates forward motion
    local linear_speed = (self.left_speed + self.right_speed) / 2
    local angular_vel = (self.right_speed - self.left_speed) / WHEEL_BASE

    -- pprint({rot_changes = angular_vel * dt})
    local rot = go.get_rotation()
    rot = rot * vmath.quat_rotation_z(angular_vel * dt)
    go.set_rotation(rot)

    go.set_rotation(vmath.conj(rot), "body")

    local p = go.get_position()
    p = p + vmath.rotate(rot, UP * linear_speed * dt)
    go.set_position(p)

    -- switch animation when facing direction changes
    local dir = get_direction(rot)
    if dir ~= self.current_anim then
      self.current_anim = dir
      msg.post("body#body", "play_animation", { id = hash(dir) })
    end
end

function on_input(self, action_id, action)
    local held = action.pressed or not action.released
    if     action_id == hash("a_key") then self.left_fwd  = held
    elseif action_id == hash("q_key") then self.left_rev  = held
    elseif action_id == hash("d_key") then self.right_fwd = held
    elseif action_id == hash("e_key") then self.right_rev = held
    end
end