require "spec"
require "../../src/engine/physics"

describe Physics do
  it "test basic tick usage" do
    time = Time.utc
    one_second = Time::Span.new(seconds: 1)
    t1 = Physics::Tick.new(n: 0, time: time, last: nil)
    t2 = Physics::Tick.new(n: 1, time: time + one_second, last: t1)

    t2.timelaps.should eq(one_second)
  end

  it "test basic timer" do
    timer = Physics::Tick::Timer.new
    start_time = timer.tick.time
    timer.tick.n.should eq(0)
    12.times { timer.timeskip!(seconds: 1) }
    timer.tick.n.should eq(12)
    timer.tick.time.should eq(start_time + Time::Span.new(seconds: 12))
  end

  it "accelerate an object" do
    a_ball_movement = Vector[0.0, 0.0, 0.0]
    a_engine_accelerate = Vector[20.0, 0.0, 0.0]
    timer = Physics::Tick::Timer.new

    timer.timeskip!(seconds: 1)
    Physics.accelerate_by_one_tick!(timer.tick, a_ball_movement, a_engine_accelerate)
    a_ball_movement[0].should eq(20.0)
    a_ball_movement[1].should eq(0.0)

    timer.timeskip!(milliseconds: 500)
    Physics.accelerate_by_one_tick!(timer.tick, a_ball_movement, a_engine_accelerate)
    a_ball_movement[0].should eq(30.0)
    a_ball_movement[1].should eq(0.0)

    timer.timeskip!(milliseconds: 500)
    Physics.accelerate_by_one_tick!(timer.tick, a_ball_movement, a_engine_accelerate)
    a_ball_movement[0].should eq(40.0)
    a_ball_movement[1].should eq(0.0)

    a_engine_stopped = Vector[0.0, 0.0, 0.0]
    timer.timeskip!(milliseconds: 500)
    Physics.accelerate_by_one_tick!(timer.tick, a_ball_movement, a_engine_stopped)
    a_ball_movement[0].should eq(40.0)
    a_ball_movement[1].should eq(0.0)

    a_retro_pulse = Vector[0.0, 0.1, 0.0]
    timer.timeskip!(milliseconds: 500)
    Physics.accelerate_by_one_tick!(timer.tick, a_ball_movement, a_retro_pulse)
    a_ball_movement[0].should eq(40.0)
    a_ball_movement[1].should eq(0.05)
  end

  it "move and accelerate an object" do
    a_ball_position = Vector[0.0, 0.0, 0.0]
    a_ball_movement = Vector[0.0, 0.0, 0.0]
    a_engine_accelerate = Vector[20.0, 0.0, 0.0]
    timer = Physics::Tick::Timer.new

    timer.timeskip!(seconds: 1)
    Physics.move_by_one_tick!(timer.tick, a_ball_position, a_ball_movement, a_engine_accelerate)
    a_ball_position[0].should eq(10.0)
    a_ball_position[1].should eq(0.0)
    a_ball_movement[0].should eq(20.0)
    a_ball_movement[1].should eq(0.0)

    timer.timeskip!(seconds: 1)
    Physics.move_by_one_tick!(timer.tick, a_ball_position, a_ball_movement, a_engine_accelerate)
    a_ball_position[0].should eq(40.0)
    a_ball_position[1].should eq(0.0)
    a_ball_movement[0].should eq(40.0)
    a_ball_movement[1].should eq(0.0)

    # cut a second in 2 computations gives the same result as one second
    timer.timeskip!(milliseconds: 500)
    Physics.move_by_one_tick!(timer.tick, a_ball_position, a_ball_movement, a_engine_accelerate)
    a_ball_position[0].should eq(62.5)
    a_ball_position[1].should eq(0.0)
    a_ball_movement[0].should eq(50.0)
    a_ball_movement[1].should eq(0.0)

    timer.timeskip!(milliseconds: 500)
    Physics.move_by_one_tick!(timer.tick, a_ball_position, a_ball_movement, a_engine_accelerate)
    a_ball_position[0].should eq(90.0)
    a_ball_position[1].should eq(0.0)
    a_ball_movement[0].should eq(60.0)
    a_ball_movement[1].should eq(0.0)

    a_gravity_generator = Vector[0.0, 0.0, 0.0]
    a_retro_pulse = Vector[0.0, 0.1, 0.0]
    star_gravity = Vector[-0.01, 0.02, 0.01]
    total_forces = a_gravity_generator + a_retro_pulse + star_gravity
    timer.timeskip!(seconds: 1)
    Physics.move_by_one_tick!(timer.tick, a_ball_position, a_ball_movement, total_forces)
    a_ball_position[0].round(3).should eq(149.995)
    a_ball_position[1].round(3).should eq(0.06)
    a_ball_position[2].round(3).should eq(0.005)
    a_ball_movement[0].round(3).should eq(59.99)
    a_ball_movement[1].round(3).should eq(0.12)
    a_ball_movement[2].round(3).should eq(0.01)
  end
end

# v0  = 40 m/s
# a   = 20 m/s²
# t1  = 0.5 s
# v1  = v0 + a * t1 = 40 + 20 * 0.6 = 50 m/s (m/s + m/s^2*s = m/s)
# [v] = (v0 + v1) / 2 = (40 + 50) / 2 = 45   (m/s + m/s)
# d   = [v] * (t1 - t0) = 45 * 0.5 = 22.5 (m/s * s)