"""Unit: `Clock`, `SystemClock` and `FakeClock` — the injectable time contract.

`FakeClock` is the test clock every time-dependent server object is driven
with (phase 05: "no test sleeps — all time via `FakeClock`"); these tests
lock its behaviour so that contract is honest.
"""

from __future__ import annotations

import pytest

from undercooked.server.clock import Clock, FakeClock, SystemClock

pytestmark = pytest.mark.unit


def test_system_clock_reads_a_monotonic_clock() -> None:
    clock = SystemClock()
    first = clock.now()
    second = clock.now()
    assert second >= first


def test_system_clock_accepts_a_substitute_time_function() -> None:
    values = iter([1.0, 2.5, 7.25])
    clock = SystemClock(lambda: next(values))
    assert clock.now() == 1.0
    assert clock.now() == 2.5
    assert clock.now() == 7.25


def test_fake_clock_starts_at_the_given_instant() -> None:
    assert FakeClock().now() == 0.0
    assert FakeClock(start=12.5).now() == 12.5


def test_fake_clock_advances_and_reports_the_new_instant() -> None:
    clock = FakeClock(start=10.0)
    assert clock.advance(5.0) == 15.0
    assert clock.now() == 15.0
    assert clock.advance(0.5) == 15.5
    assert clock.advance(0) == 15.5


def test_fake_clock_never_goes_backwards() -> None:
    clock = FakeClock()
    with pytest.raises(ValueError, match="cannot go backwards"):
        clock.advance(-1.0)
    assert clock.now() == 0.0


def test_both_clocks_satisfy_the_protocol() -> None:
    """The `Clock` protocol is a structural contract: both implementations
    must be usable anywhere a `Clock` is demanded."""
    clocks: list[Clock] = [SystemClock(), FakeClock()]
    for clock in clocks:
        assert isinstance(clock.now(), float)