"""The parent chain: walking it, and refusing to close it (milestone #254 step 1).

Pure functions over a `{id: parent_id}` literal, so a whole hierarchy is one line
of setup and no database is involved. That is the reason the cascade lives in its
own import-free module — see services/design_cascade.py.
"""
from scribe.services.design_cascade import ancestry, would_cycle


# --- ancestry ---------------------------------------------------------------

def test_ancestry_returns_the_chain_deepest_first():
    """Deepest first because that is the order resolution consumes it in: the
    system being resolved wins over its parent, which wins over the root."""
    parents = {1: None, 2: 1, 3: 2}
    assert ancestry(3, parents) == [3, 2, 1]


def test_a_root_is_its_own_whole_chain():
    """A family system has no parent, and that is an ordinary state — not an
    incomplete one. It must resolve to exactly itself."""
    assert ancestry(1, {1: None}) == [1]


def test_a_missing_parent_truncates_rather_than_raising():
    """A parent that was soft-deleted (or filtered out of the caller's scope) is
    a shorter chain, not a failed request. The alternative — raising — would make
    one deleted system break every descendant's rendering."""
    assert ancestry(3, {3: 2}) == [3, 2]


def test_a_system_absent_from_the_map_still_yields_itself():
    assert ancestry(9, {}) == [9]


def test_ancestry_terminates_on_a_cycle_instead_of_hanging():
    """LOAD-BEARING, and the reason a visited-set exists even though writes are
    guarded. A loop introduced by a direct DB edit or a future bug must degrade
    to a truncated chain: truncation shows up in the result, a hang shows up as
    an outage. Every id appears exactly once."""
    parents = {1: 3, 2: 1, 3: 2}
    chain = ancestry(1, parents)
    assert chain == [1, 3, 2]
    assert len(chain) == len(set(chain))


def test_ancestry_terminates_on_a_self_parent():
    assert ancestry(1, {1: 1}) == [1]


# --- would_cycle ------------------------------------------------------------

def test_clearing_the_parent_never_cycles():
    """None means "make this a root", which is always safe."""
    assert would_cycle(2, None, {1: None, 2: 1}) is False


def test_a_system_cannot_be_its_own_parent():
    assert would_cycle(1, 1, {1: None}) is True


def test_an_ordinary_reparent_is_allowed():
    """family <- app is the shape the whole model exists for; it must not trip
    the guard."""
    assert would_cycle(2, 1, {1: None, 2: None}) is False


def test_a_direct_swap_is_refused():
    """A -> B, then B -> A. The two-system case, and the one a UI produces first
    because both systems are on screen together."""
    parents = {1: None, 2: 1}
    assert would_cycle(1, 2, parents) is True


def test_an_indirect_loop_is_refused():
    """Three deep: root <- mid <- leaf, then root's parent set to leaf. Catching
    this is what makes the check a chain walk rather than a parent comparison."""
    parents = {1: None, 2: 1, 3: 2}
    assert would_cycle(1, 3, parents) is True


def test_reparenting_onto_a_sibling_subtree_is_allowed():
    """Two branches off one root. Moving one under the other is legitimate — the
    guard must refuse loops, not reorganisation."""
    parents = {1: None, 2: 1, 3: 1}
    assert would_cycle(3, 2, parents) is False


def test_the_guard_survives_a_hierarchy_that_is_already_corrupt():
    """If a cycle somehow already exists, the guard still has to answer rather
    than spin — the write path is exactly where such a hierarchy gets repaired."""
    parents = {1: 2, 2: 1, 3: None}
    assert would_cycle(3, 1, parents) is False
    assert would_cycle(1, 2, parents) is True