"""Keyset paging for lists that load as you scroll (#5211): a cursor round-trips,
a mangled one starts over, and a page says whether another follows."""
from datetime import datetime, timezone
from types import SimpleNamespace as NS

from sqlalchemy import Column, DateTime, MetaData, String, Table, select
from sqlalchemy.dialects import postgresql

from steward.core.paging import cursor, keyset, next_page, parse_cursor

AT = datetime(2026, 10, 7, 3, 0, 5, 123456, tzinfo=timezone.utc)


def test_a_cursor_round_trips_and_a_mangled_one_starts_over():
    assert parse_cursor(cursor(AT, "0190ab|cd")) == (AT, "0190ab|cd")   # an id may hold the separator
    assert parse_cursor(None) is None and parse_cursor("") is None
    assert parse_cursor("not a cursor") is None and parse_cursor("yesterday|x") is None


def test_a_page_says_whether_another_follows():
    rows = [NS(at=AT, id=str(i)) for i in range(5)]
    page, after = next_page(rows, lambda r: (r.at, r.id), size=3)
    assert [r.id for r in page] == ["0", "1", "2"] and after == cursor(AT, "2")
    page, after = next_page(rows[:3], lambda r: (r.at, r.id), size=3)
    assert len(page) == 3 and after is None                             # exactly a page: the end


def test_the_query_reads_one_row_past_the_page_older_than_the_cursor():
    t = Table("t", MetaData(), Column("at", DateTime(timezone=True)), Column("id", String))
    first = str(keyset(select(t), t.c.at, t.c.id, None, size=10).compile(dialect=postgresql.dialect()))
    assert "WHERE" not in first and "ORDER BY t.at DESC, t.id DESC" in first and "LIMIT" in first
    later = str(keyset(select(t), t.c.at, t.c.id, cursor(AT, "x"), size=10)
                .compile(dialect=postgresql.dialect()))
    assert "WHERE (t.at, t.id) < (" in later