siftlode/backend/tests/test_shuffle_walk.py

"""A seeded shuffle walks a stored random key instead of sorting the whole scope. DB lane.

Every video carries a random number drawn once (`Video.shuffle_key`). A seed picks a point on that
ring; the feed's Shuffle sort pages along the ring from there and wraps, and the Home surprise reads
a window of it and shuffles that window by the seed (`discover.queries.yt_surprise`). Ordering the
scope by md5(id || seed) cost ~350 ms per call on prod and grew with every Shuffle page.
"""

from contextlib import contextmanager
from datetime import datetime, timedelta, timezone

from sqlalchemy import event

from app import feedquery
from app.catalog.models import Channel, Subscription, Video
from app.discover import queries
from app.discover.queries import _FEED_BASE

NOW = datetime.now(timezone.utc)
KEYS = {"k05": 0.05, "k20": 0.20, "k40": 0.40, "k60": 0.60, "k80": 0.80, "k95": 0.95}


def _seed(db, make_user, keys=KEYS):
    user = make_user()
    db.add(Channel(id="UCring", title="Ring"))
    db.flush()
    db.add(Subscription(user_id=user.id, channel_id="UCring"))
    db.add_all(
        [
            Video(
                id=vid,
                channel_id="UCring",
                title=vid,
                duration_seconds=20 * 60,
                published_at=NOW - timedelta(days=i + 1),
                shuffle_key=key,
            )
            for i, (vid, key) in enumerate(keys.items())
        ]
    )
    db.commit()
    return user


def _ring(seed, keys=KEYS):
    """The ring order from the seed's point: the keys at or after it ascending, then the rest."""
    r = feedquery.shuffle_start(seed)
    return sorted(keys, key=lambda v: (keys[v] < r, keys[v]))


def _seed_landing_between(lo, hi):
    return next(s for s in range(1000) if lo < feedquery.shuffle_start(s) < hi)


def test_the_walk_starts_at_the_seeds_point_and_wraps(db, make_user):
    user = _seed(db, make_user)
    query, _ = feedquery.filtered_query(db, user, **_FEED_BASE)
    seed = _seed_landing_between(0.45, 0.55)  # starts at k60, wraps to k05 after k95
    got = [r.id for r in feedquery.shuffle_walk(db, query, seed, 6)]
    assert got == ["k60", "k80", "k95", "k05", "k20", "k40"]
    assert [r.id for r in feedquery.shuffle_walk(db, query, seed, 4)] == got[:4]


def test_the_feed_shuffle_pages_along_the_ring(db, make_user, as_user):
    user = _seed(db, make_user)
    client = as_user(user)
    for seed in (_seed_landing_between(0.1, 0.15), _seed_landing_between(0.85, 0.9)):
        walked, cursor = [], None
        for _ in range(5):
            url = f"/api/feed?sort=shuffle&seed={seed}&limit=2" + (
                f"&cursor={cursor}" if cursor else ""
            )
            body = client.get(url).json()
            walked += [it["id"] for it in body["items"]]
            cursor = body["next_cursor"]
            if not body["has_more"]:
                break
        assert walked == _ring(seed), seed


def test_a_new_seed_opens_the_shuffle_elsewhere(db, make_user, as_user):
    user = _seed(db, make_user)
    client = as_user(user)
    first = {
        seed: client.get(f"/api/feed?sort=shuffle&seed={seed}&limit=2").json()["items"][0]["id"]
        for seed in (_seed_landing_between(0.1, 0.15), _seed_landing_between(0.65, 0.7))
    }
    assert sorted(first.values()) == ["k20", "k80"]


@contextmanager
def _statements(db):
    seen: list[str] = []

    def grab(conn, cursor, statement, *args):
        seen.append(statement)

    engine = db.get_bind()
    event.listen(engine, "before_cursor_execute", grab)
    try:
        yield seen
    finally:
        event.remove(engine, "before_cursor_execute", grab)


def _draws_of_videos(seen):
    return [s for s in seen if "FROM videos" in s and "LIMIT" in s]


def test_neither_draw_orders_the_scope_by_a_hash(db, make_user):
    # Each draw reads the videos as an ascending range of the key (what ix_videos_shuffle_key serves),
    # never ordered by a hash or by the ring's CASE (which no index can serve).
    user = _seed(db, make_user)
    with _statements(db) as surprise:
        queries.yt_surprise(db, user, "any", 7, 3)
    with _statements(db) as page:
        feedquery.feed_page(
            db, user, dict(_FEED_BASE), sort="shuffle", seed=7, limit=2, cursor=None, lang="en"
        )
    for seen in (surprise, page):
        draws = _draws_of_videos(seen)
        assert draws
        for s in draws:
            assert "md5(" not in s.lower()
            assert "ORDER BY videos.shuffle_key, videos.id" in s.replace("\n", " "), s


def test_the_surprise_reads_a_window_of_the_ring(db, make_user, monkeypatch):
    user = _seed(db, make_user)
    monkeypatch.setattr(queries, "SURPRISE_WINDOW", 3)
    for seed in range(20):
        drawn = {r["id"] for r in queries.yt_surprise(db, user, "any", seed, 3)}
        assert drawn == set(_ring(seed)[:3]), seed


def test_a_wide_gap_on_the_ring_does_not_make_a_favourite(db, make_user):
    # One video after an empty half of the ring, twenty packed into the other half. Taking the first
    # video after the seed's point would deal the lonely one about half the time; the window is
    # shuffled by the seed, so it leads about as often as any other.
    keys = {"lonely": 0.5} | {f"p{i:02}": 0.51 + i * 0.02 for i in range(20)}
    user = _seed(db, make_user, keys)
    leads = [queries.yt_surprise(db, user, "any", s, 1)[0]["id"] for s in range(90)]
    assert leads.count("lonely") < 15, leads.count("lonely")


def test_a_new_video_gets_its_own_key(db, make_user):
    _seed(db, make_user, {})
    db.add_all([Video(id=f"n{i}", channel_id="UCring", title="n") for i in range(3)])
    db.commit()
    keys = [db.get(Video, f"n{i}").shuffle_key for i in range(3)]
    assert len(set(keys)) == 3 and all(0 <= k < 1 for k in keys)