"""Trending recommendation heuristic service.

docs/planning/discovery.md (task 33706f401ad9).
"""

from sqlalchemy import case, func, select, text
from app.models.world_visit import WorldVisit


def get_trending_subquery(days: int = 7):
    """Build a subquery that computes a decay-weighted trending score per world_id.

    Decay weights:
    - Visited within 24 hours: weight 3.0
    - Visited within 3 days: weight 2.0
    - Visited within 7 days: weight 1.0
    """
    trending_weight = case(
        (WorldVisit.visited_at >= func.now() - text("interval '1 day'"), 3.0),
        (WorldVisit.visited_at >= func.now() - text("interval '3 days'"), 2.0),
        else_=1.0,
    )

    interval_str = f"interval '{days} days'"
    subq = (
        select(
            WorldVisit.world_id.label("trending_world_id"),
            func.coalesce(func.sum(trending_weight), 0.0).label("trending_score"),
        )
        .where(WorldVisit.visited_at >= func.now() - text(interval_str))
        .group_by(WorldVisit.world_id)
        .subquery("trending_scores")
    )
    return subq