import numpy as np
import pandas as pd
import logging

logging.basicConfig(level=logging.INFO, format="%(asctime)s - %(message)s")
log = logging.getLogger(__name__)


def precision_at_k(recommended, relevant, k):
    if k == 0 or len(relevant) == 0:
        return 0.0
    hits = sum(1 for item in recommended[:k] if item in relevant)
    return hits / k


def recall_at_k(recommended, relevant, k):
    if k == 0 or len(relevant) == 0:
        return 0.0
    hits = sum(1 for item in recommended[:k] if item in relevant)
    return hits / len(relevant)


def ndcg_at_k(recommended, relevant, k):
    if k == 0 or len(relevant) == 0:
        return 0.0
    dcg = sum(
        1.0 / np.log2(i + 2)
        for i, item in enumerate(recommended[:k])
        if item in relevant
    )
    n_relevant = min(len(relevant), k)
    idcg = sum(1.0 / np.log2(i + 2) for i in range(n_relevant))
    return dcg / idcg if idcg > 0 else 0.0


def hit_rate_at_k(recommended, relevant, k):
    return float(any(item in relevant for item in recommended[:k]))


def evaluate_model(model, test_df, train_df, k_values=None, n_users=None):
    if k_values is None:
        k_values = [5, 10, 20]
    max_k = max(k_values)

    train_users = set(train_df["user_idx"].unique())
    test_users  = test_df[test_df["user_idx"].isin(train_users)]["user_idx"].unique()

    if n_users is not None:
        test_users = test_users[:n_users]

    log.info(f"Evaluating {len(test_users):,} users at K={k_values}")

    ground_truth = (
        test_df[test_df["user_idx"].isin(test_users)]
        .groupby("user_idx")["item_idx"]
        .apply(set).to_dict()
    )

    results = {k: {"precision": [], "recall": [], "ndcg": [], "hit_rate": []}
               for k in k_values}

    skipped = 0
    for user_idx in test_users:
        relevant = ground_truth.get(user_idx, set())
        if not relevant:
            skipped += 1
            continue
        try:
            recs = model.recommend(user_idx, k=max_k)
        except Exception:
            skipped += 1
            continue
        for k in k_values:
            results[k]["precision"].append(precision_at_k(recs, relevant, k))
            results[k]["recall"].append(recall_at_k(recs, relevant, k))
            results[k]["ndcg"].append(ndcg_at_k(recs, relevant, k))
            results[k]["hit_rate"].append(hit_rate_at_k(recs, relevant, k))

    if skipped > 0:
        log.warning(f"Skipped {skipped} users")

    summary = {}
    for k in k_values:
        summary[f"precision@{k}"] = float(np.mean(results[k]["precision"]))
        summary[f"recall@{k}"]    = float(np.mean(results[k]["recall"]))
        summary[f"ndcg@{k}"]      = float(np.mean(results[k]["ndcg"]))
        summary[f"hit_rate@{k}"]  = float(np.mean(results[k]["hit_rate"]))

    summary["n_users_evaluated"] = int(len(test_users) - skipped)
    return summary


def catalog_coverage(model, test_users, n_items, k=10):
    recommended_items = set()
    for u in test_users:
        try:
            recs = model.recommend(u, k=k)
            recommended_items.update(recs)
        except Exception:
            continue
    return len(recommended_items) / n_items


def intra_list_diversity(model, test_users, item_feats, k=10, sample=200):
    feat_map = item_feats.set_index("item_idx")["popularity_score"].to_dict()
    diversities = []
    for u in list(test_users)[:sample]:
        try:
            recs = model.recommend(u, k=k)
        except Exception:
            continue
        scores = np.array([feat_map.get(i, 0.0) for i in recs])
        if len(scores) < 2:
            continue
        diffs = np.abs(scores[:, None] - scores[None, :])
        div   = diffs[np.triu_indices(len(scores), k=1)].mean()
        diversities.append(div)
    return float(np.mean(diversities)) if diversities else 0.0