# -*- coding: utf-8 -*-
"""BM25 backend selection with a small local fallback."""
import math
from collections import Counter

import numpy as np


class SimpleBM25Okapi(object):
    """Small BM25Okapi-compatible fallback used when rank_bm25 is unavailable."""

    def __init__(self, corpus, k1=1.5, b=0.75, epsilon=0.25):
        self.corpus = [list(doc or []) for doc in corpus]
        self.k1 = k1
        self.b = b
        self.epsilon = epsilon
        self.corpus_size = len(self.corpus)
        self.doc_len = [len(doc) for doc in self.corpus]
        self.avgdl = float(sum(self.doc_len)) / self.corpus_size if self.corpus_size else 0.0
        self.doc_freqs = [Counter(doc) for doc in self.corpus]
        self.idf = self._calc_idf()

    def _calc_idf(self):
        nd = {}
        for freqs in self.doc_freqs:
            for word in freqs:
                nd[word] = nd.get(word, 0) + 1

        idf = {}
        negative_idfs = []
        for word, freq in nd.items():
            value = math.log(self.corpus_size - freq + 0.5) - math.log(freq + 0.5)
            idf[word] = value
            if value < 0:
                negative_idfs.append(value)

        average_idf = sum(idf.values()) / len(idf) if idf else 0.0
        eps = self.epsilon * average_idf
        for word in idf:
            if idf[word] < 0:
                idf[word] = eps
        return idf

    def get_scores(self, query):
        scores = np.zeros(self.corpus_size)
        if not query or not self.corpus_size or self.avgdl <= 0:
            return scores

        for token in query:
            token_idf = self.idf.get(token)
            if token_idf is None:
                continue
            for i, freqs in enumerate(self.doc_freqs):
                freq = freqs.get(token, 0)
                if freq == 0:
                    continue
                denominator = freq + self.k1 * (1 - self.b + self.b * self.doc_len[i] / self.avgdl)
                scores[i] += token_idf * freq * (self.k1 + 1) / denominator
        return scores


def get_bm25_okapi():
    try:
        from rank_bm25 import BM25Okapi
        return BM25Okapi
    except ImportError:
        return SimpleBM25Okapi