from __future__ import annotations

import re
from collections import Counter


def ngram_tokenize(text: str, n: int = 2) -> list[tuple[str, ...]]:
    """공백을 제거한 문자 단위 n-gram 목록"""
    tokens = list(re.sub(r"\s+", "", text))
    return [tuple(tokens[i:i + n]) for i in range(len(tokens) - n + 1)]


def calculate_rouge_n(reference: str, hypothesis: str, n: int = 2) -> float:
    """N-gram 기반 ROUGE 점수 계산 (계획서 p.24 수식: recall)"""
    ref_ngrams = ngram_tokenize(reference, n)
    hyp_ngrams = ngram_tokenize(hypothesis, n)

    if not ref_ngrams or not hyp_ngrams:
        return 0.0

    ref_counter = Counter(ref_ngrams)
    hyp_counter = Counter(hyp_ngrams)

    overlap = 0
    for ngram in hyp_counter:
        overlap += min(hyp_counter[ngram], ref_counter.get(ngram, 0))

    recall = overlap / len(ref_ngrams) if len(ref_ngrams) > 0 else 0
    return recall