"""Frozen translation success() — SFT eval and RL reward must call this."""

from __future__ import annotations

import re

CHRF_MIN = 40.0
COPY_CHRF_MAX = 80.0
_CJK = re.compile(r"[\u4e00-\u9fff]")


def _detect_lang(text: str) -> str | None:
    sample = text.strip()
    if not sample:
        return None
    try:
        from langdetect import detect

        tag = detect(sample)
    except Exception:
        if _CJK.search(sample):
            return "zh"
        if any(c.isascii() and c.isalpha() for c in sample):
            return "en"
        return None
    if tag.startswith("zh"):
        return "zh"
    return tag[:2]


def _chrf_ngram(hyp: str, ref: str, max_n: int = 4) -> float:
    """Count-based char n-gram F (β=2), 0–100. Not set-overlap unigrams."""
    from collections import Counter

    hyp, ref = hyp.strip(), ref.strip()
    if not hyp or not ref:
        return 0.0
    scores: list[float] = []
    for n in range(1, max_n + 1):
        if len(hyp) < n or len(ref) < n:
            scores.append(0.0)
            continue
        hc = Counter(hyp[i : i + n] for i in range(len(hyp) - n + 1))
        rc = Counter(ref[i : i + n] for i in range(len(ref) - n + 1))
        overlap = sum((hc & rc).values())
        prec = overlap / max(sum(hc.values()), 1)
        rec = overlap / max(sum(rc.values()), 1)
        if prec + rec == 0:
            scores.append(0.0)
            continue
        beta2 = 4.0
        scores.append((1.0 + beta2) * prec * rec / (beta2 * prec + rec))
    return 100.0 * sum(scores) / max(len(scores), 1)


def _chrf(hyp: str, ref: str) -> float:
    """chrF++ in 0–100. Falls back to count-based char n-grams if sacrebleu is missing."""
    if not hyp or not ref:
        return 0.0
    try:
        from sacrebleu.metrics import CHRF

        return float(CHRF(word_order=2).sentence_score(hyp, [ref]).score)
    except Exception:
        return _chrf_ngram(hyp, ref)


def translation_success(
    src: str,
    hyp: str,
    ref: str | None = None,
    *,
    target_lang: str,
    chrf_min: float = CHRF_MIN,
    copy_chrf_max: float = COPY_CHRF_MAX,
) -> bool:
    """Binary task success for zh↔en instruction translation.

    1. non-empty hyp, no instruction leak prefix
    2. langid(hyp) matches target_lang (zh / en)
    3. hyp is not a copy of src
    4. if ref is given, chrF(hyp, ref) >= chrf_min
    """
    hyp = hyp.strip()
    src = src.strip()
    if not hyp:
        return False
    leak = ("翻译如下", "translate to", "translation:", "译文：")
    head = hyp[:40].lower()
    if any(p in head or p in hyp[:20] for p in leak):
        return False
    want = "zh" if target_lang.startswith("zh") else "en"
    got = _detect_lang(hyp)
    if got != want:
        return False
    if src and _chrf(hyp, src) >= copy_chrf_max:
        return False
    if hyp == src:
        return False
    if ref is not None and _chrf(hyp, ref.strip()) < chrf_min:
        return False
    return True