"""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(hyp: str, ref: str) -> float:
    """chrF++ in 0–100. Falls back to char unigram F 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:
        hyp_c, ref_c = list(hyp), list(ref)
        if not hyp_c:
            return 0.0
        ref_set = set(ref_c)
        overlap = sum(1 for c in hyp_c if c in ref_set)
        prec = overlap / len(hyp_c)
        rec = overlap / max(len(ref_c), 1)
        if prec + rec == 0:
            return 0.0
        return 100.0 * 2 * prec * rec / (prec + rec)


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