hyungi_document_server/app/services/search/evidence_service.py

"""Evidence extraction 서비스 (Phase 3.2).

reranker 결과 chunks 에서 query-relevant span 을 구조적으로 추출한다.

## 설계 (EV-A: Rule + LLM span select)

```
reranked results
    ↓
[rule filter]   score >= 0.25, max_per_doc=2, top MAX_EVIDENCE_CANDIDATES
    ↓
[snippet 재윈도우]  _extract_window(full, query, 800) — LLM 입력용
    ↓
[1 batched LLM call]  gemma-4 via get_mlx_gate() (single inference)
    ↓
[post-process]
  - relevance >= 0.5 필터
  - span too-short (< 80자) → _extract_window(full, query, 120) 로 재확장
  - span too-long (> 300자) → cut
  - doc-group ordering (검색 결과 doc 순서 유지, doc 내부만 relevance desc)
  - n 재부여 (1..N)
    ↓
EvidenceItem 리스트
```

## 영구 룰

- **LLM 호출은 1번만** (batched). 순차 호출 절대 금지.
- **B-2 변경**: evidence 추출은 triage(4B Ollama) 로 전환 — Ollama 는 concurrent
  OK 라 `get_mlx_gate()` 불필요. primary(26B MLX) 는 synthesis 전용 보호.
- 기존 analyzer / synthesis 의 `get_mlx_gate()` 공유는 유지 — 26B 경로에만 적용.
- **fallback span 도 query 중심 window**. `full_snippet[:200]` 같은 "앞에서부터
  자르기" 절대 금지. 조용한 품질 붕괴 (citation 은 멀쩡한데 실제 span 이 query
  와 무관) 대표 사례.
- **Span too-short 보정 필수**: `len(span) < 80` 이면 자동 확장. "짧을수록
  정확" 이 아니라 **짧으면 위험** — synthesis LLM 이 문맥 부족으로 이어 만들기
  (soft hallucination) 를 한다.
- **Evidence ordering 은 doc-group 유지**. 전역 relevance desc 정렬 금지.
  answer 는 [1][2][3] 순서로 생성되고 그 순서가 문맥 흐름을 결정한다.

## 확장 여지 (지금은 비활성)

`EVIDENCE_FAST_PATH_THRESHOLD` 가 `None` 이 아니고 `results[0].rerank_score >=
THRESHOLD` 이면 LLM 호출 스킵 후 rule-only 경로로 즉시 반환. Activation 조건:
(1) evidence LLM 호출 비율 > 80%, (2) /ask 평균 latency > 15s, (3) rerank
top1 p50 > 0.75. 셋 다 충족해야 켠다.
"""

from __future__ import annotations

import asyncio
import re
import time
from dataclasses import dataclass, field
from typing import TYPE_CHECKING

from ai.client import AIClient, _load_prompt, parse_json_response
from core.utils import setup_logger

from .rerank_service import _extract_window

if TYPE_CHECKING:
    from api.search import SearchResult

logger = setup_logger("evidence")

# ─── 상수 (plan 영구 룰) ─────────────────────────────────
EVIDENCE_MIN_RERANK = 0.25  # 1차 rule cut — rerank score 이 미만은 제외
MAX_EVIDENCE_CANDIDATES = 6  # LLM 입력 상한
MAX_PER_DOC = 2
CANDIDATE_SNIPPET_CHARS = 1200  # LLM 이 볼 원문 창 크기 (800→1200, 6×1200자≈2880tok < gemma-4 4096 예산)
MIN_EVIDENCE_FOR_SYNTHESIS = 3  # sparse evidence supplement 임계값

MIN_RELEVANCE_KEEP = 0.5  # LLM 출력 필터
SPAN_MIN_CHARS = 80  # 이 미만이면 window enlarge
SPAN_ENLARGE_TARGET = 120  # enlarge 시 재윈도우 target_chars
SPAN_MAX_CHARS = 300  # 이 초과면 cut (synthesis token budget 보호)

LLM_TIMEOUT_MS = 15000
PROMPT_VERSION = "v2-triage"   # B-2: primary(26B MLX) → triage(4B Ollama) 전환

# 확장 여지 — None 이면 비활성 (baseline). 실측 후 0.8 등으로 켠다.
EVIDENCE_FAST_PATH_THRESHOLD: float | None = None


# ─── 반환 타입 ───────────────────────────────────────────


@dataclass(slots=True)
class EvidenceItem:
    """LLM 또는 rule fallback 이 추출한 단일 evidence span.

    n 은 doc-group ordering + relevance 정렬 후 1부터 재부여된다.
    `full_snippet` 은 **synthesis 프롬프트에 절대 포함 금지** — debug / citation
    원문 보기 전용.
    """

    n: int  # 1-based, synthesis 프롬프트의 [n] 과 매핑
    chunk_id: int | None
    doc_id: int
    title: str | None
    section_title: str | None
    span_text: str  # LLM 추출 (또는 rule fallback) span, 80~300자
    relevance: float  # LLM 0~1 (fallback 시 rerank_score 복사)
    rerank_score: float  # raw reranker 점수
    full_snippet: str  # 원본 800자 (debug/citation 전용, synthesis 금지)
    source: str = "llm"  # "llm" | "supplement" | "rule_fallback"


# ─── 프롬프트 로딩 (module 초기화 1회) ───────────────────
try:
    EVIDENCE_PROMPT = _load_prompt("evidence_extract.txt")
except FileNotFoundError:
    EVIDENCE_PROMPT = ""
    logger.warning(
        "evidence_extract.txt not found — evidence_service will always use rule-only fallback"
    )


# ─── Helper: candidates → LLM 입력 블록 ──────────────────


def _build_numbered_candidates(
    candidates: list["SearchResult"], query: str
) -> tuple[str, list[str]]:
    """LLM 프롬프트의 {numbered_candidates} 블록 + 재윈도우된 full_snippet 리스트.

    Returns:
        (block_str, full_snippets) — full_snippets[i] 는 1-based n=i+1 의 원문
    """
    lines: list[str] = []
    full_snippets: list[str] = []
    for i, c in enumerate(candidates, 1):
        title = (c.title or "").strip()
        raw_text = c.snippet or ""
        full = _extract_window(raw_text, query, target_chars=CANDIDATE_SNIPPET_CHARS)
        full_snippets.append(full)
        lines.append(f"[{i}] title: {title} / text: {full}")
    return "\n".join(lines), full_snippets


# ─── Helper: span length 보정 ───────────────────────────


def _normalize_span(span: str, full: str, query: str) -> tuple[str, bool]:
    """span 을 SPAN_MIN_CHARS ~ SPAN_MAX_CHARS 범위로 보정.

    Returns:
        (normalized_span, was_expanded)
        - was_expanded=True 이면 "short_span_expanded" 로그 대상
    """
    s = (span or "").strip()
    expanded = False
    if len(s) < SPAN_MIN_CHARS:
        # soft hallucination 방어 — query 중심으로 window 재확장
        s = _extract_window(full, query, target_chars=SPAN_ENLARGE_TARGET)
        expanded = True
    if len(s) > SPAN_MAX_CHARS:
        s = s[:SPAN_MAX_CHARS]
    return s, expanded


# ─── Helper: doc-group ordering ─────────────────────────


def _apply_doc_group_ordering(
    items: list[EvidenceItem],
    results: list["SearchResult"],
) -> list[EvidenceItem]:
    """검색 결과 doc 순서 유지 + doc 내부만 relevance desc + n 재부여.

    answer 는 [1][2][3] 순서로 생성되고 그 순서가 문맥 흐름을 결정한다.
    전역 relevance desc 정렬은 "doc A span1 → doc B span1 → doc A span2"
    처럼 튀면서 읽기 이상한 답변을 만든다.
    """
    if not items:
        return []
    doc_order: dict[int, int] = {}
    for idx, r in enumerate(results):
        if r.id not in doc_order:
            doc_order[r.id] = idx
    # 정렬: (doc 순서, -relevance)
    items.sort(
        key=lambda it: (doc_order.get(it.doc_id, 9999), -it.relevance)
    )
    # n 재부여
    for new_n, it in enumerate(items, 1):
        it.n = new_n
    return items


# ─── Helper: rule-only fallback ─────────────────────────


def _build_rule_only_evidence(
    candidates: list["SearchResult"],
    full_snippets: list[str],
    query: str,
) -> list[EvidenceItem]:
    """LLM 실패/timeout 시 rule-only 경로.

    ⚠ `full_snippet[:200]` 같은 앞자르기 금지. 반드시 `_extract_window` 로
    query 중심 윈도우를 만든다. relevance 는 rerank_score 복사.
    """
    items: list[EvidenceItem] = []
    for i, (c, full) in enumerate(zip(candidates, full_snippets), 1):
        span = _extract_window(full, query, target_chars=200)
        # 정규화 (보통 여기서는 SPAN_MIN_CHARS 이상이지만 안전장치)
        span, _expanded = _normalize_span(span, full, query)
        items.append(
            EvidenceItem(
                n=i,
                chunk_id=c.chunk_id,
                doc_id=c.id,
                title=c.title,
                section_title=c.section_title,
                span_text=span,
                relevance=float(c.rerank_score or c.score or 0.0),
                rerank_score=float(c.rerank_score or c.score or 0.0),
                full_snippet=full,
                source="rule_fallback",
            )
        )
    return items


# ─── Core: extract_evidence ─────────────────────────────


async def extract_evidence(
    query: str,
    results: list["SearchResult"],
    ai_client: AIClient | None = None,
) -> tuple[list[EvidenceItem], str | None]:
    """reranked results → EvidenceItem 리스트.

    Returns:
        (items, skip_reason)
        skip_reason ∈ {None, "empty_retrieval", "all_low_rerank", "fast_path",
                       "llm_timeout_fallback_rule", "llm_error_fallback_rule",
                       "parse_failed_fallback_rule", "all_llm_rejected"}
        - skip_reason 이 None 이 아니어도 items 는 비어있지 않을 수 있다
          (fallback/fast_path 경로).
    """
    if not results:
        return [], "empty_retrieval"

    # ── 1차 rule filter: rerank_score >= EVIDENCE_MIN_RERANK + max_per_doc ──
    candidates: list["SearchResult"] = []
    per_doc: dict[int, int] = {}
    for r in results:
        raw_score = r.rerank_score if r.rerank_score is not None else r.score
        if raw_score is None or raw_score < EVIDENCE_MIN_RERANK:
            continue
        if per_doc.get(r.id, 0) >= MAX_PER_DOC:
            continue
        candidates.append(r)
        per_doc[r.id] = per_doc.get(r.id, 0) + 1
        if len(candidates) >= MAX_EVIDENCE_CANDIDATES:
            break

    if not candidates:
        return [], "all_low_rerank"

    # ── Fast-path (현재 비활성) ─────────────────────────
    if EVIDENCE_FAST_PATH_THRESHOLD is not None:
        # ⚠ display score 가 아니라 raw rerank_score 로 판단.
        #    normalize_display_scores 를 거친 r.score 는 frontend 용 리스케일
        #    값이라 distribution drift 가능. fast-path 는 reranker raw 신호가 안전.
        top_rerank = (
            results[0].rerank_score if results[0].rerank_score is not None else 0.0
        )
        if top_rerank is not None and top_rerank >= EVIDENCE_FAST_PATH_THRESHOLD:
            _block, full_snippets = _build_numbered_candidates(candidates, query)
            items = _build_rule_only_evidence(candidates, full_snippets, query)
            items = _apply_doc_group_ordering(items, results)
            logger.info(
                "evidence fast_path query=%r candidates=%d kept=%d top_rerank=%.2f",
                query[:80], len(candidates), len(items), top_rerank,
            )
            return items, "fast_path"

    # ── LLM 호출 준비 ───────────────────────────────────
    if not EVIDENCE_PROMPT:
        # 프롬프트 미로딩 → rule-only
        _block, full_snippets = _build_numbered_candidates(candidates, query)
        items = _build_rule_only_evidence(candidates, full_snippets, query)
        items = _apply_doc_group_ordering(items, results)
        logger.warning(
            "evidence prompt_not_loaded → rule fallback query=%r kept=%d",
            query[:80], len(items),
        )
        return items, "llm_error_fallback_rule"

    block, full_snippets = _build_numbered_candidates(candidates, query)
    prompt = EVIDENCE_PROMPT.replace("{query}", query).replace(
        "{numbered_candidates}", block
    )

    client_owned = False
    if ai_client is None:
        ai_client = AIClient()
        client_owned = True

    t_start = time.perf_counter()
    raw: str | None = None
    llm_error: str | None = None

    try:
        # B-2: evidence 추출은 4B triage (Ollama concurrent OK) — MLX gate 경유 불필요.
        # primary(26B) 는 synthesis 전용으로 MLX gate 보호.
        async with asyncio.timeout(LLM_TIMEOUT_MS / 1000):
            raw = await ai_client.call_triage(prompt)
    except asyncio.TimeoutError:
        llm_error = "timeout"
    except Exception as exc:
        llm_error = f"llm_error:{type(exc).__name__}"
    finally:
        if client_owned:
            try:
                await ai_client.close()
            except Exception:
                pass

    elapsed_ms = (time.perf_counter() - t_start) * 1000

    # ── LLM 실패 → rule fallback ────────────────────────
    if llm_error is not None:
        items = _build_rule_only_evidence(candidates, full_snippets, query)
        items = _apply_doc_group_ordering(items, results)
        logger.warning(
            "evidence LLM %s → rule fallback query=%r candidates=%d kept=%d elapsed_ms=%.0f",
            llm_error, query[:80], len(candidates), len(items), elapsed_ms,
        )
        return items, "llm_timeout_fallback_rule" if llm_error == "timeout" else "llm_error_fallback_rule"

    parsed = parse_json_response(raw or "")
    if not isinstance(parsed, dict) or not isinstance(parsed.get("items"), list):
        items = _build_rule_only_evidence(candidates, full_snippets, query)
        items = _apply_doc_group_ordering(items, results)
        logger.warning(
            "evidence parse_failed → rule fallback query=%r raw=%r elapsed_ms=%.0f",
            query[:80], (raw or "")[:200], elapsed_ms,
        )
        return items, "parse_failed_fallback_rule"

    # ── LLM 출력 파싱 ──────────────────────────────────
    short_span_expanded = 0
    llm_items: list[EvidenceItem] = []
    for entry in parsed["items"]:
        if not isinstance(entry, dict):
            continue
        try:
            n_raw = int(entry.get("n", 0))
        except (TypeError, ValueError):
            continue
        if n_raw < 1 or n_raw > len(candidates):
            continue
        try:
            relevance = float(entry.get("relevance", 0.0) or 0.0)
        except (TypeError, ValueError):
            relevance = 0.0
        if relevance < MIN_RELEVANCE_KEEP:
            continue
        span_raw = entry.get("span")
        if not isinstance(span_raw, str) or not span_raw.strip():
            continue

        candidate = candidates[n_raw - 1]
        full = full_snippets[n_raw - 1]
        span, expanded = _normalize_span(span_raw, full, query)
        if expanded:
            short_span_expanded += 1

        llm_items.append(
            EvidenceItem(
                n=n_raw,  # doc-group ordering 에서 재부여됨
                chunk_id=candidate.chunk_id,
                doc_id=candidate.id,
                title=candidate.title,
                section_title=candidate.section_title,
                span_text=span,
                relevance=relevance,
                rerank_score=float(
                    candidate.rerank_score
                    if candidate.rerank_score is not None
                    else (candidate.score or 0.0)
                ),
                full_snippet=full,
            )
        )

    # ── LLM 이 전부 reject → rule fallback ──────────────
    if not llm_items:
        items = _build_rule_only_evidence(candidates, full_snippets, query)
        items = _apply_doc_group_ordering(items, results)
        logger.warning(
            "evidence all_llm_rejected → rule fallback query=%r elapsed_ms=%.0f",
            query[:80], elapsed_ms,
        )
        return items, "all_llm_rejected"

    # ── Sparse evidence supplement (Phase 3.5b) ────────
    # dead zone 해소: LLM kept 1~2 + candidates 충분 → rule-only 보충
    supplement_skip = None
    if 0 < len(llm_items) < MIN_EVIDENCE_FOR_SYNTHESIS and len(candidates) >= MIN_EVIDENCE_FOR_SYNTHESIS:
        llm_n_set = {it.n for it in llm_items}
        supplement_count = MIN_EVIDENCE_FOR_SYNTHESIS - len(llm_items)
        supplemented = 0

        # substring + critical token 필터 준비
        query_tokens = re.findall(r'[가-힣]{2,}|[a-zA-Z]{3,}', query)
        _IMPORTANT_SUFFIXES = {"조건", "기준", "요건", "처벌", "벌칙",
                               "정의", "차이", "절차", "방법", "계산"}
        critical_tokens = [
            t for t in query_tokens
            if len(t) >= 3 or any(s in t for s in _IMPORTANT_SUFFIXES)
        ]

        for idx, (c, full) in enumerate(zip(candidates, full_snippets), 1):
            if idx in llm_n_set or supplement_count <= 0:
                continue
            span = _extract_window(full, query, target_chars=200)
            span, _ = _normalize_span(span, full, query)

            # substring match (recall)
            has_match = any(qt in span for qt in query_tokens)
            # critical token check (precision)
            has_critical = (
                any(ct in span for ct in critical_tokens)
                if critical_tokens else has_match
            )
            if not (has_match and has_critical):
                continue

            llm_items.append(
                EvidenceItem(
                    n=idx,
                    chunk_id=c.chunk_id,
                    doc_id=c.id,
                    title=c.title,
                    section_title=c.section_title,
                    span_text=span,
                    relevance=float(c.rerank_score or c.score or 0.0) * 0.8,
                    rerank_score=float(c.rerank_score or c.score or 0.0),
                    full_snippet=full,
                    source="supplement",
                )
            )
            supplemented += 1
            supplement_count -= 1

        if supplemented > 0:
            supplement_skip = "sparse_evidence_supplemented"
            logger.info(
                "evidence sparse_supplement query=%r llm_kept=%d supplemented=%d total=%d",
                query[:80], len(llm_items) - supplemented, supplemented, len(llm_items),
            )

    # ── doc-group ordering + n 재부여 ───────────────────
    llm_items = _apply_doc_group_ordering(llm_items, results)

    logger.info(
        "evidence ok query=%r candidates=%d kept=%d short_span_expanded=%d elapsed_ms=%.0f",
        query[:80], len(candidates), len(llm_items), short_span_expanded, elapsed_ms,
    )
    return llm_items, supplement_skip