"""POST /api/chat — a RAG chat turn streamed over SSE (PLAN §3/§4).

Flow (LOCKED A7/A15): embed the question → pgvector cosine top-K chunks →
the **honesty gate** (A8: best score < ``BOR_RELEVANCE_THRESHOLD`` ⇒
deflection) → locked persona prompt (PLAN §6) → ``turbo`` streamed as
``delta`` events → final ``done`` event (``deflected``, ``sources``,
``suggestions``) + ``query_log`` row + the per-turn log line (PLAN §9).
Mid-stream failures become a structured ``error`` event; a pre-stream DB
outage is a plain 503 JSON.

Honesty gate: a weak retrieval (score strictly below the threshold — or
an empty KB) flips the turn to deflection mode: the LOW prompt carries
weak-hit *titles only* (never document content) plus deterministic
"Maybe try" chips, and the ``done`` event / ``query_log`` row record
``deflected=true`` with the weak score.
"""
from __future__ import annotations

import json
import logging
import time
from collections.abc import AsyncIterator, Sequence
from dataclasses import dataclass
from typing import Any

from fastapi import APIRouter, Depends
from fastapi.responses import JSONResponse, StreamingResponse
from sqlalchemy.orm import Session

from app.config import Settings, get_settings
from app.db import db_available, get_db
from app.models import Document, QueryLog
from app.rag.llm import EmbeddingError, LLMClient, LLMError
from app.rag.prompts import build_deflect_prompt, build_high_prompt
from app.rag.retriever import RetrievedChunk, retrieve, select_documents, weak_hit_titles
from app.rag.suggestions import derive_suggestions
from app.schemas import ChatDoneEvent, ChatRequest, SourceRef

logger = logging.getLogger("app.chat")
router = APIRouter(tags=["chat"])

#: Streaming hints: no proxy buffering, no client caching (PLAN A15).
SSE_HEADERS = {"Cache-Control": "no-cache", "X-Accel-Buffering": "no"}

_llm: LLMClient | None = None


def get_llm() -> LLMClient:
    """Shared LLM client (FastAPI dependency so tests can override it)."""
    global _llm
    if _llm is None:
        _llm = LLMClient(get_settings())
    return _llm


def sse_event(payload: dict[str, Any]) -> str:
    """Serialize one SSE frame: ``data: <json>\\n\\n`` (PLAN §4)."""
    return f"data: {json.dumps(payload, ensure_ascii=False)}\n\n"


@dataclass
class TurnPlan:
    """What one chat turn sends to the LLM and reports on ``done``."""

    top_score: float
    deflected: bool
    system_prompt: str
    docs: list[Document]  # cited sources (weak hits when deflected)
    suggestions: list[str]  # "Maybe try" chips (deflected turns only)


def plan_turn(chunks: Sequence[RetrievedChunk], settings: Settings) -> TurnPlan:
    """Apply the honesty gate (A8) and assemble prompt + context for a turn.

    * ``top_score >= threshold`` → grounded: HIGH prompt with the full
      top-N documents, no suggestions. A score exactly at the threshold
      is an answer — the gate is strict (``score < threshold``).
    * ``top_score < threshold`` (or no hits at all) → deflected: LOW
      prompt (``DEFLECT_MODE``) with weak-hit titles only — never document
      content — plus deterministic alternative-question chips derived
      from those titles.
    """
    top_score = chunks[0].score if chunks else 0.0
    if top_score >= settings.relevance_threshold:
        docs = select_documents(
            chunks, n=settings.top_n_docs, max_chars=settings.max_context_chars
        )
        return TurnPlan(top_score, False, build_high_prompt(docs), docs, [])
    titles = weak_hit_titles(chunks)
    return TurnPlan(
        top_score,
        True,
        build_deflect_prompt(titles),
        select_documents(chunks, n=settings.top_n_docs, max_chars=settings.max_context_chars),
        derive_suggestions(titles, settings.suggestions),
    )


@router.post("/chat")
async def chat(
    request: ChatRequest,
    db: Session = Depends(get_db),  # noqa: B008
    llm: LLMClient = Depends(get_llm),  # noqa: B008
):
    """One chat turn: SSE stream of ``delta`` events + a final ``done``."""
    if not db_available():
        return JSONResponse(
            status_code=503,
            content={
                "detail": (
                    "The knowledge base is offline — start Postgres with "
                    "`podman compose up -d db`, then ask again."
                )
            },
        )

    started = time.monotonic()

    async def stream() -> AsyncIterator[str]:
        # 1. Embed the question.
        t0 = time.monotonic()
        try:
            question_vec = await llm.embed_one(request.message)
        except EmbeddingError as e:
            logger.error("chat: embedding failed question=%r — %s", request.message, e)
            yield sse_event(
                {
                    "type": "error",
                    "detail": "I couldn't reach the embedding model — please try again.",
                }
            )
            return
        embed_ms = int((time.monotonic() - t0) * 1000)

        # 2. Retrieve top-K chunks, then the honesty gate (A8) picks the
        #    HIGH (grounded) or LOW (deflected) prompt + context.
        settings = get_settings()
        try:
            chunks = retrieve(db, question_vec)
            plan = plan_turn(chunks, settings)
        except Exception:  # noqa: BLE001 — DB failure mid-turn
            logger.exception("chat: retrieval failed question=%r", request.message)
            yield sse_event(
                {
                    "type": "error",
                    "detail": "The knowledge base went offline mid-question — is Postgres up?",
                }
            )
            return
        source_paths = [f"{d.source}/{d.path}" for d in plan.docs]
        messages = [
            {"role": "system", "content": plan.system_prompt},
            {"role": "user", "content": request.message},
        ]

        # 3. Stream the answer (grounded, or an honest deflection).
        try:
            async for piece in llm.chat_stream(messages):
                yield sse_event({"type": "delta", "text": piece})
        except LLMError as e:
            logger.error("chat: LLM stream failed question=%r — %s", request.message, e)
            yield sse_event(
                {"type": "error", "detail": "The chat model dropped the connection — try again?"}
            )
            return

        # 4. Durable record + required per-turn log line (PLAN §9).
        total_ms = int((time.monotonic() - started) * 1000)
        try:
            db.add(
                QueryLog(
                    question=request.message,
                    top_score=plan.top_score,
                    chunk_hits=len(chunks),
                    deflected=plan.deflected,
                    sources=", ".join(source_paths),
                    latency_ms=total_ms,
                )
            )
            db.commit()
        except Exception:  # noqa: BLE001 — the answer already went out
            logger.exception("chat: failed to write query_log question=%r", request.message)

        logger.info(
            "question=%r embed_ms=%d top_score=%.3f threshold=%.2f deflected=%s "
            "sources=%r total_ms=%d",
            request.message,
            embed_ms,
            plan.top_score,
            settings.relevance_threshold,
            plan.deflected,
            source_paths,
            total_ms,
        )
        yield sse_event(
            ChatDoneEvent(
                deflected=plan.deflected,
                sources=[
                    SourceRef(source=d.source, path=d.path, title=d.title) for d in plan.docs
                ],
                suggestions=plan.suggestions,
            ).model_dump()
        )

    return StreamingResponse(stream(), media_type="text/event-stream", headers=SSE_HEADERS)