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

Flow (LOCKED A7/A15): embed the question → hybrid retrieval (cosine
top-N ∪ Postgres FTS top-N, RRF-fused) → the **honesty gate** → 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.

Thinking (phase 17, PLAN §4 extension, owner permission 2026-08-23): the
model's reasoning arrives ahead of the answer and is streamed as
``thinking`` events before the ``delta`` events of the same turn. Each
turn's thinking is counted in the per-turn log line
(``thinking_chars=N``); ``BOR_STREAM_THINKING=0`` suppresses the
``thinking`` frames (the pieces are still counted).

Honesty gate (A8, revised 2026-08-21): LOW — deflection — only when the
best cosine is strictly below ``BOR_RELEVANCE_THRESHOLD`` **and** no
candidate chunk FTS-matches the question (``fts_hits == 0``). A
name-your-tool question with weak vector overlap but a lexical hit still
gets a grounded answer. Deflection mode carries weak-hit *titles only*
(never document content) plus deterministic "Maybe try" chips, and the
``done`` event / ``query_log`` row record ``deflected=true``, the weak
score and the ``fts_hits`` count.

Steering (phase 15): the owner's stored tuning notes are loaded per turn
(oldest first) and injected into the system prompt as a ``<tuning>``
section — both the HIGH and the LOW prompt carry it. The per-turn log
line records ``tuning=N`` (the number of injected notes).

Summaries (phase 30): a lite-model summary chunk's parent *is* the
source document, so a summary hit resolves to the full source document
through the unchanged chunk→document mapping (A7 revised) — context
assembly is untouched. ``TurnPlan.summary_hits`` counts the hit chunks
with ``is_summary`` whose parent document landed in the selected
top-N context, and the per-turn log line records ``summary_hits=N``
after ``fts_hits`` (PLAN §9 line extension).

KB overview (phase 31): the lite-generated outline of the knowledge
base (single ``kb_overview`` row) is read per turn (one indexed PK
lookup — no LLM call) and injected into **both** prompts as the
``<knowledge_base>`` section, ordered ``<relevance>`` →
``<knowledge_base>`` → ``<tuning>`` → mode body. With an empty row the
prompts stay byte-identical to the pre-phase text (phase 15
convention); ``TurnPlan.kb_chars`` records the length of the stored
outline (0 when absent) and the per-turn log line records
``kb_chars=N`` after ``tuning=N`` (PLAN §9 line extension).
"""
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.api.steering import load_steering_notes
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,
    StreamPiece,  # noqa: F401  (phase 17 typing: chat_stream yields StreamPiece)
)
from app.rag.overview import load_kb_overview
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,
    ChatErrorEvent,
    ChatRequest,
    ChatThinkingEvent,
    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  # best cosine across candidates (query_log.top_score)
    fts_hits: int  # lexical (OR-tsquery) candidates matched
    deflected: bool
    system_prompt: str
    docs: list[Document]  # cited sources (weak hits when deflected)
    suggestions: list[str]  # "Maybe try" chips (deflected turns only)
    tuning_count: int = 0  # steering notes injected into the system prompt
    #: Hit chunks with ``is_summary`` whose parent document made it into
    #: *docs* (phase 30; per-turn log line ``summary_hits=N``).
    summary_hits: int = 0
    #: Length of the stored KB overview injected as the
    #: ``<knowledge_base>`` section (phase 31; per-turn log line
    #: ``kb_chars=N``). 0 when no non-empty row exists.
    kb_chars: int = 0


def plan_turn(
    chunks: Sequence[RetrievedChunk],
    settings: Settings,
    notes: Sequence[str] | None = None,
    kb_overview: str | None = None,
) -> TurnPlan:
    """Apply the honesty gate (A8, revised) and assemble prompt + context.

    * **HIGH (grounded)** when ``best_cosine >= threshold`` **or**
      ``fts_hits > 0``: HIGH prompt with the full top-N documents, no
      suggestions. A cosine exactly at the threshold is an answer — the
      gate is strict (``< threshold``).
    * **LOW (deflected)** only when ``best_cosine < threshold`` **and**
      ``fts_hits == 0`` (or no hits at all): LOW prompt (``DEFLECT_MODE``)
      with weak-hit titles only — never document content — plus
      deterministic alternative-question chips derived from those titles.

    ``top_score`` (stored in ``query_log``) is the best cosine, so the
    gate input is always a pure vector-similarity number; the lexical
    signal is recorded separately as ``fts_hits``.

    *notes* are the owner's steering notes (phase 15, oldest first):
    when non-empty, both the HIGH and the LOW prompt carry the
    ``<tuning>`` section; with no notes the prompts are unchanged.

    *kb_overview* is the stored KB outline (phase 31, one PK lookup per
    turn): when non-empty, both prompts carry the ``<knowledge_base>``
    section (between ``<relevance>`` and ``<tuning>``) and
    ``kb_chars`` records the outline's length; with no outline the
    prompts are byte-identical to the pre-phase text and ``kb_chars``
    is 0.

    ``summary_hits`` (phase 30) counts the hit chunks with
    ``is_summary`` whose parent document is among the selected
    top-N documents — both the HIGH and the LOW branch record it.
    """
    steering = list(notes or [])
    kb_text = (kb_overview or "").strip()
    kb_chars = len(kb_text)
    best_cosine = max((c.cosine for c in chunks), default=0.0)
    fts_hits = sum(1 for c in chunks if c.fts_hit)
    docs = select_documents(chunks, n=settings.top_n_docs)
    selected_ids = {d.id for d in docs}
    summary_hits = sum(1 for c in chunks if c.is_summary and c.document.id in selected_ids)
    if best_cosine >= settings.relevance_threshold or fts_hits > 0:
        return TurnPlan(
            best_cosine,
            fts_hits,
            False,
            build_high_prompt(docs, notes=steering, kb_overview=kb_text),
            docs,
            [],
            len(steering),
            summary_hits,
            kb_chars,
        )
    titles = weak_hit_titles(chunks)
    return TurnPlan(
        best_cosine,
        fts_hits,
        True,
        build_deflect_prompt(titles, notes=steering, kb_overview=kb_text),
        docs,
        derive_suggestions(titles, settings.suggestions),
        len(steering),
        summary_hits,
        kb_chars,
    )


@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:
            embed_ms = int((time.monotonic() - t0) * 1000)
            total_ms = int((time.monotonic() - started) * 1000)
            logger.error(
                "chat: question=%r embed_ms=%d total_ms=%d — embedding failed: %s",
                request.message,
                embed_ms,
                total_ms,
                e,
            )
            yield sse_event(
                ChatErrorEvent(
                    detail="I couldn't reach the embedding model — please try again."
                ).model_dump()
            )
            return
        embed_ms = int((time.monotonic() - t0) * 1000)

        # 2. Retrieve top-K chunks, load the owner's steering notes
        #    (phase 15), then the honesty gate (A8) picks the HIGH
        #    (grounded) or LOW (deflected) prompt + context.
        settings = get_settings()
        try:
            steering_notes = load_steering_notes(db)
            # KB overview (phase 31): one indexed PK lookup per turn — the
            # outline is generated at import time, never per chat turn.
            kb_overview = load_kb_overview(db)
            chunks = retrieve(db, request.message, question_vec)
            plan = plan_turn(chunks, settings, notes=steering_notes, kb_overview=kb_overview)
        except Exception:  # noqa: BLE001 — DB failure mid-turn
            logger.exception(
                "chat: retrieval failed question=%r total_ms=%d",
                request.message,
                int((time.monotonic() - started) * 1000),
            )
            yield sse_event(
                ChatErrorEvent(
                    detail="The knowledge base went offline mid-question — is Postgres up?"
                ).model_dump()
            )
            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).
        #    Phase 17: thinking pieces stream as ``thinking`` events
        #    ahead of the ``delta`` events (PLAN §4 extension); the
        #    kill-switch (``BOR_STREAM_THINKING=0``) suppresses the
        #    frames, not the counting.
        thinking_chars = 0
        try:
            async for piece in llm.chat_stream(messages):  # StreamPiece (phase 17)
                if piece.kind == "thinking":
                    thinking_chars += len(piece.text)
                    if settings.stream_thinking:
                        yield sse_event(ChatThinkingEvent(text=piece.text).model_dump())
                else:
                    yield sse_event({"type": "delta", "text": piece.text})
        except LLMError as e:
            logger.error(
                "chat: LLM stream failed question=%r total_ms=%d — %s",
                request.message,
                int((time.monotonic() - started) * 1000),
                e,
            )
            yield sse_event(
                ChatErrorEvent(
                    detail="The chat model dropped the connection — try again?"
                ).model_dump()
            )
            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,
                    fts_hits=plan.fts_hits,
                    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 fts_hits=%d summary_hits=%d tuning=%d "
            "kb_chars=%d threshold=%.2f deflected=%s sources=%r thinking_chars=%d total_ms=%d",
            request.message,
            embed_ms,
            plan.top_score,
            plan.fts_hits,
            plan.summary_hits,
            plan.tuning_count,
            plan.kb_chars,
            settings.relevance_threshold,
            plan.deflected,
            source_paths,
            thinking_chars,
            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)