"""Chat creation, messaging and the streaming reply endpoint."""

from __future__ import annotations

import asyncio
import logging
import time
from collections.abc import AsyncIterator

from fastapi import APIRouter, Depends, Form, HTTPException, Request, status
from fastapi.responses import HTMLResponse, Response, StreamingResponse
from sqlalchemy.orm import Session as DBSession

from lembas.api.deps import Db, RequiredUser, require_permission
from lembas.db.models import ROLE_ASSISTANT, ROLE_USER, Chat, Message, User
from lembas.db.session import session_scope
from lembas.security import permissions
from lembas.services import chat as chat_service
from lembas.services import sse
from lembas.services.llm.openai_client import (
    LLMError,
    delta_reasoning,
    delta_text,
    stream_chat,
)
from lembas.services.markdown import escape_text, render_markdown
from lembas.services.reasoning import REASONING, ReasoningSplitter
from lembas.web.templating import render, templates

log = logging.getLogger(__name__)

router = APIRouter(prefix="/api/chats", tags=["chats"])


def _owned_chat(db: DBSession, chat_id: str, user_id: str) -> Chat:
    chat = db.get(Chat, chat_id)
    # 404 rather than 403 for someone else's chat: whether a given id exists is
    # not information this endpoint should hand out.
    if chat is None or chat.user_id != user_id:
        raise HTTPException(status.HTTP_404_NOT_FOUND, "That chat no longer exists.")
    return chat


@router.post("", dependencies=[Depends(require_permission("chat.create"))])
async def create_chat(db: Db, user: RequiredUser, folder_id: str = Form("")) -> Response:
    chosen = chat_service.default_model(db, user)
    chat = Chat(
        user_id=user.id,
        folder_id=folder_id or None,
        model_id=chosen[0] if chosen else "",
        connection_id=chosen[1] if chosen else None,
    )
    db.add(chat)
    db.commit()

    # HX-Redirect rather than a swap: a new chat is a new URL, and the address
    # bar has to follow so the chat can be reloaded or bookmarked.
    response = Response(status_code=status.HTTP_204_NO_CONTENT)
    response.headers["HX-Redirect"] = f"/chat/{chat.id}"
    return response


@router.post("/{chat_id}/messages")
async def post_message(
    request: Request,
    db: Db,
    user: RequiredUser,
    chat_id: str,
    content: str = Form(...),
) -> Response:
    """Persist the user's turn and hand back the pair of bubbles.

    The assistant bubble comes back empty, carrying the sse-connect attribute
    that opens the stream below. Splitting it this way means the POST returns
    immediately and the slow part is a separate, resumable connection.
    """
    chat = _owned_chat(db, chat_id, user.id)

    content = content.strip()
    if not content:
        return Response(status_code=status.HTTP_204_NO_CONTENT)

    user_message = chat_service.create_message(db, chat, ROLE_USER, content)
    assistant_message = chat_service.create_message(
        db, chat, ROLE_ASSISTANT, "", complete_=False, model_id=chat.model_id
    )

    # `user` is required by the shared message template, which renders both
    # roles; without it the user bubble's initial blows up.
    return templates.TemplateResponse(
        request,
        "chat/_turn.html",
        {
            "request": request,
            "user_message": user_message,
            "assistant_message": assistant_message,
            "chat": chat,
            "user": user,
        },
    )


@router.get("/{chat_id}/messages/{message_id}/stream")
async def stream_message(
    db: Db,
    user: RequiredUser,
    chat_id: str,
    message_id: str,
) -> Response:
    """Stream the assistant's reply as server-sent events.

    Emits `token` events carrying escaped text, then a single `done` event
    carrying the finished bubble rendered from Markdown, then `close`.
    """
    chat = _owned_chat(db, chat_id, user.id)
    message = db.get(Message, message_id)
    if message is None or message.chat_id != chat.id:
        raise HTTPException(status.HTTP_404_NOT_FOUND, "That message no longer exists.")

    return StreamingResponse(
        _generate(chat.id, message.id),
        media_type="text/event-stream",
        headers={
            "Cache-Control": "no-cache, no-transform",
            "Connection": "keep-alive",
            # nginx buffers proxied responses by default, which turns a stream
            # into one delivery at the end. This is the documented opt-out.
            "X-Accel-Buffering": "no",
        },
    )


async def _generate(chat_id: str, message_id: str) -> AsyncIterator[str]:
    """Drive one completion and frame it as SSE.

    Opens its own database session rather than using the request's: streaming
    outlives the request handler, and the dependency-scoped session may already
    be closed by the time the first token arrives.
    """
    accumulated: list[str] = []
    thinking: list[str] = []
    error: str | None = None
    reasoning_ms = 0

    with session_scope() as db:
        chat = db.get(Chat, chat_id)
        message = db.get(Message, message_id)
        if chat is None or message is None:
            yield sse.event("close", "")
            return

        first_user_text = ""
        # Handles models that emit <think> tags inline in content rather than
        # using the reasoning_content field.
        splitter = ReasoningSplitter()
        started = time.monotonic()
        reasoning_started: float | None = None

        try:
            endpoint, model_id = chat_service.resolve_endpoint(db, chat)
            payload = chat_service.build_request(db, chat, upto=message)
            first_user_text = next(
                (m["content"] for m in reversed(payload["messages"]) if m["role"] == ROLE_USER),
                "",
            )

            async for chunk in stream_chat(endpoint, payload):
                # A dedicated reasoning field is unambiguous; take it as-is.
                thought = delta_reasoning(chunk)
                if thought:
                    if reasoning_started is None:
                        reasoning_started = time.monotonic()
                    thinking.append(thought)
                    yield sse.event("reasoning", escape_text(thought))
                    await asyncio.sleep(0)

                text = delta_text(chunk)
                if not text:
                    continue

                for kind, piece in splitter.feed(text):
                    if kind == REASONING:
                        if reasoning_started is None:
                            reasoning_started = time.monotonic()
                        thinking.append(piece)
                        yield sse.event("reasoning", escape_text(piece))
                    else:
                        # First answer token ends the thinking phase.
                        if reasoning_started is not None and not reasoning_ms:
                            reasoning_ms = int((time.monotonic() - reasoning_started) * 1000)
                        accumulated.append(piece)
                        yield sse.event("token", escape_text(piece))
                    # Hand control back so the event is flushed rather than
                    # batched behind a fast generator.
                    await asyncio.sleep(0)

            for kind, piece in splitter.flush():
                if kind == REASONING:
                    thinking.append(piece)
                    yield sse.event("reasoning", escape_text(piece))
                else:
                    accumulated.append(piece)
                    yield sse.event("token", escape_text(piece))

        except LLMError as exc:
            error = exc.message
            log.info("generation failed for chat %s: %s", chat_id, exc.message)
        except asyncio.CancelledError:
            # The reader navigated away or closed the tab. Keep whatever was
            # produced so the partial reply is still there on reload.
            message.content = "".join(accumulated)
            message.reasoning = "".join(thinking)
            message.complete = True
            db.commit()
            raise
        except Exception as exc:  # noqa: BLE001 - must not kill the stream silently
            error = "Something went wrong while generating this reply."
            log.exception("unexpected generation failure for chat %s: %s", chat_id, exc)

        if reasoning_started is not None and not reasoning_ms:
            # Reasoning ran to the end without an answer following it.
            reasoning_ms = int((time.monotonic() - reasoning_started) * 1000)

        message.content = "".join(accumulated)
        message.reasoning = "".join(thinking)
        message.reasoning_ms = reasoning_ms
        message.error = error or ""
        message.complete = True
        log.debug(
            "chat %s: %d chars answer, %d chars reasoning, %.1fs total",
            chat_id,
            len(message.content),
            len(message.reasoning),
            time.monotonic() - started,
        )

        if not chat.title_generated and (accumulated or error):
            chat.title = (
                await chat_service.generate_title(
                    endpoint, model_id, first_user_text, message.content
                )
                if not error and first_user_text
                else chat_service.fallback_title(first_user_text)
            )
            chat.title_generated = True

        db.commit()

        final_html = templates.get_template("chat/_message.html").render(
            {
                "message": message,
                "body_html": render_markdown(message.content),
                "chat": chat,
                # Passed even though an assistant bubble never reads it: the
                # template shares both roles, and a missing `user` would only
                # blow up on whichever branch is not being exercised here.
                "user": db.get(User, chat.user_id),
            }
        )
        title_html = templates.get_template("chat/_title_oob.html").render(
            {"chat": chat}
        )

    yield sse.event("done", final_html + title_html)
    yield sse.event("close", "")


@router.patch("/{chat_id}")
async def update_chat(request: Request, db: Db, user: RequiredUser, chat_id: str) -> Response:
    """Partially update a chat.

    The raw form is read rather than declaring Form() parameters because
    FastAPI substitutes the default for an empty form value, which makes
    "field absent" and "field submitted empty" indistinguishable. That
    difference is exactly what this endpoint needs: an empty system prompt or
    temperature means *clear it*, not *leave it alone*.
    """
    chat = _owned_chat(db, chat_id, user.id)
    allowed = permissions.resolve(db, user)
    form = await request.form()

    if "title" in form:
        cleaned = str(form["title"]).strip()[:300]
        if cleaned:
            chat.title = cleaned
            # An explicit rename must not be overwritten by auto-titling later.
            chat.title_generated = True

    if "folder_id" in form:
        chat.folder_id = str(form["folder_id"]) or None

    model_id = str(form.get("model_id", "")).strip()

    if model_id:
        if not allowed.get("chat.model_select"):
            raise HTTPException(
                status.HTTP_403_FORBIDDEN, "You may not change the model for a chat."
            )
        # Checked against what this user can reach, not merely what exists --
        # otherwise the picker is advisory and a crafted request bypasses it.
        match = next(
            (m for m in chat_service.available_models(db, user) if m.model_id == model_id),
            None,
        )
        if match is None:
            raise HTTPException(status.HTTP_403_FORBIDDEN, "That model is not available to you.")
        chat.model_id = model_id
        chat.connection_id = match.connection_id

    if "system_prompt" in form:
        if not allowed.get("chat.system_prompt"):
            raise HTTPException(
                status.HTTP_403_FORBIDDEN, "You may not set a system prompt."
            )
        chat.system_prompt = str(form["system_prompt"]).strip()[:8000]

    submitted_params = {name: form[name] for name in _PARAM_RANGES if name in form}
    if submitted_params:
        if not allowed.get("chat.params"):
            raise HTTPException(
                status.HTTP_403_FORBIDDEN, "You may not change sampling parameters."
            )
        chat.params_json = {
            **(chat.params_json or {}),
            **_clean_params(**{k: str(v) for k, v in submitted_params.items()}),
        }

    db.commit()
    return Response(status_code=status.HTTP_204_NO_CONTENT)


# Bounds are the ones every provider agrees on. Out-of-range values are
# dropped rather than clamped: silently changing what someone typed is worse
# than ignoring it, and the form shows what actually stuck on reload.
_PARAM_RANGES: dict[str, tuple[type, float, float]] = {
    "temperature": (float, 0.0, 2.0),
    "top_p": (float, 0.0, 1.0),
    "max_tokens": (int, 1, 1_000_000),
}


def _clean_params(**submitted: str | None) -> dict[str, float | int | None]:
    """Parse sampling parameters, dropping anything unusable.

    An empty string means "unset this and let the provider default apply", so
    it maps to None rather than being ignored.
    """
    cleaned: dict[str, float | int | None] = {}
    for name, raw in submitted.items():
        if raw is None:
            continue
        if not raw.strip():
            cleaned[name] = None
            continue
        caster, low, high = _PARAM_RANGES[name]
        try:
            value = caster(raw)
        except (TypeError, ValueError):
            continue
        if low <= value <= high:
            cleaned[name] = value
    return cleaned


@router.delete("/{chat_id}", dependencies=[Depends(require_permission("chat.delete"))])
async def delete_chat(db: Db, user: RequiredUser, chat_id: str) -> Response:
    chat = _owned_chat(db, chat_id, user.id)
    db.delete(chat)
    db.commit()

    response = Response(status_code=status.HTTP_204_NO_CONTENT)
    response.headers["HX-Redirect"] = "/chat"
    return response


@router.get("/{chat_id}/messages/{message_id}/raw")
async def raw_message(db: Db, user: RequiredUser, chat_id: str, message_id: str) -> HTMLResponse:
    """The unrendered Markdown of a message, for the copy button."""
    _owned_chat(db, chat_id, user.id)
    message = db.get(Message, message_id)
    if message is None or message.chat_id != chat_id:
        raise HTTPException(status.HTTP_404_NOT_FOUND, "That message no longer exists.")
    return HTMLResponse(escape_text(message.content))


@router.post("/{chat_id}/messages/{message_id}/regenerate")
async def regenerate(
    request: Request,
    db: Db,
    user: RequiredUser,
    chat_id: str,
    message_id: str,
) -> Response:
    """Discard an assistant reply and produce a fresh one in its place."""
    chat = _owned_chat(db, chat_id, user.id)
    message = db.get(Message, message_id)
    if message is None or message.chat_id != chat.id or message.role != ROLE_ASSISTANT:
        raise HTTPException(status.HTTP_404_NOT_FOUND, "That reply no longer exists.")

    message.content = ""
    message.error = ""
    message.complete = False
    message.model_id = chat.model_id
    db.commit()

    return templates.TemplateResponse(
        request,
        "chat/_message.html",
        {"request": request, "message": message, "chat": chat, "body_html": "", "user": user},
    )


__all__ = ["render", "router"]