"""Background auto-review worker.

A daemon thread polls the PR queue on an interval and pre-reviews any new or
changed non-draft PR into the SQLite cache, so a review is ready by the time you
open it in the dashboard. Nothing is ever posted automatically.

Change detection is two-level to keep token spend down:
  1. cheap gate  - skip if the PR's ``updated_at`` is unchanged since the last
     ready review (no diff fetched at all);
  2. precise gate - if it did change, fetch the diff and skip the LLM call when
     the diff's SHA-256 matches (e.g. the PR was only commented on).

``run_cycle`` takes its GitHub client, reviewer, and store as parameters so it is
directly testable with fakes; the ``Worker`` class only owns the thread/timing.
"""

from __future__ import annotations

import hashlib
import json
import logging
import random
import threading
import time
from concurrent.futures import ThreadPoolExecutor, as_completed
from typing import Any, Callable

# Keep a review for a closed/merged PR this long before hard-deleting it, so an
# open dashboard doesn't lose a review the moment the PR merges.
CLOSED_GRACE_SECONDS = 600

_log = logging.getLogger("pr_reviewer.worker")


def _sha256(text: str) -> str:
    return hashlib.sha256(text.encode("utf-8", "ignore")).hexdigest()


def _is_rate_limited(exc: Exception) -> bool:
    resp = getattr(exc, "response", None)
    return getattr(resp, "status_code", None) == 429


def _review_with_backoff(
    reviewer: Any,
    pr: dict[str, Any],
    diff: str,
    *,
    retries: int = 3,
    base: float = 2.0,
    sleep: Callable[[float], None] = time.sleep,
) -> dict[str, Any]:
    """Call the reviewer, backing off on HTTP 429 with exponential delay +
    jitter. Non-429 errors (and the final 429) propagate to the caller."""
    for attempt in range(retries + 1):
        try:
            return reviewer.review(pr, diff)
        except Exception as e:  # noqa: BLE001 - re-raised below unless retriable
            if _is_rate_limited(e) and attempt < retries:
                sleep(base * (2**attempt) + random.uniform(0, 1))
                continue
            raise


def _review_one(
    gh: Any,
    reviewer: Any,
    store: Any,
    pr: dict[str, Any],
    sleep: Callable[[float], None],
) -> str:
    """Review a single PR and persist the outcome. Returns an outcome label and
    never raises (failures are stored as ``error`` rows)."""
    owner, repo, number = pr["owner"], pr["repo"], pr["number"]
    meta = {
        "pr_title": pr.get("title"),
        "pr_author": pr.get("author"),
        "pr_url": pr.get("url"),
        "pr_updated_at": pr.get("updated_at"),
        "pr_created_at": pr.get("created_at"),
        "pr_node_id": pr.get("node_id"),
    }
    try:
        diff = gh.pr_diff(owner, repo, number)
        diff_hash = _sha256(diff)
        existing = store.get(owner, repo, number)
        if (
            existing
            and existing.get("diff_hash") == diff_hash
            and existing.get("review_json")
        ):
            # Diff unchanged (updated_at moved for a non-code reason) and we
            # already have a review: keep it, just mark ready and refresh metadata
            # so the cheap gate catches it next time. (Status is "reviewing" here
            # because run_cycle marks queued PRs before handing them off.)
            store.upsert(owner, repo, number, status="ready", **meta)
            return "unchanged"
        review = _review_with_backoff(reviewer, pr, diff, sleep=sleep)
        store.upsert(
            owner,
            repo,
            number,
            status="ready",
            diff_hash=diff_hash,
            review_json=json.dumps(review),
            error=None,
            attempts=0,
            **meta,
        )
        return "reviewed"
    except Exception as e:  # noqa: BLE001 - stored as an error row, surfaced in UI
        existing = store.get(owner, repo, number)
        attempts = (existing.get("attempts", 0) if existing else 0) + 1
        store.upsert(
            owner,
            repo,
            number,
            status="error",
            error=str(e),
            attempts=attempts,
            **meta,
        )
        return "error"


def run_cycle(
    gh: Any,
    reviewer: Any,
    store: Any,
    *,
    concurrency: int = 2,
    max_attempts: int = 3,
    grace_seconds: float = CLOSED_GRACE_SECONDS,
    sleep: Callable[[float], None] = time.sleep,
    handbook: Any = None,
) -> dict[str, int]:
    """One poll cycle: refresh handbook guidance if stale, refresh the queue,
    close/purge departed PRs, and review every new or changed non-draft PR
    (bounded concurrency). Returns per-outcome counts."""
    if handbook is not None:
        handbook.refresh_if_stale()  # daily in-process trigger; no-op when fresh

    prs = gh.search_prs()
    active = [p for p in prs if not p.get("draft")]
    live_keys = {(p["owner"], p["repo"], p["number"]) for p in active}
    store.mark_missing_closed(live_keys)
    store.purge_closed(grace_seconds)

    todo: list[dict[str, Any]] = []
    for p in active:
        existing = store.get(p["owner"], p["repo"], p["number"])
        if existing:
            same_update = existing.get("pr_updated_at") == p.get("updated_at")
            if existing.get("status") == "ready" and same_update:
                # Cheap gate: no re-review. But backfill lightweight PR metadata
                # (e.g. node_id/created_at added in a later release) when it
                # actually differs, so features like auto-merge have current data
                # without re-running the model. No-op write when nothing differs.
                if existing.get("pr_node_id") != p.get("node_id") or existing.get(
                    "pr_created_at"
                ) != p.get("created_at"):
                    store.upsert(
                        p["owner"],
                        p["repo"],
                        p["number"],
                        status="ready",
                        pr_node_id=p.get("node_id"),
                        pr_created_at=p.get("created_at"),
                    )
                continue
            if (
                existing.get("status") == "error"
                and same_update
                and existing.get("attempts", 0) >= max_attempts
            ):
                continue  # keeps failing: wait until the PR itself changes
        todo.append(p)

    # Mark queued PRs as reviewing up front so the UI reflects in-flight work.
    for p in todo:
        store.upsert(
            p["owner"],
            p["repo"],
            p["number"],
            status="reviewing",
            pr_title=p.get("title"),
            pr_author=p.get("author"),
            pr_url=p.get("url"),
            pr_updated_at=p.get("updated_at"),
            pr_created_at=p.get("created_at"),
            pr_node_id=p.get("node_id"),
        )

    results = {
        "reviewed": 0,
        "unchanged": 0,
        "error": 0,
        "skipped": len(active) - len(todo),
    }
    if todo:
        with ThreadPoolExecutor(max_workers=max(1, concurrency)) as ex:
            futures = [
                ex.submit(_review_one, gh, reviewer, store, p, sleep) for p in todo
            ]
            for fut in as_completed(futures):
                results[fut.result()] += 1
    return results


class Worker:
    """Owns the poll thread and its timing; delegates the work to ``run_cycle``."""

    def __init__(
        self, cfg: Any, gh: Any, reviewer: Any, store: Any, handbook: Any = None
    ) -> None:
        self.cfg = cfg
        self.gh = gh
        self.reviewer = reviewer
        self.store = store
        self.handbook = handbook
        self._stop = threading.Event()
        self._wake = threading.Event()
        self._thread: threading.Thread | None = None

    def start(self) -> None:
        n = self.store.reset_stale_reviewing()
        if n:
            _log.info("reset %d interrupted review(s) from a prior run", n)
        self._thread = threading.Thread(
            target=self._loop, name="review-worker", daemon=True
        )
        self._thread.start()

    def trigger(self) -> None:
        """Ask the loop to run a cycle now (non-blocking)."""
        self._wake.set()

    def stop(self) -> None:
        self._stop.set()
        self._wake.set()
        if self._thread:
            self._thread.join(timeout=5)

    def _loop(self) -> None:
        while not self._stop.is_set():
            try:
                res = run_cycle(
                    self.gh,
                    self.reviewer,
                    self.store,
                    concurrency=self.cfg.WORKER_CONCURRENCY,
                    max_attempts=self.cfg.MAX_REVIEW_ATTEMPTS,
                    handbook=self.handbook,
                )
                _log.info("review cycle: %s", res)
            except Exception:
                _log.exception("review cycle failed")
            self._wake.wait(timeout=self.cfg.POLL_INTERVAL)
            self._wake.clear()