"""Prometheus metrics for FastAPI + optional DB query metrics."""

from __future__ import annotations

import time
from typing import Optional

from fastapi import FastAPI, Request, Response

try:
    from prometheus_client import Counter, Histogram, generate_latest, CONTENT_TYPE_LATEST
except Exception:  # pragma: no cover
    Counter = None  # type: ignore
    Histogram = None  # type: ignore
    generate_latest = None  # type: ignore
    CONTENT_TYPE_LATEST = "text/plain; version=0.0.4"  # type: ignore


_instrumented = False
_db_instrumented = False


def _get_route_template(request: Request) -> str:
    route = request.scope.get("route")
    if route is not None and hasattr(route, "path"):
        return str(route.path)
    return request.url.path


def instrument_app(app: FastAPI, *, metrics_path: str = "/metrics", enable_db_metrics: bool = False) -> None:
    """
    Add Prometheus request metrics and a `/metrics` endpoint.

    - Low-cardinality path labels by using FastAPI route templates.
    - Optional SQLAlchemy query timing metrics (off by default).
    """
    global _instrumented
    if _instrumented:
        return
    _instrumented = True

    if Counter is None or Histogram is None:
        # Dependency not installed; keep app working without metrics.
        return

    http_requests_total = Counter(
        "http_requests_total",
        "Total HTTP requests",
        ["method", "path", "status"],
    )
    http_request_duration_seconds = Histogram(
        "http_request_duration_seconds",
        "HTTP request duration (seconds)",
        ["method", "path"],
        buckets=(0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10),
    )

    @app.middleware("http")
    async def _metrics_middleware(request: Request, call_next):
        start = time.perf_counter()
        response: Optional[Response] = None
        try:
            response = await call_next(request)
            return response
        finally:
            duration = time.perf_counter() - start
            path = _get_route_template(request)
            method = request.method
            status = str(getattr(response, "status_code", 500))
            http_requests_total.labels(method=method, path=path, status=status).inc()
            http_request_duration_seconds.labels(method=method, path=path).observe(duration)

    @app.get(metrics_path, include_in_schema=False)
    async def _metrics_endpoint():
        return Response(generate_latest(), media_type=CONTENT_TYPE_LATEST)

    if enable_db_metrics:
        _instrument_db_metrics()


def _instrument_db_metrics() -> None:
    """Attach SQLAlchemy event listeners to track query latencies."""
    global _db_instrumented
    if _db_instrumented:
        return
    _db_instrumented = True

    if Counter is None or Histogram is None:
        return

    from sqlalchemy import event
    from app.database import engine

    db_queries_total = Counter(
        "db_queries_total",
        "Total DB queries executed",
        ["dialect"],
    )
    db_query_duration_seconds = Histogram(
        "db_query_duration_seconds",
        "DB query duration (seconds)",
        ["dialect"],
        buckets=(0.001, 0.0025, 0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5),
    )

    dialect = engine.sync_engine.dialect.name

    @event.listens_for(engine.sync_engine, "before_cursor_execute")
    def _before_cursor_execute(conn, cursor, statement, parameters, context, executemany):  # type: ignore[no-untyped-def]
        conn.info.setdefault("_query_start_time", []).append(time.perf_counter())

    @event.listens_for(engine.sync_engine, "after_cursor_execute")
    def _after_cursor_execute(conn, cursor, statement, parameters, context, executemany):  # type: ignore[no-untyped-def]
        start_list = conn.info.get("_query_start_time") or []
        if not start_list:
            return
        start = start_list.pop()
        duration = time.perf_counter() - start
        db_queries_total.labels(dialect=dialect).inc()
        db_query_duration_seconds.labels(dialect=dialect).observe(duration)