import os
import threading
import time

import mysql.connector
from mysql.connector import Error, pooling


def _get_required_env(name: str) -> str:
    value = os.getenv(name)
    if value is None or value == "":
        raise RuntimeError(f"Missing required environment variable: {name}")
    return value


# Process-wide MySQL connection pool. Gunicorn forks workers; each worker
# gets its own pool (the global is rebuilt per process at first use).
_pool: "pooling.MySQLConnectionPool | None" = None
_pool_lock = threading.Lock()


def _build_pool() -> "pooling.MySQLConnectionPool":
    config = {
        "host": _get_required_env("DB_HOST"),
        "user": _get_required_env("DB_USER"),
        "password": _get_required_env("DB_PASSWORD"),
        "database": _get_required_env("DB_NAME"),
        "port": int(_get_required_env("DB_PORT")),
        "connection_timeout": int(_get_required_env("DB_CONNECTION_TIMEOUT")),
        "pool_name": "cm_pool",
        "pool_size": int(os.getenv("DB_POOL_SIZE", "8")),
        "pool_reset_session": True,
    }
    return pooling.MySQLConnectionPool(**config)


def _get_pool() -> "pooling.MySQLConnectionPool":
    """Lazily build the per-process pool with retry, then memoize."""
    global _pool
    if _pool is not None:
        return _pool
    with _pool_lock:
        if _pool is not None:
            return _pool
        retries = max(1, int(_get_required_env("DB_CONNECT_RETRIES")))
        delay = float(_get_required_env("DB_CONNECT_RETRY_DELAY"))
        last_err: "Exception | None" = None
        for attempt in range(1, retries + 1):
            try:
                _pool = _build_pool()
                return _pool
            except Error as e:
                last_err = e
                if attempt < retries:
                    print(
                        f"MySQL pool init failed ({e}); "
                        f"retry {attempt}/{retries} in {delay}s..."
                    )
                    time.sleep(delay)
        raise RuntimeError(
            f"Failed to build MySQL pool after {retries} attempts: {last_err}"
        )


def verify_tables_once() -> None:
    """Run once at app startup to confirm schema is present.

    Previously the DB() constructor ran two SHOW TABLES LIKE queries on
    every request — wasted round-trips. Now the check happens once when
    create_app() boots the WSGI app; subsequent requests just rent a
    connection from the pool.
    """
    conn = _get_pool().get_connection()
    try:
        cursor = conn.cursor()
        try:
            cursor.execute("SHOW TABLES LIKE 'acc'")
            if not cursor.fetchone():
                raise Exception("Table 'acc' does not exist")
            cursor.execute("SHOW TABLES LIKE 'user'")
            if not cursor.fetchone():
                raise Exception("Table 'user' does not exist")
        finally:
            cursor.close()
    finally:
        conn.close()


class DB:
    """Thin handle backed by the process-wide MySQL pool.

    Constructing DB() is now ~free — it just touches the (cached) pool.
    Each query()/execute() rents a connection from the pool and returns
    it on completion via conn.close() (which the pool intercepts and
    releases the connection back instead of actually closing it).

    Pool size caps in-flight queries per worker; tune with DB_POOL_SIZE
    (default 8). Two gunicorn workers × pool_size 8 = 16 max
    connections — comfortably under MySQL's default max_connections.
    """

    def __init__(self):
        _get_pool()

    def query(self, sql, params=None):
        conn = _get_pool().get_connection()
        try:
            cursor = conn.cursor(dictionary=True)
            try:
                cursor.execute(sql, params or ())
                return cursor.fetchall()
            finally:
                cursor.close()
        except Error as e:
            print(f"Error executing query: {e}")
            return []
        finally:
            conn.close()

    def execute(self, sql, params=None):
        conn = _get_pool().get_connection()
        try:
            cursor = conn.cursor()
            try:
                cursor.execute(sql, params or ())
                conn.commit()
                return True
            finally:
                cursor.close()
        except Error as e:
            print(f"Error executing query: {e}")
            return False
        finally:
            conn.close()