"""
app/mail.py
───────────
Adaptive SMTP delivery engine.

Three mutually-exclusive modes are selected at runtime based on environment:

  Mode A (Authenticated TLS relay)
    All three MAIL_RELAYHOST + MAIL_RELAYHOST_USERNAME + MAIL_RELAYHOST_PASSWORD
    must be set.  The backend connects with STARTTLS and authenticates.

  Mode B (Simple container/host proxy relay)
    Only MAIL_RELAY_HOST is set (hostname, no credentials).
    Backend opens an unauthenticated SMTP session on port 587 to that host.
    Designed for a companion boky/postfix container in the same Docker network.

  Direct SMTP (fallback)
    No relay variables present.  Backend attempts delivery via localhost:25
    (or any MTA listening locally).
"""
from __future__ import annotations

import logging
import smtplib
from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText
from enum import Enum

from app.config import settings
from app.models import STATUS_DELIVERED, STATUS_FAILED, STATUS_RELAY

logger = logging.getLogger(__name__)


# ─────────────────────────────────────────────────────────────────────────────
#  Mode detection
# ─────────────────────────────────────────────────────────────────────────────

class MailMode(str, Enum):
    MODE_A = "authenticated_relay"
    MODE_B = "proxy_relay"
    DIRECT = "direct_smtp"


def _detect_mode() -> MailMode:
    if (
        settings.MAIL_RELAYHOST
        and settings.MAIL_RELAYHOST_USERNAME
        and settings.MAIL_RELAYHOST_PASSWORD
    ):
        return MailMode.MODE_A
    if settings.MAIL_RELAY_HOST:
        return MailMode.MODE_B
    return MailMode.DIRECT


def _build_message(to_email: str, subject: str, body: str) -> MIMEMultipart:
    msg = MIMEMultipart("alternative")
    msg["Subject"] = subject
    msg["From"]    = f"{settings.MAIL_FROM_NAME} <{settings.MAIL_FROM}>"
    msg["To"]      = to_email
    msg.attach(MIMEText(body, "plain", "utf-8"))
    return msg


# ─────────────────────────────────────────────────────────────────────────────
#  Public send function
# ─────────────────────────────────────────────────────────────────────────────

def send_email(to_email: str, subject: str, body: str) -> str:
    """
    Deliver an email to *to_email* using the configured strategy.

    Returns one of the STATUS_* constants on success.
    Raises on failure so the caller can log STATUS_FAILED.
    """
    mode = _detect_mode()
    msg  = _build_message(to_email, subject, body)
    raw  = msg.as_string()

    try:
        if mode == MailMode.MODE_A:
            # ── Authenticated TLS relay ──────────────────────────────────────
            relay = settings.MAIL_RELAYHOST
            if ":" in relay:
                host, port_str = relay.rsplit(":", 1)
                port = int(port_str)
            else:
                host, port = relay, 587

            with smtplib.SMTP(host, port, timeout=20) as srv:
                srv.ehlo()
                srv.starttls()
                srv.ehlo()
                srv.login(settings.MAIL_RELAYHOST_USERNAME, settings.MAIL_RELAYHOST_PASSWORD)
                srv.sendmail(settings.MAIL_FROM, [to_email], raw)

            logger.info("[MAIL MODE-A] → %s via %s:%d", to_email, host, port)
            return STATUS_DELIVERED

        elif mode == MailMode.MODE_B:
            # ── Unauthenticated container proxy relay ────────────────────────
            host = settings.MAIL_RELAY_HOST
            with smtplib.SMTP(host, 587, timeout=20) as srv:
                srv.ehlo()
                srv.sendmail(settings.MAIL_FROM, [to_email], raw)

            logger.info("[MAIL MODE-B] → %s via %s:587", to_email, host)
            return STATUS_RELAY

        else:
            # ── Direct localhost:25 delivery ─────────────────────────────────
            with smtplib.SMTP("localhost", 25, timeout=20) as srv:
                srv.sendmail(settings.MAIL_FROM, [to_email], raw)

            logger.info("[MAIL DIRECT] → %s via localhost:25", to_email)
            return STATUS_DELIVERED

    except Exception as exc:
        logger.error(
            "[MAIL ERROR] Failed to deliver to %s (mode=%s): %s",
            to_email, mode.value, exc,
            exc_info=True,
        )
        raise