"""Handle (localpart) validation per SPEC §4.

Rules (enforced in code, mail-grade from day one):
  - localpart: 3-30 chars, characters ``[a-z0-9._-]``
  - no leading/trailing dot, no double dot
  - case-insensitive (normalize to lowercase)
  - not in the reserved list (:mod:`app.identity.reserved`)

The stored handle is the full form ``localpart@{APP_DOMAIN}``.
"""

from __future__ import annotations

import re

from app.config import get_settings
from app.identity.reserved import is_reserved

_LOCALPART_RE = re.compile(r"^[a-z0-9._-]+$")
_MIN_LEN = 3
_MAX_LEN = 30


class InvalidHandle(ValueError):
    """Raised when a proposed localpart violates the SPEC §4 rules."""

    def __init__(self, code: str, message: str) -> None:
        super().__init__(message)
        self.code = code
        self.message = message


def normalize_localpart(localpart: str) -> str:
    """Lowercase and strip surrounding whitespace (case-insensitive handles)."""
    return localpart.strip().lower()


def validate_localpart(localpart: str) -> str:
    """Validate and normalize ``localpart``; return it or raise :class:`InvalidHandle`."""
    lp = normalize_localpart(localpart)
    if not (_MIN_LEN <= len(lp) <= _MAX_LEN):
        raise InvalidHandle("length", f"handle must be {_MIN_LEN}-{_MAX_LEN} characters")
    if not _LOCALPART_RE.match(lp):
        raise InvalidHandle("charset", "handle may contain only a-z, 0-9, '.', '_', '-'")
    if lp.startswith(".") or lp.endswith("."):
        raise InvalidHandle("dot_edge", "handle may not start or end with a dot")
    if ".." in lp:
        raise InvalidHandle("double_dot", "handle may not contain consecutive dots")
    if is_reserved(lp):
        raise InvalidHandle("reserved", "that handle is reserved")
    return lp


def full_handle(localpart: str) -> str:
    """Return the full handle ``localpart@{APP_DOMAIN}`` for a validated localpart."""
    return f"{validate_localpart(localpart)}@{get_settings().app_domain}"