"""
Slug Validation Utility

Validates slugs used for subdomains, database names, directories, and usernames.
"""

import re

# Slugs reserved for infrastructure, services, or common subdomains.
RESERVED_SLUGS = frozenset({
    'www', 'mail', 'ftp', 'admin', 'api', 'chat', 'store', 'test',
    'app', 'blog', 'dev', 'staging', 'smtp', 'pop', 'imap', 'ns1',
    'ns2', 'dns', 'cdn', 'vpn', 'ssh', 'sftp', 'webmail', 'cpanel',
    'dashboard', 'login', 'signup', 'register', 'support', 'help',
    'status', 'docs',
})

# Pattern: starts and ends with [a-z0-9], allows hyphens in middle.
SLUG_PATTERN = re.compile(r'^[a-z0-9]([a-z0-9-]*[a-z0-9])?$')

MIN_LENGTH = 3
MAX_LENGTH = 20


def validate_slug_format(slug: str) -> dict:
    """
    Validate a slug's format.

    Returns:
        dict with 'valid' (bool) and 'message' (str) keys.
    """
    if len(slug) < MIN_LENGTH:
        return {'valid': False, 'message': 'Slug must be at least 3 characters long.'}

    if len(slug) > MAX_LENGTH:
        return {'valid': False, 'message': 'Slug must be no more than 20 characters long.'}

    if not SLUG_PATTERN.match(slug):
        return {
            'valid': False,
            'message': 'Slug must contain only lowercase letters, numbers, and hyphens, '
                       'and must start and end with a letter or number.',
        }

    if '--' in slug:
        return {'valid': False, 'message': 'Slug must not contain consecutive hyphens.'}

    if slug in RESERVED_SLUGS:
        return {'valid': False, 'message': f'The slug "{slug}" is reserved and cannot be used.'}

    return {'valid': True, 'message': ''}


def slug_to_db_name(slug: str) -> str:
    """Convert a slug to a valid MySQL database name component by replacing hyphens with underscores."""
    return slug.replace('-', '_')