"""Slug value object for URL-friendly identifiers.

This module defines the Slug value object for generating and validating
URL-friendly slugs from titles. Enforces lowercase, alphanumeric, and
hyphen-only format.
"""

import re
from dataclasses import dataclass

from app.domain.value_objects.base import ValueObject


@dataclass(frozen=True, slots=True)
class Slug(ValueObject[str]):
    """URL slug value object.

    Represents a URL-friendly identifier generated from titles.
    Validates format and provides factory method for slug generation.

    Attributes:
        value: The slug string value.
        MAX_LENGTH: Maximum allowed slug length (200 characters).
        SLUG_PATTERN: Regex pattern for valid slug format.

    Raises:
        ValueError: If slug format is invalid.

    Example:
        >>> slug = Slug.from_title("My First Post!")
        >>> print(slug.value)
        'my-first-post'
    """

    MAX_LENGTH: int = 200
    SLUG_PATTERN: str = r"^[a-z0-9]+(?:-[a-z0-9]+)*$"

    def _validate(self) -> None:
        """Validate slug format.

        Ensures slug contains only lowercase letters, numbers, and hyphens.

        Raises:
            ValueError: If slug format is invalid.
        """
        if not isinstance(self.value, str):
            raise ValueError("Slug must be a string")
        if len(self.value) > self.MAX_LENGTH:
            raise ValueError(f"Slug must be at most {self.MAX_LENGTH} characters")
        if not re.match(self.SLUG_PATTERN, self.value):
            raise ValueError("Slug must contain only lowercase letters, numbers, and hyphens")

    @classmethod
    def from_title(cls, title: str) -> "Slug":
        """Generate slug from title.

        Converts title to URL-friendly format by lowercasing, removing
        special characters, and replacing spaces with hyphens.

        Args:
            title: Source title string.

        Returns:
            New Slug instance with generated value.
        """
        slug = title.lower().strip()
        slug = re.sub(r"[^a-z0-9\s-]", "", slug)
        slug = re.sub(r"[-\s]+", "-", slug)
        max_len = 200
        slug = slug[:max_len].strip("-")
        if not slug:
            slug = "post"
        return cls(value=slug)