"""
Subject taxonomy: curated hierarchical Topics and community Tags.

* ``Topic`` — a curated tree (e.g. Mathematics → Algebra → Polynomials)
  maintained by moderators; courses and problems hang off topics for
  navigation and prerequisite graphs.
* ``Tag`` — flat, community-applied labels for fine-grained discovery
  (e.g. "induction", "dynamic-programming"). ``usage_count`` is denormalized
  and maintained by the service layer for fast popular-tag queries.
"""
from django.conf import settings
from django.db import models

from apps.core.models import TimeStampedModel, UUIDModel


class Topic(UUIDModel, TimeStampedModel):
    parent = models.ForeignKey(
        "self",
        on_delete=models.PROTECT,
        null=True,
        blank=True,
        related_name="children",
    )
    name = models.CharField(max_length=120)
    slug = models.SlugField(max_length=140, unique=True)
    description = models.TextField(blank=True)
    icon = models.CharField(
        max_length=64, blank=True, help_text="Icon identifier for the frontend."
    )
    order = models.PositiveIntegerField(
        default=0, help_text="Sort position among siblings."
    )

    class Meta:
        ordering = ["order", "name"]
        indexes = [
            models.Index(fields=["parent", "order"], name="topic_parent_order_idx"),
        ]
        constraints = [
            models.CheckConstraint(
                condition=models.Q(parent__isnull=True)
                | ~models.Q(parent=models.F("id")),
                name="topic_not_own_parent",
            ),
        ]

    def __str__(self) -> str:
        return self.name


class Tag(UUIDModel, TimeStampedModel):
    name = models.CharField(max_length=80)
    slug = models.SlugField(max_length=100, unique=True)
    description = models.TextField(blank=True)
    topic = models.ForeignKey(
        Topic, on_delete=models.SET_NULL, null=True, blank=True, related_name="tags"
    )
    created_by = models.ForeignKey(
        settings.AUTH_USER_MODEL,
        on_delete=models.SET_NULL,
        null=True,
        blank=True,
        related_name="+",
    )
    is_curated = models.BooleanField(
        default=False, help_text="Curated tags are protected from renaming/merging."
    )
    usage_count = models.PositiveIntegerField(default=0, db_index=True)

    class Meta:
        ordering = ["-usage_count", "name"]

    def __str__(self) -> str:
        return self.name