from django.db import IntegrityError, transaction
from django.db.models import Exists, OuterRef
from django.utils import timezone
from django_filters import FilterSet, NumberFilter
from django_filters.rest_framework import DjangoFilterBackend
from rest_framework import generics, permissions, status, viewsets
from rest_framework.decorators import action
from rest_framework.response import Response

from config.mixins import IntegrityConflictMixin
from config.pagination import StandardResultsSetPagination

from .importers import extract_checkpoints
from .models import Resource, ResourceCheckpoint, SkillResource
from .serializers import (
    ResourceAdminSerializer,
    ResourceCheckpointAdminSerializer,
    ResourceDetailSerializer,
    ResourceListSerializer,
    SkillResourceAdminSerializer,
)


class ResourceFilterSet(FilterSet):
    skill = NumberFilter(field_name='skillresource__skill_id', distinct=True)

    class Meta:
        model = Resource
        fields = {
            'provider': ['exact'],
            'type': ['exact'],
            'difficulty_level': ['exact'],
        }


class ResourceListView(generics.ListAPIView):
    # Annotate `has_checkpoints` via an Exists subquery so the serializer can
    # read it as a plain field — avoids an N+1 from `obj.checkpoints.exists()`
    # on every row (.exists() bypasses any prefetch cache).
    queryset = Resource.objects.annotate(
        has_checkpoints=Exists(
            ResourceCheckpoint.objects.filter(resource=OuterRef('pk'))
        )
    )
    serializer_class = ResourceListSerializer
    permission_classes = [permissions.IsAuthenticated]
    filter_backends = [DjangoFilterBackend]
    filterset_class = ResourceFilterSet
    pagination_class = StandardResultsSetPagination


class ResourceDetailView(generics.RetrieveAPIView):
    queryset = Resource.objects.all().prefetch_related('checkpoints')
    serializer_class = ResourceDetailSerializer
    permission_classes = [permissions.IsAuthenticated]

    def get_serializer_context(self):
        """Preload the current user's completed-checkpoint IDs for this resource.

        Without this, the frontend checkpoint checkboxes render unchecked
        on every page load even after the user has ticked them — the biggest
        UX wart flagged in the 7b polish list.
        """
        ctx = super().get_serializer_context()
        resource_id = self.kwargs.get('pk')
        if resource_id and self.request.user.is_authenticated:
            # Import here to avoid a circular import from progress->resources.
            from apps.progress.models import UserCheckpointProgress
            ctx['completed_checkpoint_ids'] = set(
                UserCheckpointProgress.objects
                .filter(
                    user=self.request.user,
                    checkpoint__resource_id=resource_id,
                    completed_at__isnull=False,
                )
                .values_list('checkpoint_id', flat=True)
            )
        return ctx


class ResourceAdminViewSet(IntegrityConflictMixin, viewsets.ModelViewSet):
    """Admin-only CRUD on Resource. Cascade (model-level) cleans up linked
    SkillResource and ResourceCheckpoint rows on delete — but a delete is
    refused (409) while any student has tracked progress on the resource, so
    deleting never silently discards a learner's history.
    """
    queryset = Resource.objects.all().order_by('title')
    serializer_class = ResourceAdminSerializer
    permission_classes = [permissions.IsAuthenticated, permissions.IsAdminUser]
    pagination_class = StandardResultsSetPagination
    conflict_detail = 'A resource with this URL already exists.'

    def destroy(self, request, *args, **kwargs):
        resource = self.get_object()
        # Import locally (matches ResourceDetailView's pattern) and to keep the
        # resources<->progress dependency one-directional. UserProgress is the
        # sufficient signal: CheckpointToggleView always get_or_creates a
        # UserProgress before any UserCheckpointProgress, so UCP ⟹ UP.
        from apps.progress.models import UserProgress
        if UserProgress.objects.filter(resource=resource).exists():
            return Response(
                {'detail': (
                    'Cannot delete: one or more students have tracked progress '
                    'on this resource. Unlink it from roles instead.'
                )},
                status=status.HTTP_409_CONFLICT,
            )
        return super().destroy(request, *args, **kwargs)


class ResourceCheckpointAdminViewSet(viewsets.ModelViewSet):
    """Admin-only CRUD on ResourceCheckpoint.

    Accepts `?resource=<id>` for scoped fetches. The `bulk` action mirrors
    the Django admin's textarea paste at apps/resources/admin.py:58-92 —
    splits the input on newlines and creates N rows transactionally.
    """
    queryset = ResourceCheckpoint.objects.all().order_by('resource_id', 'order_index')
    serializer_class = ResourceCheckpointAdminSerializer
    permission_classes = [permissions.IsAuthenticated, permissions.IsAdminUser]
    pagination_class = None

    def get_queryset(self):
        qs = super().get_queryset()
        resource_id = self.request.query_params.get('resource')
        if resource_id:
            # Guard non-numeric input: filtering on a non-int pk raises a
            # ValueError at query prep (→ 500). isascii() also rejects Unicode
            # digits (e.g. '²') that isdigit() accepts but int() can't parse.
            qs = qs.filter(resource_id=resource_id) if (
                str(resource_id).isascii() and str(resource_id).isdigit()
            ) else qs.none()
        return qs

    @action(detail=False, methods=['post'], url_path='bulk')
    def bulk(self, request):
        resource_id = request.data.get('resource')
        text = (request.data.get('bulk') or '').strip()
        # `source` records provenance for imported lists (jsonld/html/youtube_api).
        # bulk_create below bypasses model `choices` validation (Django choices
        # are app-level, not a DB constraint), so validate it explicitly here.
        source = request.data.get('source') or 'manual'
        if source not in dict(ResourceCheckpoint.SOURCE_CHOICES):
            return Response(
                {'detail': f'Invalid source {source!r}.'},
                status=status.HTTP_400_BAD_REQUEST,
            )
        if not resource_id:
            return Response(
                {'detail': 'resource is required.'},
                status=status.HTTP_400_BAD_REQUEST,
            )
        try:
            resource = Resource.objects.get(pk=resource_id)
        except Resource.DoesNotExist:
            return Response(
                {'detail': 'Resource not found.'},
                status=status.HTTP_404_NOT_FOUND,
            )
        except (ValueError, TypeError):
            # Non-numeric resource id in the body → ValueError at pk lookup.
            return Response(
                {'detail': 'resource must be a numeric id.'},
                status=status.HTTP_400_BAD_REQUEST,
            )
        lines = [line.strip() for line in text.splitlines() if line.strip()]
        if not lines:
            return Response(
                {'detail': 'No non-empty lines provided.'},
                status=status.HTTP_400_BAD_REQUEST,
            )
        # Match Django admin behavior (apps/resources/admin.py:66-77): refuse
        # bulk-append when checkpoints already exist. Edit existing rows via
        # the per-row CRUD endpoint instead of bulk-adding.
        if resource.checkpoints.exists():
            return Response(
                {'detail': (
                    'Resource already has checkpoints. Edit individual rows '
                    'via the checkpoint endpoints instead of bulk-adding.'
                )},
                status=status.HTTP_409_CONFLICT,
            )
        # The exists() check above and the bulk_create below are a TOCTOU: two
        # concurrent bulk pastes both see no checkpoints, then collide on
        # unique_together(resource, order_index). Catch the loser's
        # IntegrityError outside the atomic (clean rollback) → 409 not 500.
        # Stamp extraction time for non-manual (imported) lists; manual stays null.
        extracted_at = timezone.now() if source != 'manual' else None
        try:
            with transaction.atomic():
                created = ResourceCheckpoint.objects.bulk_create([
                    ResourceCheckpoint(
                        resource=resource,
                        order_index=i,
                        title=line,
                        source=source,
                        extracted_at=extracted_at,
                    )
                    for i, line in enumerate(lines, start=1)
                ])
        except IntegrityError:
            return Response(
                {'detail': (
                    'Checkpoints were just added for this resource. Edit '
                    'individual rows via the checkpoint endpoints instead.'
                )},
                status=status.HTTP_409_CONFLICT,
            )
        serializer = self.get_serializer(created, many=True)
        return Response(serializer.data, status=status.HTTP_201_CREATED)

    @action(detail=False, methods=['post'], url_path='import')
    def import_outline(self, request):
        """Fetch a resource's course page and return a *draft* checkpoint list.

        Read-only: writes nothing. The admin reviews/edits the returned titles,
        then persists them through the `bulk` action (which validates `source`).
        `extract_checkpoints` never raises — a blocked/empty page comes back as
        an empty list + a human `note` so the admin falls back to manual.
        """
        resource_id = request.data.get('resource')
        if not resource_id:
            return Response(
                {'detail': 'resource is required.'},
                status=status.HTTP_400_BAD_REQUEST,
            )
        try:
            resource = Resource.objects.get(pk=resource_id)
        except Resource.DoesNotExist:
            return Response(
                {'detail': 'Resource not found.'},
                status=status.HTTP_404_NOT_FOUND,
            )
        except (ValueError, TypeError):
            return Response(
                {'detail': 'resource must be a numeric id.'},
                status=status.HTTP_400_BAD_REQUEST,
            )
        result = extract_checkpoints(resource.url)
        return Response(
            {
                'source': result.source,
                'provider': result.provider,
                'note': result.note,
                'checkpoints': [
                    {
                        'order_index': c.order_index,
                        'title': c.title,
                        'url_fragment': c.url_fragment,
                        'estimated_minutes': c.estimated_minutes,
                    }
                    for c in result.checkpoints
                ],
            },
            status=status.HTTP_200_OK,
        )


class SkillResourceAdminViewSet(IntegrityConflictMixin, viewsets.ModelViewSet):
    """Admin-only CRUD on the Skill <-> Resource junction (with relevance)."""
    queryset = SkillResource.objects.all().select_related('skill', 'resource')
    serializer_class = SkillResourceAdminSerializer
    permission_classes = [permissions.IsAuthenticated, permissions.IsAdminUser]
    pagination_class = None
    conflict_detail = 'This skill is already linked to the resource.'

    def get_queryset(self):
        qs = super().get_queryset()
        resource_id = self.request.query_params.get('resource')
        if resource_id:
            # Guard non-numeric input: filtering on a non-int pk raises a
            # ValueError at query prep (→ 500). isascii() also rejects Unicode
            # digits (e.g. '²') that isdigit() accepts but int() can't parse.
            qs = qs.filter(resource_id=resource_id) if (
                str(resource_id).isascii() and str(resource_id).isdigit()
            ) else qs.none()
        return qs