"""Pace calculation service for reading progress vs meeting targets."""

from __future__ import annotations

import logging
from datetime import datetime, timedelta
from typing import TypedDict

from django.db.models import QuerySet
from django.utils import timezone

from apps.books.models import EBook, ReadingProgress
from apps.groups.models import (
    GroupMeeting,
    GroupMembership,
    NotificationPreference,
    PaceNotification,
    NotificationStatus,
)

logger = logging.getLogger(__name__)


class PaceStatus(TypedDict):
    meeting_id: int
    meeting_title: str
    scheduled_at: str
    target_section: int
    target_section_label: str
    current_section: int
    ahead_behind_delta: int
    status: str  # "ahead", "behind", "on_track", "completed"
    days_until_meeting: int
    sections_to_catch_up: int
    ebook_id: int
    ebook_title: str


def get_relevant_ebooks_for_user(user) -> QuerySet[EBook]:
    """Return ebooks that belong to groups the user is a member of."""
    group_ids = (
        GroupMembership.objects.filter(user=user)
        .values_list("group_id", flat=True)
    )
    return (
        EBook.objects.filter(reading_groups__in=group_ids)
        .select_related("reading_progress")
        .prefetch_related("chapters")
        .distinct()
    )


def calculate_pace(
    user,
    ebook: EBook | None = None,
    meeting: GroupMeeting | None = None,
) -> list[PaceStatus]:
    """Calculate reading pace for a user's upcoming meetings.

    Compares the user's current reading progress (chapter index) against
    the target section for each upcoming meeting within 3 days.
    """
    now = timezone.now()
    three_days_from_now = now + timedelta(days=3)

    # Find upcoming meetings for groups the user belongs to
    user_group_ids = (
        GroupMembership.objects.filter(user=user)
        .values_list("group_id", flat=True)
    )
    meetings_qs = GroupMeeting.objects.filter(
        group_id__in=user_group_ids,
        scheduled_at__gte=now,
        scheduled_at__lte=three_days_from_now,
    ).select_related("group", "group__ebook")

    if meeting:
        meetings_qs = meetings_qs.filter(pk=meeting.pk)

    meetings = list(meetings_qs)

    results: list[PaceStatus] = []

    for m in meetings:
        group_ebook = m.group.ebook
        ebooks_to_check: list[EBook] = []

        if ebook:
            ebooks_to_check = [ebook]
        elif group_ebook:
            ebooks_to_check = [group_ebook]
        else:
            # Check all ebooks in the group
            ebooks_to_check = list(
                EBook.objects.filter(reading_groups=m.group)
                .select_related("reading_progress")
                .prefetch_related("chapters")
            )

        for eb in ebooks_to_check:
            pace = _compute_single_pace(user, m, eb, now)
            if pace:
                results.append(pace)

    return results


def _compute_single_pace(
    user,
    meeting: GroupMeeting,
    ebook: EBook,
    now: datetime,
) -> PaceStatus | None:
    """Compute pace for one user + meeting + ebook combination."""
    try:
        progress = ebook.reading_progress
    except ReadingProgress.DoesNotExist:
        # No progress yet — user hasn't started reading
        return None

    # Current section: based on last_page (chapter index) or computed from progress %
    if progress.last_page > 0:
        current_section = progress.last_page
    elif progress.current_position > 0:
        chapter_count = ebook.chapters.count() or ebook.page_count or 1
        current_section = max(1, int((progress.current_position / 100) * chapter_count))
    else:
        current_section = 0

    target_section = meeting.target_section or 0
    if target_section == 0:
        return None

    delta = current_section - target_section
    days_until = max(0, (meeting.scheduled_at - now).days)

    if current_section >= target_section:
        if current_section >= target_section and progress.current_position > 0:
            status = "completed"
        else:
            status = "ahead"
    elif delta >= -1:
        status = "on_track"
    else:
        status = "behind"

    return PaceStatus(
        meeting_id=meeting.pk,
        meeting_title=meeting.title or meeting.target_section_label,
        scheduled_at=meeting.scheduled_at.isoformat(),
        target_section=target_section,
        target_section_label=meeting.target_section_label,
        current_section=current_section,
        ahead_behind_delta=delta,
        status=status,
        days_until_meeting=days_until,
        sections_to_catch_up=max(0, target_section - current_section),
        ebook_id=ebook.pk,
        ebook_title=ebook.title,
    )


def should_send_notification(
    user,
    meeting: GroupMeeting,
    ebook: EBook,
) -> bool:
    """Check if a pace notification should be sent based on user preferences."""
    pref, _ = NotificationPreference.objects.get_or_create(user=user)

    if not pref.pace_notifications_enabled:
        return False

    # Check if notification was already sent today
    today = timezone.now().date()
    existing = PaceNotification.objects.filter(
        user=user,
        meeting=meeting,
        ebook=ebook,
        created_at__date=today,
    ).exclude(status=NotificationStatus.DISMISSED)

    if existing.exists():
        return False

    # Frequency check
    if pref.frequency == "weekly":
        week_ago = today - timedelta(days=7)
        recent = PaceNotification.objects.filter(
            user=user,
            meeting=meeting,
            ebook=ebook,
            created_at__date__gte=week_ago,
        ).exclude(status=NotificationStatus.DISMISSED)
        if recent.exists():
            return False

    return True


def get_reminder_meetings(user) -> list[GroupMeeting]:
    """Get meetings that are within 24 hours for reminder notifications."""
    now = timezone.now()
    tomorrow = now + timedelta(days=1)

    user_group_ids = (
        GroupMembership.objects.filter(user=user)
        .values_list("group_id", flat=True)
    )

    return list(
        GroupMeeting.objects.filter(
            group_id__in=user_group_ids,
            scheduled_at__gte=now,
            scheduled_at__lte=tomorrow,
        )
        .select_related("group", "group__ebook")
        .order_by("scheduled_at")
    )