from fastapi import APIRouter, Depends, HTTPException, Query
from sqlalchemy.ext.asyncio import AsyncSession
from sqlalchemy import select, and_
from typing import Optional, List
from datetime import datetime, date, timedelta
import calendar

from app.database import get_db
from app.models.todo import Todo
from app.schemas.todo import TodoCreate, TodoUpdate, TodoResponse
from app.routers.auth import get_current_user, get_current_settings
from app.models.user import User
from app.models.settings import Settings

router = APIRouter()


def _calculate_recurrence(
    recurrence_rule: str,
    current_due_date: date | None,
    first_day_of_week: int = 0,
) -> tuple[datetime | None, date | None]:
    """Calculate reset_at and next_due_date for a recurring todo.

    Args:
        recurrence_rule: "daily", "weekly", or "monthly"
        current_due_date: The todo's current due date (may be None)
        first_day_of_week: 0=Sunday, 1=Monday
    Returns:
        (reset_at, next_due_date) or (None, None) if rule is invalid
    """
    today = date.today()

    if recurrence_rule == "daily":
        reset_date = today + timedelta(days=1)
        next_due = reset_date

    elif recurrence_rule == "weekly":
        # Find the start of the next week based on first_day_of_week setting.
        # Python weekday(): Monday=0 ... Sunday=6
        # Setting: 0=Sunday, 1=Monday
        target_weekday = 6 if first_day_of_week == 0 else 0  # Python weekday for start
        days_ahead = (target_weekday - today.weekday()) % 7
        if days_ahead == 0:
            days_ahead = 7  # Always push to *next* week, even if completed on the first day
        reset_date = today + timedelta(days=days_ahead)

        if current_due_date:
            # Preserve the day-of-week: place it in the reset week
            dow_offset = (current_due_date.weekday() - target_weekday) % 7
            next_due = reset_date + timedelta(days=dow_offset)
        else:
            next_due = reset_date

    elif recurrence_rule == "monthly":
        # First day of next month
        if today.month == 12:
            reset_date = date(today.year + 1, 1, 1)
        else:
            reset_date = date(today.year, today.month + 1, 1)

        if current_due_date:
            # Same day-of-month, clamped to month length
            max_day = calendar.monthrange(reset_date.year, reset_date.month)[1]
            day = min(current_due_date.day, max_day)
            next_due = date(reset_date.year, reset_date.month, day)
        else:
            next_due = reset_date
    else:
        return None, None

    reset_at = datetime(reset_date.year, reset_date.month, reset_date.day, 0, 0, 0)
    return reset_at, next_due


async def _reactivate_recurring_todos(db: AsyncSession) -> None:
    """Auto-reactivate recurring todos whose reset_at has passed.

    Uses flush (not commit) so changes are visible to the subsequent query
    within the same transaction. The caller's commit handles persistence.
    """
    now = datetime.now()
    query = select(Todo).where(
        and_(
            Todo.completed == True,
            Todo.recurrence_rule.isnot(None),
            Todo.reset_at.isnot(None),
            Todo.reset_at <= now,
        )
    ).with_for_update()
    result = await db.execute(query)
    todos = result.scalars().all()

    for todo in todos:
        todo.completed = False
        todo.completed_at = None
        if todo.next_due_date:
            todo.due_date = todo.next_due_date
        todo.reset_at = None
        todo.next_due_date = None

    if todos:
        await db.flush()


@router.get("/", response_model=List[TodoResponse])
async def get_todos(
    completed: Optional[bool] = Query(None),
    priority: Optional[str] = Query(None),
    category: Optional[str] = Query(None),
    search: Optional[str] = Query(None),
    db: AsyncSession = Depends(get_db),
    current_user: Settings = Depends(get_current_settings)
):
    """Get all todos with optional filters."""
    # Reactivate any recurring todos whose reset time has passed
    await _reactivate_recurring_todos(db)

    query = select(Todo)

    if completed is not None:
        query = query.where(Todo.completed == completed)

    if priority:
        query = query.where(Todo.priority == priority)

    if category:
        query = query.where(Todo.category == category)

    if search:
        query = query.where(Todo.title.ilike(f"%{search}%"))

    query = query.order_by(Todo.created_at.desc())

    result = await db.execute(query)
    todos = result.scalars().all()

    await db.commit()

    return todos


@router.post("/", response_model=TodoResponse, status_code=201)
async def create_todo(
    todo: TodoCreate,
    db: AsyncSession = Depends(get_db),
    current_user: Settings = Depends(get_current_settings)
):
    """Create a new todo."""
    new_todo = Todo(**todo.model_dump())
    db.add(new_todo)
    await db.commit()
    await db.refresh(new_todo)

    return new_todo


@router.get("/{todo_id}", response_model=TodoResponse)
async def get_todo(
    todo_id: int,
    db: AsyncSession = Depends(get_db),
    current_user: Settings = Depends(get_current_settings)
):
    """Get a specific todo by ID."""
    result = await db.execute(select(Todo).where(Todo.id == todo_id))
    todo = result.scalar_one_or_none()

    if not todo:
        raise HTTPException(status_code=404, detail="Todo not found")

    return todo


@router.put("/{todo_id}", response_model=TodoResponse)
async def update_todo(
    todo_id: int,
    todo_update: TodoUpdate,
    db: AsyncSession = Depends(get_db),
    current_user: Settings = Depends(get_current_settings)
):
    """Update a todo."""
    result = await db.execute(select(Todo).where(Todo.id == todo_id))
    todo = result.scalar_one_or_none()

    if not todo:
        raise HTTPException(status_code=404, detail="Todo not found")

    update_data = todo_update.model_dump(exclude_unset=True)

    # Handle completion timestamp
    if "completed" in update_data:
        if update_data["completed"] and not todo.completed:
            update_data["completed_at"] = datetime.now()
        elif not update_data["completed"]:
            update_data["completed_at"] = None
            update_data["reset_at"] = None
            update_data["next_due_date"] = None

    # Clear due_time if due_date is being removed
    if "due_date" in update_data and update_data["due_date"] is None:
        update_data["due_time"] = None

    for key, value in update_data.items():
        setattr(todo, key, value)

    # Recalculate recurrence schedule if the todo is completed and now
    # has a recurrence rule (e.g. user edited a completed todo to add
    # recurrence, or changed the rule/due_date while completed).
    if todo.completed and todo.recurrence_rule:
        reset_at, next_due = _calculate_recurrence(
            todo.recurrence_rule,
            todo.due_date,
            current_user.first_day_of_week,
        )
        todo.reset_at = reset_at
        todo.next_due_date = next_due
    elif todo.completed and not todo.recurrence_rule:
        # Recurrence removed while completed — clear schedule
        todo.reset_at = None
        todo.next_due_date = None

    await db.commit()
    await db.refresh(todo)

    return todo


@router.delete("/{todo_id}", status_code=204)
async def delete_todo(
    todo_id: int,
    db: AsyncSession = Depends(get_db),
    current_user: Settings = Depends(get_current_settings)
):
    """Delete a todo."""
    result = await db.execute(select(Todo).where(Todo.id == todo_id))
    todo = result.scalar_one_or_none()

    if not todo:
        raise HTTPException(status_code=404, detail="Todo not found")

    await db.delete(todo)
    await db.commit()

    return None


@router.patch("/{todo_id}/toggle", response_model=TodoResponse)
async def toggle_todo(
    todo_id: int,
    db: AsyncSession = Depends(get_db),
    current_user: Settings = Depends(get_current_settings)
):
    """Toggle todo completion status. For recurring todos, calculates reset schedule."""
    result = await db.execute(select(Todo).where(Todo.id == todo_id))
    todo = result.scalar_one_or_none()

    if not todo:
        raise HTTPException(status_code=404, detail="Todo not found")

    todo.completed = not todo.completed

    if todo.completed:
        todo.completed_at = datetime.now()

        # If recurring, schedule the reset
        if todo.recurrence_rule:
            reset_at, next_due = _calculate_recurrence(
                todo.recurrence_rule,
                todo.due_date,
                current_user.first_day_of_week,
            )
            todo.reset_at = reset_at
            todo.next_due_date = next_due
    else:
        # Manual uncomplete — clear recurrence scheduling
        todo.completed_at = None
        todo.reset_at = None
        todo.next_due_date = None

    await db.commit()
    await db.refresh(todo)

    return todo