from datetime import datetime, timezone
from typing import Optional, TYPE_CHECKING, List
from pydantic import BaseModel, Field, field_validator, computed_field

from app.models.todo import Priority

if TYPE_CHECKING:
    from app.schemas.category import Category
    from app.schemas.project import Project


class TodoBase(BaseModel):
    title: str = Field(..., min_length=1, max_length=200, description="Todo title")
    description: Optional[str] = Field(
        None, max_length=500, description="Todo description"
    )
    completed: bool = Field(False, description="Whether the todo is completed")
    priority: Priority = Field(Priority.MEDIUM, description="Priority level")
    category_id: Optional[int] = Field(None, description="Category ID")
    project_id: Optional[int] = Field(None, description="Project ID")
    due_date: Optional[datetime] = Field(None, description="Due date with timezone")

    @field_validator("due_date")
    @classmethod
    def validate_due_date(cls, v: Optional[datetime]) -> Optional[datetime]:
        if v is not None:
            # Ensure timezone awareness
            if v.tzinfo is None:
                v = v.replace(tzinfo=timezone.utc)
            # For creation/updates, ensure due date is in the future
            if v < datetime.now(timezone.utc):
                raise ValueError("Due date must be in the future")
        return v


class TodoCreate(TodoBase):
    tag_ids: Optional[List[int]] = Field(
        default=None, description="List of tag IDs to associate with the todo"
    )


class TodoUpdate(BaseModel):
    title: Optional[str] = Field(
        None, min_length=1, max_length=200, description="Todo title"
    )
    description: Optional[str] = Field(
        None, max_length=500, description="Todo description"
    )
    completed: Optional[bool] = Field(None, description="Whether the todo is completed")
    priority: Optional[Priority] = Field(None, description="Priority level")
    category_id: Optional[int] = Field(None, description="Category ID")
    project_id: Optional[int] = Field(None, description="Project ID")
    due_date: Optional[datetime] = Field(None, description="Due date with timezone")
    tag_ids: Optional[List[int]] = Field(
        None, description="List of tag IDs to associate with this todo"
    )

    @field_validator("due_date")
    @classmethod
    def validate_due_date(cls, v: Optional[datetime]) -> Optional[datetime]:
        if v is not None:
            # Ensure timezone awareness
            if v.tzinfo is None:
                v = v.replace(tzinfo=timezone.utc)
            # For updates, only validate future date if it's being set to a new value
            # Allow None to clear the due date
        return v


# Forward declaration to handle circular reference
class Todo(TodoBase):
    id: int
    created_at: datetime
    updated_at: Optional[datetime] = None
    category: Optional["Category"] = None
    project: Optional["Project"] = None

    @computed_field
    @property
    def is_overdue(self) -> bool:
        """Check if this todo is overdue."""
        if self.due_date is None or self.completed:
            return False
        return datetime.now(timezone.utc) > self.due_date

    @computed_field
    @property
    def days_until_due(self) -> Optional[int]:
        """Calculate the number of days until the due date.

        Returns:
            int: Number of days until due (negative if overdue)
            None: If no due date is set or todo is completed
        """
        if self.due_date is None or self.completed:
            return None

        now = datetime.now(timezone.utc)
        delta = (self.due_date - now).days
        return delta

    @field_validator("due_date", mode="before")
    @classmethod
    def ensure_timezone_aware(cls, v: Optional[datetime]) -> Optional[datetime]:
        if v is not None and v.tzinfo is None:
            v = v.replace(tzinfo=timezone.utc)
        return v

    model_config = {"from_attributes": True}


class TodoListResponse(BaseModel):
    items: list[Todo]
    total: int
    page: int
    per_page: int
    has_next: bool
    has_prev: bool


class SubtaskCreate(BaseModel):
    title: str = Field(..., min_length=1, max_length=200, description="Subtask title")
    description: Optional[str] = Field(
        None, max_length=500, description="Subtask description"
    )
    priority: Priority = Field(Priority.MEDIUM, description="Priority level")
    due_date: Optional[datetime] = Field(None, description="Due date with timezone")

    @field_validator("due_date")
    @classmethod
    def validate_due_date(cls, v: Optional[datetime]) -> Optional[datetime]:
        if v is not None:
            # Ensure timezone awareness
            if v.tzinfo is None:
                v = v.replace(tzinfo=timezone.utc)
            # For creation, ensure due date is in the future
            if v < datetime.now(timezone.utc):
                raise ValueError("Due date must be in the future")
        return v


# Update forward references
Todo.model_rebuild()