from sqlalchemy import Column, String, Integer, Enum, DateTime, ForeignKey, UniqueConstraint
from sqlalchemy.orm import relationship
from sqlalchemy.sql import func
from app.core.database import Base
import enum


class DependencyType(str, enum.Enum):
    """
    Task dependency types for Gantt chart.

    FS (Finish-to-Start): Predecessor must finish before successor starts (most common)
    SS (Start-to-Start): Predecessor must start before successor starts
    FF (Finish-to-Finish): Predecessor must finish before successor finishes
    SF (Start-to-Finish): Predecessor must start before successor finishes (rare)
    """
    FS = "FS"  # Finish-to-Start
    SS = "SS"  # Start-to-Start
    FF = "FF"  # Finish-to-Finish
    SF = "SF"  # Start-to-Finish


class TaskDependency(Base):
    """
    Represents a dependency relationship between two tasks.

    The predecessor task affects when the successor task can be scheduled,
    based on the dependency_type. This is used for Gantt chart visualization
    and date validation.
    """
    __tablename__ = "pjctrl_task_dependencies"

    __table_args__ = (
        UniqueConstraint('predecessor_id', 'successor_id', name='uq_predecessor_successor'),
    )

    id = Column(String(36), primary_key=True)
    predecessor_id = Column(
        String(36),
        ForeignKey("pjctrl_tasks.id", ondelete="CASCADE"),
        nullable=False,
        index=True
    )
    successor_id = Column(
        String(36),
        ForeignKey("pjctrl_tasks.id", ondelete="CASCADE"),
        nullable=False,
        index=True
    )
    dependency_type = Column(
        Enum("FS", "SS", "FF", "SF", name="dependency_type_enum"),
        default="FS",
        nullable=False
    )
    lag_days = Column(Integer, default=0, nullable=False)
    created_at = Column(DateTime, server_default=func.now(), nullable=False)

    # Relationships
    predecessor = relationship(
        "Task",
        foreign_keys=[predecessor_id],
        back_populates="successors"
    )
    successor = relationship(
        "Task",
        foreign_keys=[successor_id],
        back_populates="predecessors"
    )