from dataclasses import replace
from typing import List, Sequence, Set

from .todo_item import Todo


def _in_manual_order(todos: Sequence[Todo], which: str) -> List[Todo]:
    """Manual order: order ascending, ties by position (sorted is stable)."""
    seen: Set[str] = set()
    for todo in todos:
        if todo.id in seen:
            raise ValueError(f'duplicate id "{todo.id}" in the {which} list')
        seen.add(todo.id)
    return sorted(todos, key=lambda todo: todo.order)


def merge_todos(current: Sequence[Todo], incoming: Sequence[Todo]) -> List[Todo]:
    """Merge by id: incoming wins in the current todo's place, new ids follow; manual order, renumbered."""
    mine = _in_manual_order(current, "current")
    theirs = _in_manual_order(incoming, "incoming")
    by_id = {todo.id: todo for todo in theirs}
    kept = {todo.id for todo in mine}
    merged = [by_id.get(todo.id, todo) for todo in mine] + [todo for todo in theirs if todo.id not in kept]
    return [replace(todo, order=order) for order, todo in enumerate(merged)]