"""
Screen-stack navigation for the console UI.

Navigation maintains a stack of ``(screen_name, context, label)`` entries.
Screens push onto the stack when the user drills into a sub-view and pop
when they press Escape / Backspace to go back.
"""


class Navigation:
    """A simple stack-based navigator.

    Each entry is a tuple ``(screen_name, context, label)`` where
    *screen_name* identifies which screen to display, *context* carries
    any data the screen needs, and *label* is the human-readable text
    shown in the breadcrumb trail.
    """

    def __init__(self):
        self._stack: list[tuple[str, object, str]] = []

    def push(self, screen_name: str, context=None, label: str | None = None) -> None:
        """Push a new screen onto the stack.

        If *label* is ``None`` the *screen_name* is used as fallback in
        the breadcrumb.
        """
        self._stack.append((screen_name, context, label if label is not None else screen_name))

    def pop(self) -> tuple[str, object] | None:
        """Remove and return the top entry as ``(screen_name, context)``.

        Returns ``None`` if the stack is empty.
        """
        if not self._stack:
            return None
        screen_name, context, _label = self._stack.pop()
        return (screen_name, context)

    def current(self) -> tuple[str, object] | None:
        """Return the top entry as ``(screen_name, context)`` without removing it.

        Returns ``None`` if the stack is empty.
        """
        if not self._stack:
            return None
        screen_name, context, _label = self._stack[-1]
        return (screen_name, context)

    def breadcrumb(self) -> list[str]:
        """Return the list of labels from bottom to top of the stack."""
        return [label for _name, _ctx, label in self._stack]

    def clear(self) -> None:
        """Remove all entries from the stack."""
        self._stack.clear()

    def depth(self) -> int:
        """Return the number of entries on the stack."""
        return len(self._stack)