"""Resolution context — carries service and parent references during tree walk."""


class Context:
    """Evaluation frame for the resolver.

    Top-level context has ``service`` set and ``parent_object`` as None.
    Each selected/allocated object pushes a child context so children
    can reference ``parent`` and traverse its relations.
    """

    def __init__(self, service, parent_object=None):
        self.service = service
        self.parent_object = parent_object

    def child(self, obj):
        return Context(
            service=self.service,
            parent_object=obj,
        )

    @property
    def parent(self):
        return self.parent_object

    def resolve_path(self, path):
        """Resolve a dotted reference like ``parent.site.management_prefix``.

        Supported roots:
          - ``service``  → the Service instance
          - ``parent``   → the parent object (or None at service scope)
        Returns None if any segment is None or missing.
        """
        parts = path.split(".")
        root = parts[0]

        if root == "service":
            obj = self.service
        elif root == "parent":
            obj = self.parent_object
        else:
            raise KeyError(f"Unknown context root: {root!r}")

        for part in parts[1:]:
            if obj is None:
                return None
            if isinstance(obj, dict):
                obj = obj.get(part)
            else:
                obj = getattr(obj, part, None)

        return obj