"""
Union-find data structure.
"""

from networkx.utils import groups


class UnionFind:
    """Union-find data structure.

    Each unionFind instance X maintains a family of disjoint sets of
    hashable objects, supporting the following two methods:

    - X[item] returns a name for the set containing the given item.
      Each set is named by an arbitrarily-chosen one of its members; as
      long as the set remains unchanged it will keep the same name. If
      the item is not yet part of a set in X, a new singleton set is
      created for it.

    - X.union(item1, item2, ...) merges the sets containing each item
      into a single larger set.  If any item is not yet part of a set
      in X, it is added to X as one of the members of the merged set.

      Union-find data structure. Based on Josiah Carlson's code,
      http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/215912
      with significant additional changes by D. Eppstein.
      http://www.ics.uci.edu/~eppstein/PADS/UnionFind.py

    """

    def __init__(self, elements=None):
        """Create a new empty union-find structure.

        If *elements* is an iterable, this structure will be initialized
        with the discrete partition on the given set of elements.

        """
        if elements is None:
            elements = ()
        self.parents = {}
        self.weights = {}
        for x in elements:
            self.weights[x] = 1
            self.parents[x] = x

    def __getitem__(self, object):
        """Find and return the name of the set containing the object."""

        # check for previously unknown object
        if object not in self.parents:
            self.parents[object] = object
            self.weights[object] = 1
            return object

        # find path of objects leading to the root
        path = [object]
        root = self.parents[object]
        while root != path[-1]:
            path.append(root)
            root = self.parents[root]

        # compress the path and return
        for ancestor in path:
            self.parents[ancestor] = root
        return root

    def __iter__(self):
        """Iterate through all items ever found or unioned by this structure.

        """
        return iter(self.parents)

    def to_sets(self):
        """Iterates over the sets stored in this structure.

        For example::

            >>> partition = UnionFind("xyz")
            >>> sorted(map(sorted, partition.to_sets()))
            [['x'], ['y'], ['z']]
            >>> partition.union("x", "y")
            >>> sorted(map(sorted, partition.to_sets()))
            [['x', 'y'], ['z']]

        """
        # Ensure fully pruned paths
        for x in self.parents.keys():
            _ = self[x]  # Evaluated for side-effect only

        yield from groups(self.parents).values()

    def union(self, *objects):
        """Find the sets containing the objects and merge them all."""
        # Find the heaviest root according to its weight.
        roots = iter(sorted({self[x] for x in objects}, key=lambda r: self.weights[r]))
        try:
            root = next(roots)
        except StopIteration:
            return

        for r in roots:
            self.weights[root] += self.weights[r]
            self.parents[r] = root
