import option::none;
import option::some;


// A very naive implementation of union-find with unsigned integer nodes.
// Maintains the invariant that the root of a node is always equal to or less
// than the node itself.
type node = option::t[uint];

type ufind = rec(mutable node[mutable] nodes);

fn make() -> ufind {
    ret rec(mutable nodes=~[mutable]);
}

fn make_set(&ufind ufnd) -> uint {
    auto idx = ivec::len(ufnd.nodes);
    ufnd.nodes += ~[mutable none[uint]];
    ret idx;
}


/// Creates sets as necessary to ensure that least `n` sets are present in the
/// data structure.
fn grow(&ufind ufnd, uint n) {
    while (set_count(ufnd) < n) { make_set(ufnd); }
}

fn find(&ufind ufnd, uint n) -> uint {
    alt (ufnd.nodes.(n)) {
        case (none) { ret n; }
        case (some(?m)) { auto m_ = m; be find(ufnd, m_); }
    }
}

fn union(&ufind ufnd, uint m, uint n) {
    auto m_root = find(ufnd, m);
    auto n_root = find(ufnd, n);
    if (m_root < n_root) {
        ufnd.nodes.(n_root) = some[uint](m_root);
    } else if (m_root > n_root) { ufnd.nodes.(m_root) = some[uint](n_root); }
}

fn set_count(&ufind ufnd) -> uint { ret ivec::len[node](ufnd.nodes); }


// Removes all sets with IDs greater than or equal to the given value.
fn prune(&ufind ufnd, uint n) {
    // TODO: Use "slice" once we get rid of "mutable?"

    auto len = ivec::len[node](ufnd.nodes);
    while (len != n) { ivec::pop[node](ufnd.nodes); len -= 1u; }
}