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 = {nodes: @mutable [mutable node]};

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

fn make_set(ufnd: &ufind) -> uint {
    let 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(ufnd: &ufind, n: uint) {
    while set_count(ufnd) < n { make_set(ufnd); }
}

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

fn union(ufnd: &ufind, m: uint, n: uint) {
    let m_root = find(ufnd, m);
    let 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(ufnd: &ufind) -> uint { ret ivec::len[node](*ufnd.nodes); }


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

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