Stuff Guava is Missing

import static com.google.common.base.Optional.of;
import static com.google.common.collect.Iterables.concat;
import static com.google.common.collect.Iterables.filter;
import static com.google.common.collect.Iterables.isEmpty;
import static com.google.common.collect.Iterables.transform;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Comparator;
import com.google.common.base.Function;
import com.google.common.base.Optional;
import com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import com.google.common.collect.ImmutableSortedSet;
import com.google.common.collect.Lists;
import com.google.common.collect.Multimaps;
import com.google.common.collect.Sets;
import edu.umd.cs.findbugs.annotations.NonNull;


/**
 * A Bunch of generic collection stuff that really should have been in Guava.
 */
public class CollectionUtil {

  private CollectionUtil() {
  }

  /**
   * Flattens a list of lists into a single list.
   * Like flatMap in Scala or bind in Haskell, but restricted to Iterables only thanks to Java's laughable type system.
   */
  public static <A, B> Iterable<B> flatTransform(Iterable<A> i, Function<? super A, ? extends Iterable<B>> f) {
    return concat(transform(i, f));
  }

  /**
   * Groups elements into a list of lists, with grouping determined by the given function.
   *
   * @param i the elements to group
   * @param f determines the property to group by
   * @return A list of grouped elements.
   */
  public static <A, B> Collection<? extends Collection<? extends A>> group(Iterable<? extends A> i,
      Function<A, B> f) {
    return Multimaps.index(i, f).asMap().values();
  }

  /**
   * Left Fold.
   * (a, b, c, d), initial -> f(f(f(f(initial,a), b), c), d)
   * http://en.wikipedia.org/wiki/Fold_(higher-order_function)
   */
  public static <A, B> A fold(final Iterable<? extends B> gen, final A initial,
      final Function2<? super A, ? super B, ? extends A> function) {
    final Iterator<? extends B> it = gen.iterator();
    if (!it.hasNext()) {
      return initial;
    }
    A acc = initial;
    while (it.hasNext()) {
      acc = function.apply(acc, it.next());
    }
    return acc;
  }

  /**
   * A transformation function of arity-2 (two arguments) from A and B to C.
   */
  public interface Function2<A, B, C> {
    C apply(A a, B b);
  }

  /**
   * Sums Integers.
   * @param  ints a collection of {@code Integer} objects
   * @return sum of ints
   * @throws NullPointerException if {@code ints} or any of its elements is null
   */
  public static int sum(@NonNull Iterable<Integer> ints) {
    int sum = 0;
    for (int anInt : ints) {
      sum += anInt;
    }
    return sum;
  }


  /**
   * Returns true if all {@code B}s are equal
   */
  public static <A, B> boolean allEqual(Iterable<A> iterable, Function<A, B> function) {
    return allEqual(transform(iterable, function));
  }

  /**
   * Returns true if all elements in the List are equal
   */
  public static boolean allEqual(Iterable<?> iterable) {
    return Sets.newHashSet(iterable).size() < 2;
  }

  /**
   * Returns the count of unique {@code A}s according to supplied comparator
   */
  public static <A> int countDistinct(Iterable<A> iterable, Comparator<? super A> comparator) {
    return ImmutableSortedSet.copyOf(comparator, iterable).size();
  }

  /**
   * Returns a list with duplicates removed according to the given comparator.
   */
  public static <A> Iterable<A> distinct(Iterable<A> iterable, Comparator<? super A> comparator) {
    return ImmutableSortedSet.copyOf(comparator, iterable);
  }

  /**
   * Determines if the given iterable contains no elements.
   */
  public static boolean isNotEmpty(Iterable<?> iterable) {
    return !isEmpty(iterable);
  }

  /**
   * Returns true if the predicate holds for at least one of the elements of this iterable, false otherwise
   * (false for the empty list).
   */
  public static <A> boolean exists(Iterable<A> iterable, Predicate<A> predicate) {
    return isNotEmpty(filter(iterable, predicate));
  }

  /**
   * Returns true if the iterable has at least one element whose class is {@code type} or a subclass of {@code type}.
   */
  public static boolean exists(Iterable<?> iterable, Class<?> type) {
    return isNotEmpty(filter(iterable, type));
  }

  /**
   * Syntactic sugar for Guava's Predicates.in()
   */
  public static <T> Predicate<T> in(T... target) {
    return Predicates.in(Lists.newArrayList(target));
  }

  /**
   * Simply returns a new Iterable with each element wrapped in an Optional.
   */
  public static <A> Iterable<Optional<A>> asOptionalList(Iterable<A> iterable) {
    Collection<Optional<A>> list = new ArrayList<Optional<A>>();
    for (A a : iterable) {
      list.add(of(a));
    }
    return list;
  }

  /**
   * Creates a Guava multimap using the input map.
   */
  public static <K, V> Multimap<K, V> createMultiMap(Map<K, ? extends Iterable<V>> input) {
    Multimap<K, V> multimap = ArrayListMultimap.create();
    for (Map.Entry<K, ? extends Iterable<V>> entry : input.entrySet()) {
      multimap.putAll(entry.getKey(), entry.getValue());
    }
    return multimap;
  }

  /**
   * Creates an Immutable Guava multimap using the input map.
   */
  public static <K, V> ImmutableMultimap<K, V> createImmutableMultiMap(Map<K, ? extends Iterable<V>> input) {
    ImmutableMultimap.Builder<K, V> builder = ImmutableMultimap.builder();
    for (Map.Entry<K, ? extends Iterable<V>> entry : input.entrySet()) {
      builder.putAll(entry.getKey(), entry.getValue());
    }
    return builder.build();
  }

  public static <K, V> Map<K, Collection<V>> combine(Map<K, Collection<V>> map1, Map<K, Collection<V>> map2) {
    return combine(createMultiMap(map1), createMultiMap(map2)).asMap();
  }

  public static <K, V> Multimap<K, V> combine(Multimap<K, V> map1, Multimap<K, V> map2) {
    ArrayListMultimap<K, V> map = ArrayListMultimap.create();
    map.putAll(map1);
    map.putAll(map2);
    return map;
  }
}