✨ add cartesianProduct; combine combinations file into collections

src/main/scala/org/lemon/advent/lib/collections.scala

@@ -15,3 +15,39 @@ extension [A](it: Iterator[A])
     * @return the nth element of the iterator, or None if there is no nth element
     */
   def nthOption(n: Int): Option[A] = it.drop(n).nextOption
+
+extension [A](it: Iterable[A])
+  /** Returns an iterator view of the Cartesian product of this iterable and another.
+    * Also known as the cross product
+    *
+    * @param rhs the other iterable
+    * @return all pairs of elements (a, b) where a is from this iterable and b is from the other iterable
+    */
+  def cartesianProduct[B](rhs: Iterable[B]): Iterator[(A, B)] =
+    for a <- it.iterator; b <- rhs.iterator yield (a, b)
+
+  /** Returns an iterator of all unordered pairs of elements in the iterable.
+    * If the collection contains duplicates, the pairs will not be unique.
+    *
+    * @return iterator of all unordered pairs
+    */
+  def pairs: Iterator[(A, A)] =
+    for
+      (x, i) <- it.iterator.zipWithIndex
+      (y, j) <- it.iterator.zipWithIndex
+      if i < j
+    yield (x, y)
+
+  /** Returns an iterator of all unordered triples of elements in the iterable.
+    * If the collection contains duplicates, the triples will not be unique.
+    *
+    * @return iterator of all unordered triples
+    */
+  def triples: Iterator[(A, A, A)] =
+    for
+      (x, i) <- it.iterator.zipWithIndex
+      (y, j) <- it.iterator.zipWithIndex
+      (z, k) <- it.iterator.zipWithIndex
+      if i < j
+      if j < k
+    yield (x, y, z)