Generalize combinations() to Iterable

This makes it applicable to more collections
such as ranges.

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/extensions/Combinations.kt

@@ -6,7 +6,7 @@ import java.lang.Math.pow
 /**
  * @return All pairwise combinations of list elements without the reflexive ones.
  */
-fun <E> List<E>.combinations() = sequence {
+fun <E> Iterable<E>.combinations() = sequence {
     forEachIndexed { i, a ->
         forEachIndexed { j, b ->
             if (i != j) {

src/test/kotlin/de/ronny_h/aoc/extensions/CombinationsTest.kt

@@ -32,6 +32,18 @@ class CombinationsTest : StringSpec({
                 )
     }
 
+    "a range with three elements yields all six combinations" {
+        (1..3).combinations().toList() shouldBe
+                listOf(
+                    1 to 2,
+                    1 to 3,
+                    2 to 1,
+                    2 to 3,
+                    3 to 1,
+                    3 to 2
+                )
+    }
+
     "a list with non-unique elements yields non-unique combinations but still skips the identical ones" {
         listOf(1, 1, 2).combinations().toList() shouldBe
                 listOf(