permutationsOf() yields all possible permutations of the given list elements

Author: rhaendel

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

@@ -24,3 +24,18 @@ fun <A, B> combinationsOf(first: List<A>, second: List<B>) = sequence {
         }
     }
 }
+
+/**
+ * @return A sequence of lists containing all permutations of the original one.
+ */
+fun <E> permutationsOf(list: List<E>): Sequence<List<E>> = sequence {
+    if (list.isEmpty()) {
+        yield(emptyList())
+    }
+
+    for (i in list.indices) {
+        permutationsOf(list - list[i]).forEach { smallerPermutation ->
+            yield(smallerPermutation + list[i])
+        }
+    }
+}

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

@@ -1,6 +1,9 @@
 package de.ronny_h.aoc.extensions
 
 import io.kotest.core.spec.style.StringSpec
+import io.kotest.data.blocking.forAll
+import io.kotest.data.row
+import io.kotest.matchers.collections.shouldContainExactlyInAnyOrder
 import io.kotest.matchers.shouldBe
 
 class CombinationsTest : StringSpec({
@@ -58,4 +61,25 @@ class CombinationsTest : StringSpec({
                     2 to "b",
                 )
     }
+
+    "permutationsOf() yields all possible permutations of the given list elements" {
+        forAll(
+            row(emptyList(), listOf(emptyList())),
+            row(listOf(1), listOf(listOf(1))),
+            row(listOf(1, 2), listOf(listOf(1, 2), listOf(2, 1))),
+            row(
+                listOf(1, 2, 3),
+                listOf(
+                    listOf(1, 2, 3),
+                    listOf(1, 3, 2),
+                    listOf(2, 1, 3),
+                    listOf(2, 3, 1),
+                    listOf(3, 1, 2),
+                    listOf(3, 2, 1)
+                )
+            ),
+        ) { input, permutations ->
+            permutationsOf(input).toList() shouldContainExactlyInAnyOrder permutations
+        }
+    }
 })