Add BoundedEnumerable.reverse

Fixes typelevel#3769
ashleymercer · Feb 14, 2021 · 534cee4 · 534cee4
1 parent a8d77cf
commit 534cee4
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diff --git a/kernel-laws/shared/src/test/scala/cats/kernel/laws/LawTests.scala b/kernel-laws/shared/src/test/scala/cats/kernel/laws/LawTests.scala
@@ -229,6 +229,15 @@ class Tests extends TestsConfig with DisciplineSuite {
   checkAll("BoundedEnumerable[Int]", BoundedEnumerableTests[Int].boundedEnumerable)
   checkAll("BoundedEnumerable[Char]", BoundedEnumerableTests[Char].boundedEnumerable)
   checkAll("BoundedEnumerable[Long]", BoundedEnumerableTests[Long].boundedEnumerable)
+  checkAll("BoundedEnumerable.reverse(BoundedEnumerable[Int])",
+           BoundedEnumerableTests(BoundedEnumerable.reverse(BoundedEnumerable[Int])).boundedEnumerable
+  )
+  checkAll(
+    "BoundedEnumerable.reverse(BoundedEnumerable.reverse(BoundedEnumerable[Int]))",
+    BoundedEnumerableTests(
+      BoundedEnumerable.reverse(BoundedEnumerable.reverse(BoundedEnumerable[Int]))
+    ).boundedEnumerable
+  )
 
   checkAll("Monoid[String]", MonoidTests[String].monoid)
   checkAll("Monoid[String]", SerializableTests.serializable(Monoid[String]))

diff --git a/kernel/src/main/scala/cats/kernel/Enumerable.scala b/kernel/src/main/scala/cats/kernel/Enumerable.scala
@@ -79,6 +79,21 @@ object BoundedEnumerable {
     cats.kernel.instances.char.catsKernelStdOrderForChar
 
   @inline def apply[A](implicit e: BoundedEnumerable[A]): BoundedEnumerable[A] = e
+
+  /**
+   * Defines a `BoundedEnumerable[A]` from the given enumerable such that
+   * all arrows / successor functions switch direction.
+   */
+  def reverse[@sp A](e: BoundedEnumerable[A]): BoundedEnumerable[A] =
+    new BoundedEnumerable[A] {
+      override def order: Order[A] = Order.reverse(e.order)
+
+      override def partialNext(a: A): Option[A] = e.partialPrevious(a)
+      override def partialPrevious(a: A): Option[A] = e.partialNext(a)
+
+      override def minBound: A = e.maxBound
+      override def maxBound: A = e.minBound
+    }
 }
 
 trait LowerBoundedEnumerable[@sp A] extends PartialNextLowerBounded[A] with Next[A] {