Add method init and last to NonEmptyChain like NonEmptyList.

core/src/main/scala/cats/data/Chain.scala

@@ -47,11 +47,47 @@ sealed abstract class Chain[+A] {
     result
   }
 
+  /**
+   * Returns the init and last of this Chain if non empty, none otherwise. Amortized O(1).
+   */
+  final def initLast: Option[(Chain[A], A)] = {
+    var c: Chain[A] = this
+    val lefts = new collection.mutable.ArrayBuffer[Chain[A]]
+    // scalastyle:off null
+    var result: Option[(Chain[A], A)] = null
+    while (result eq null) {
+      c match {
+        case Singleton(a) =>
+          val pre =
+            if (lefts.isEmpty) nil
+            else lefts.reduceLeft((x, y) => Append(x, y))
+          result = Some(pre -> a)
+        case Append(l, r) => c = r; lefts += l
+        case Wrap(seq) =>
+          val init = fromSeq(seq.init)
+          val pre =
+            if (lefts.isEmpty) init
+            else lefts.reduceLeft((x, y) => Append(x, y)) ++ init
+          result = Some((pre, seq.last))
+        case Empty =>
+          // Empty is only top level, it is never internal to an Append
+          result = None
+      }
+    }
+    // scalastyle:on null
+    result
+  }
+
   /**
    * Returns the head of this Chain if non empty, none otherwise. Amortized O(1).
    */
   def headOption: Option[A] = uncons.map(_._1)
 
+  /**
+   * Returns the last of this Chain if non empty, none otherwise. Amortized O(1).
+   */
+  final def lastOption: Option[A] = initLast.map(_._2)
+
   /**
    * Returns true if there are no elements in this collection.
    */

core/src/main/scala/cats/data/NonEmptyChain.scala

@@ -191,15 +191,30 @@ class NonEmptyChainOps[A](private val value: NonEmptyChain[A]) extends AnyVal {
   final def uncons: (A, Chain[A]) = toChain.uncons.get
 
   /**
-   * Returns the first element of this chain.
+   * Returns the init and last of this NonEmptyChain. Amortized O(1).
+   */
+  final def initLast: (Chain[A], A) = toChain.initLast.get
+
+  /**
+   * Returns the first element of this NonEmptyChain. Amortized O(1).
    */
   final def head: A = uncons._1
 
   /**
-   * Returns all but the first element of this chain.
+   * Returns all but the first element of this NonEmptyChain. Amortized O(1).
    */
   final def tail: Chain[A] = uncons._2
 
+  /**
+   * Returns all but the last element of this NonEmptyChain. Amortized O(1).
+   */
+  final def init: Chain[A] = initLast._1
+
+  /**
+   * Returns the last element of this NonEmptyChain. Amortized O(1).
+   */
+  final def last: A = initLast._2
+
   /**
    * Tests if some element is contained in this chain.
    * {{{

tests/src/test/scala/cats/tests/ChainSuite.scala

@@ -68,6 +68,12 @@ class ChainSuite extends CatsSuite {
     }
   }
 
+  test("lastOption") {
+    forAll { (c: Chain[Int]) =>
+      c.lastOption should ===(c.toList.lastOption)
+    }
+  }
+
   test("size is consistent with toList.size") {
     forAll { (ci: Chain[Int]) =>
       ci.size.toInt should ===(ci.toList.size)

tests/src/test/scala/cats/tests/NonEmptyChainSuite.scala

@@ -139,4 +139,16 @@ class NonEmptyChainSuite extends CatsSuite {
       ci.distinct.toList should ===(ci.toList.distinct)
     }
   }
+
+  test("init") {
+    forAll { ci: NonEmptyChain[Int] =>
+      ci.init.toList should ===(ci.toList.init)
+    }
+  }
+
+  test("last") {
+    forAll { ci: NonEmptyChain[Int] =>
+      ci.last should ===(ci.toList.last)
+    }
+  }
 }