Added Chain#distinct and NonEmptyChain#distinct to match `List#di…

…stinct` and `NonEmptyList#distinct` (#2579)

* Chain#distinct

* NonEmptyChain#distinct

* fmt

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

@@ -5,6 +5,7 @@ import Chain._
 
 import scala.annotation.tailrec
 import scala.collection.immutable.SortedMap
+import scala.collection.immutable.TreeSet
 import scala.collection.mutable.ListBuffer
 
 /**
@@ -358,6 +359,24 @@ sealed abstract class Chain[+A] {
       iterX.hasNext == iterY.hasNext
     }
 
+  /**
+   * Remove duplicates. Duplicates are checked using `Order[_]` instance.
+   */
+  def distinct[AA >: A](implicit O: Order[AA]): Chain[AA] = {
+    implicit val ord = O.toOrdering
+
+    var alreadyIn = TreeSet.empty[AA]
+
+    foldLeft(Chain.empty[AA]) { (elementsSoFar, b) =>
+      if (alreadyIn.contains(b)) {
+        elementsSoFar
+      } else {
+        alreadyIn += b
+        elementsSoFar :+ b
+      }
+    }
+  }
+
   def show[AA >: A](implicit AA: Show[AA]): String = {
     val builder = new StringBuilder("Chain(")
     var first = true

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

@@ -372,6 +372,24 @@ class NonEmptyChainOps[A](val value: NonEmptyChain[A]) extends AnyVal {
   /** Reverses this `NonEmptyChain` */
   final def reverse: NonEmptyChain[A] =
     create(toChain.reverse)
+
+  /**
+   * Remove duplicates. Duplicates are checked using `Order[_]` instance.
+   */
+  final def distinct[AA >: A](implicit O: Order[AA]): NonEmptyChain[AA] = {
+    implicit val ord = O.toOrdering
+
+    var alreadyIn = TreeSet(head: AA)
+
+    foldLeft(NonEmptyChain(head: AA)) { (elementsSoFar, b) =>
+      if (alreadyIn.contains(b)) {
+        elementsSoFar
+      } else {
+        alreadyIn += b
+        elementsSoFar :+ b
+      }
+    }
+  }
 }
 
 sealed abstract private[data] class NonEmptyChainInstances extends NonEmptyChainInstances1 {

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

@@ -174,4 +174,10 @@ class ChainSuite extends CatsSuite {
       }
     }
   }
+
+  test("Chain#distinct is consistent with List#distinct") {
+    forAll { a: Chain[Int] =>
+      a.distinct.toList should ===(a.toList.distinct)
+    }
+  }
 }

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

@@ -133,4 +133,10 @@ class NonEmptyChainSuite extends CatsSuite {
       ci.reverse.toChain should ===(ci.toChain.reverse)
     }
   }
+
+  test("NonEmptyChain#distinct is consistent with List#distinct") {
+    forAll { ci: NonEmptyChain[Int] =>
+      ci.distinct.toList should ===(ci.toList.distinct)
+    }
+  }
 }