Add missing Chain#distinctBy method

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

@@ -27,8 +27,8 @@ import cats.kernel.instances.StaticMethods
 
 import scala.annotation.tailrec
 import scala.collection.immutable.SortedMap
-import scala.collection.immutable.TreeSet
 import scala.collection.immutable.{IndexedSeq => ImIndexedSeq}
+import scala.collection.mutable
 import scala.collection.mutable.ListBuffer
 
 import Chain.{
@@ -151,6 +151,10 @@ sealed abstract class Chain[+A] extends ChainCompat[A] {
    */
   final def nonEmpty: Boolean = !isEmpty
 
+  // Quick check whether the chain is either empty or contains one element only.
+  @inline private def isEmptyOrSingleton: Boolean =
+    isEmpty || this.isInstanceOf[Chain.Singleton[_]]
+
   /**
    * Concatenates this with `c` in O(1) runtime.
    */
@@ -760,19 +764,59 @@ sealed abstract class Chain[+A] extends ChainCompat[A] {
 
   /**
    * Remove duplicates. Duplicates are checked using `Order[_]` instance.
+   * 
+   * Example:
+   * {{{
+   * scala> import cats.data.Chain
+   * scala> val chain = Chain(1, 2, 2, 3)
+   * scala> chain.distinct
+   * res0: cats.data.Chain[Int] = Chain(1, 2, 3)
+   * }}}
    */
   def distinct[AA >: A](implicit O: Order[AA]): Chain[AA] = {
-    implicit val ord: Ordering[AA] = O.toOrdering
-
-    var alreadyIn = TreeSet.empty[AA]
+    if (isEmptyOrSingleton) this
+    else {
+      implicit val ord: Ordering[AA] = O.toOrdering
+
+      val bldr = Vector.newBuilder[AA]
+      val seen = mutable.TreeSet.empty[AA]
+      val it = iterator
+      while (it.hasNext) {
+        val next = it.next()
+        if (seen.add(next))
+          bldr += next
+      }
+      // Result can contain a single element only.
+      Chain.fromSeq(bldr.result())
+    }
+  }
 
-    foldLeft(Chain.empty[AA]) { (elementsSoFar, b) =>
-      if (alreadyIn.contains(b)) {
-        elementsSoFar
-      } else {
-        alreadyIn += b
-        elementsSoFar :+ b
+  /**
+   * Remove duplicates by a predicate. Duplicates are checked using `Order[_]` instance.
+   * 
+   * Example:
+   * {{{
+   * scala> import cats.data.Chain
+   * scala> val chain = Chain(1, 2, 3, 4)
+   * scala> chain.distinctBy(_ / 2)
+   * res0: cats.data.Chain[Int] = Chain(1, 2, 4)
+   * }}}
+   */
+  def distinctBy[B](f: A => B)(implicit O: Order[B]): Chain[A] = {
+    if (isEmptyOrSingleton) this
+    else {
+      implicit val ord: Ordering[B] = O.toOrdering
+
+      val bldr = Vector.newBuilder[A]
+      val seen = mutable.TreeSet.empty[B]
+      val it = iterator
+      while (it.hasNext) {
+        val next = it.next()
+        if (seen.add(f(next)))
+          bldr += next
       }
+      // Result can contain a single element only.
+      Chain.fromSeq(bldr.result())
     }
   }
 

tests/shared/src/test/scala-2.12/cats/tests/compat/ScalaVersionSpecificSyntax.scala

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2015 Typelevel
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package cats.tests.compat
+
+import scala.collection.immutable.Seq
+
+private[tests] trait ScalaVersionSpecificSyntax {
+  implicit final private[tests] def catsTestsCompatSeqOps[C[a] <: Seq[a], A](self: C[A]) = new SeqOps[C, A](self)
+}

tests/shared/src/test/scala-2.12/cats/tests/compat/SeqOps.scala

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2015 Typelevel
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package cats.tests.compat
+
+import scala.collection.generic.CanBuildFrom
+import scala.collection.immutable.Seq
+import scala.collection.mutable
+
+final private[tests] class SeqOps[C[a] <: Seq[a], A] private[compat] (private val self: C[A]) extends AnyVal {
+
+  // Scala v2.12.x does not have `distinctBy` implemented.
+  // Therefore this implementation is copied (and adapted) from Scala Library v2.13.8 sources:
+  // https://github.com/scala/scala/blob/v2.13.8/src/library/scala/collection/immutable/StrictOptimizedSeqOps.scala#L26-L39
+  def distinctBy[B](f: A => B)(implicit cbf: CanBuildFrom[C[A], A, C[A]]): C[A] = {
+    if (self.lengthCompare(1) <= 0) self
+    else {
+      val builder = cbf()
+      val seen = mutable.HashSet.empty[B]
+      val it = self.iterator
+      var different = false
+      while (it.hasNext) {
+        val next = it.next()
+        if (seen.add(f(next))) builder += next else different = true
+      }
+      if (different) builder.result() else self
+    }
+  }
+}

tests/shared/src/test/scala-2.13+/cats/tests/compat/ScalaVersionSpecificSyntax.scala

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2015 Typelevel
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package cats.tests.compat
+
+private[tests] trait ScalaVersionSpecificSyntax {}

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

@@ -380,6 +380,12 @@ class ChainSuite extends CatsSuite {
     }
   }
 
+  test("Chain#distinctBy is consistent with List#distinctBy") {
+    forAll { (a: Chain[Int], f: Int => String) =>
+      assertEquals(a.distinctBy(f).toList, a.toList.distinctBy(f))
+    }
+  }
+
   test("=== is consistent with == (issue #2540)") {
     assertEquals(Chain.one(1) |+| Chain.one(2) |+| Chain.one(3), Chain.fromSeq(List(1, 2, 3)))
 

tests/shared/src/test/scala/cats/tests/package.scala

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2015 Typelevel
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package cats
+
+package object tests extends cats.tests.compat.ScalaVersionSpecificSyntax