From ade69ec0e93cb1182d734624deaf2cf744831c55 Mon Sep 17 00:00:00 2001
From: Ilya Gorbunov <ilya.gorbunov@jetbrains.com>
Date: Fri, 10 Apr 2015 21:00:21 +0300
Subject: [PATCH] Provide the emptySequence() method. sequenceOf() with no
 arguments is equivalent to emptySequence(). #KT-7233 Fixed

---
 .../stdlib/src/kotlin/collections/Sequence.kt    | 16 +++++++++++++++-
 .../stdlib/test/collections/SequenceTest.kt      | 13 +++++++------
 2 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/libraries/stdlib/src/kotlin/collections/Sequence.kt b/libraries/stdlib/src/kotlin/collections/Sequence.kt
index a2d51fe4197..581564795c2 100644
--- a/libraries/stdlib/src/kotlin/collections/Sequence.kt
+++ b/libraries/stdlib/src/kotlin/collections/Sequence.kt
@@ -53,7 +53,7 @@ public fun<T> Enumeration<T>.asSequence(): Sequence<T> = this.iterator().sequenc
 /**
  * Creates a sequence that returns the specified values.
  */
-public fun <T> sequenceOf(vararg elements: T): Sequence<T> = elements.asSequence()
+public fun <T> sequenceOf(vararg elements: T): Sequence<T> = if (elements.isEmpty()) emptySequence() else elements.asSequence()
 
 /**
  * Creates a sequence that returns all values in the specified [progression].
@@ -62,6 +62,20 @@ public fun <T> sequenceOf(progression: Progression<T>): Sequence<T> = object : S
     override fun iterator(): Iterator<T> = progression.iterator()
 }
 
+/**
+ * Returns an empty sequence.
+ */
+public fun <T> emptySequence(): Sequence<T> = EmptySequence
+
+private object EmptySequence : Sequence<Nothing> {
+    override fun iterator(): Iterator<Nothing> = EmptySequenceIterator
+}
+
+private object EmptySequenceIterator : Iterator<Nothing> {
+    override fun next(): Nothing = throw NoSuchElementException("Sequence is empty.")
+    override fun hasNext(): Boolean = false
+}
+
 deprecated("Use FilteringSequence<T> instead")
 public class FilteringStream<T>(stream: Stream<T>, sendWhen: Boolean = true, predicate: (T) -> Boolean)
 : Stream<T> by FilteringSequence<T>(stream.toSequence(), sendWhen, predicate)
diff --git a/libraries/stdlib/test/collections/SequenceTest.kt b/libraries/stdlib/test/collections/SequenceTest.kt
index eebcdfd8e4d..1b09f563d0c 100644
--- a/libraries/stdlib/test/collections/SequenceTest.kt
+++ b/libraries/stdlib/test/collections/SequenceTest.kt
@@ -20,15 +20,16 @@ fun fibonacci(): Sequence<Int> {
 public class SequenceTest {
 
     test fun filterEmptySequence() {
-        val sequence = sequenceOf<String>()
-        assertEquals(0, sequence.filter { false }.count())
-        assertEquals(0, sequence.filter { true }.count())
+        for (sequence in listOf(emptySequence<String>(), sequenceOf<String>())) {
+            assertEquals(0, sequence.filter { false }.count())
+            assertEquals(0, sequence.filter { true }.count())
+        }
     }
 
     test fun mapEmptySequence() {
-        val sequence = sequenceOf<String>()
-        assertEquals(0, sequence.map { false }.count())
-        assertEquals(0, sequence.map { true }.count())
+        for (sequence in listOf(emptySequence<String>(), sequenceOf<String>())) {
+            assertEquals(0, sequence.map { true }.count())
+        }
     }
 
     test fun requireNoNulls() {