From a665b2183c3bb3d5f043df3f7fb9fffc9ead53e8 Mon Sep 17 00:00:00 2001
From: Ilya Gorbunov <ilya.gorbunov@jetbrains.com>
Date: Fri, 22 Apr 2016 20:51:44 +0300
Subject: [PATCH] Optimize operations to return special collection
 implementations when result is empty or has single element. #KT-9990

---
 libraries/stdlib/src/generated/_Arrays.kt     | 72 +++++++++----------
 .../stdlib/src/generated/_Collections.kt      | 16 ++---
 .../src/templates/Filtering.kt                | 16 ++---
 .../src/templates/Ordering.kt                 |  6 +-
 4 files changed, 55 insertions(+), 55 deletions(-)

diff --git a/libraries/stdlib/src/generated/_Arrays.kt b/libraries/stdlib/src/generated/_Arrays.kt
index a3c5c08c72a..ef31447cffc 100644
--- a/libraries/stdlib/src/generated/_Arrays.kt
+++ b/libraries/stdlib/src/generated/_Arrays.kt
@@ -2526,7 +2526,7 @@ public fun <T> Array<out T>.drop(n: Int): List<T> {
     for (index in n..size - 1) {
         list.add(this[index])
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -2542,7 +2542,7 @@ public fun ByteArray.drop(n: Int): List<Byte> {
     for (index in n..size - 1) {
         list.add(this[index])
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -2558,7 +2558,7 @@ public fun ShortArray.drop(n: Int): List<Short> {
     for (index in n..size - 1) {
         list.add(this[index])
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -2574,7 +2574,7 @@ public fun IntArray.drop(n: Int): List<Int> {
     for (index in n..size - 1) {
         list.add(this[index])
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -2590,7 +2590,7 @@ public fun LongArray.drop(n: Int): List<Long> {
     for (index in n..size - 1) {
         list.add(this[index])
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -2606,7 +2606,7 @@ public fun FloatArray.drop(n: Int): List<Float> {
     for (index in n..size - 1) {
         list.add(this[index])
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -2622,7 +2622,7 @@ public fun DoubleArray.drop(n: Int): List<Double> {
     for (index in n..size - 1) {
         list.add(this[index])
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -2638,7 +2638,7 @@ public fun BooleanArray.drop(n: Int): List<Boolean> {
     for (index in n..size - 1) {
         list.add(this[index])
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -2654,7 +2654,7 @@ public fun CharArray.drop(n: Int): List<Char> {
     for (index in n..size - 1) {
         list.add(this[index])
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -3547,7 +3547,7 @@ public fun CharArray.slice(indices: IntRange): List<Char> {
  */
 public fun <T> Array<out T>.slice(indices: Iterable<Int>): List<T> {
     val size = indices.collectionSizeOrDefault(10)
-    if (size == 0) return listOf()
+    if (size == 0) return emptyList()
     val list = ArrayList<T>(size)
     for (index in indices) {
         list.add(get(index))
@@ -3560,7 +3560,7 @@ public fun <T> Array<out T>.slice(indices: Iterable<Int>): List<T> {
  */
 public fun ByteArray.slice(indices: Iterable<Int>): List<Byte> {
     val size = indices.collectionSizeOrDefault(10)
-    if (size == 0) return listOf()
+    if (size == 0) return emptyList()
     val list = ArrayList<Byte>(size)
     for (index in indices) {
         list.add(get(index))
@@ -3573,7 +3573,7 @@ public fun ByteArray.slice(indices: Iterable<Int>): List<Byte> {
  */
 public fun ShortArray.slice(indices: Iterable<Int>): List<Short> {
     val size = indices.collectionSizeOrDefault(10)
-    if (size == 0) return listOf()
+    if (size == 0) return emptyList()
     val list = ArrayList<Short>(size)
     for (index in indices) {
         list.add(get(index))
@@ -3586,7 +3586,7 @@ public fun ShortArray.slice(indices: Iterable<Int>): List<Short> {
  */
 public fun IntArray.slice(indices: Iterable<Int>): List<Int> {
     val size = indices.collectionSizeOrDefault(10)
-    if (size == 0) return listOf()
+    if (size == 0) return emptyList()
     val list = ArrayList<Int>(size)
     for (index in indices) {
         list.add(get(index))
@@ -3599,7 +3599,7 @@ public fun IntArray.slice(indices: Iterable<Int>): List<Int> {
  */
 public fun LongArray.slice(indices: Iterable<Int>): List<Long> {
     val size = indices.collectionSizeOrDefault(10)
-    if (size == 0) return listOf()
+    if (size == 0) return emptyList()
     val list = ArrayList<Long>(size)
     for (index in indices) {
         list.add(get(index))
@@ -3612,7 +3612,7 @@ public fun LongArray.slice(indices: Iterable<Int>): List<Long> {
  */
 public fun FloatArray.slice(indices: Iterable<Int>): List<Float> {
     val size = indices.collectionSizeOrDefault(10)
-    if (size == 0) return listOf()
+    if (size == 0) return emptyList()
     val list = ArrayList<Float>(size)
     for (index in indices) {
         list.add(get(index))
@@ -3625,7 +3625,7 @@ public fun FloatArray.slice(indices: Iterable<Int>): List<Float> {
  */
 public fun DoubleArray.slice(indices: Iterable<Int>): List<Double> {
     val size = indices.collectionSizeOrDefault(10)
-    if (size == 0) return listOf()
+    if (size == 0) return emptyList()
     val list = ArrayList<Double>(size)
     for (index in indices) {
         list.add(get(index))
@@ -3638,7 +3638,7 @@ public fun DoubleArray.slice(indices: Iterable<Int>): List<Double> {
  */
 public fun BooleanArray.slice(indices: Iterable<Int>): List<Boolean> {
     val size = indices.collectionSizeOrDefault(10)
-    if (size == 0) return listOf()
+    if (size == 0) return emptyList()
     val list = ArrayList<Boolean>(size)
     for (index in indices) {
         list.add(get(index))
@@ -3651,7 +3651,7 @@ public fun BooleanArray.slice(indices: Iterable<Int>): List<Boolean> {
  */
 public fun CharArray.slice(indices: Iterable<Int>): List<Char> {
     val size = indices.collectionSizeOrDefault(10)
-    if (size == 0) return listOf()
+    if (size == 0) return emptyList()
     val list = ArrayList<Char>(size)
     for (index in indices) {
         list.add(get(index))
@@ -3853,7 +3853,7 @@ public fun <T> Array<out T>.take(n: Int): List<T> {
             break
         list.add(item)
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -3870,7 +3870,7 @@ public fun ByteArray.take(n: Int): List<Byte> {
             break
         list.add(item)
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -3887,7 +3887,7 @@ public fun ShortArray.take(n: Int): List<Short> {
             break
         list.add(item)
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -3904,7 +3904,7 @@ public fun IntArray.take(n: Int): List<Int> {
             break
         list.add(item)
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -3921,7 +3921,7 @@ public fun LongArray.take(n: Int): List<Long> {
             break
         list.add(item)
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -3938,7 +3938,7 @@ public fun FloatArray.take(n: Int): List<Float> {
             break
         list.add(item)
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -3955,7 +3955,7 @@ public fun DoubleArray.take(n: Int): List<Double> {
             break
         list.add(item)
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -3972,7 +3972,7 @@ public fun BooleanArray.take(n: Int): List<Boolean> {
             break
         list.add(item)
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -3989,7 +3989,7 @@ public fun CharArray.take(n: Int): List<Char> {
             break
         list.add(item)
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -4003,7 +4003,7 @@ public fun <T> Array<out T>.takeLast(n: Int): List<T> {
     val list = ArrayList<T>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -4017,7 +4017,7 @@ public fun ByteArray.takeLast(n: Int): List<Byte> {
     val list = ArrayList<Byte>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -4031,7 +4031,7 @@ public fun ShortArray.takeLast(n: Int): List<Short> {
     val list = ArrayList<Short>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -4045,7 +4045,7 @@ public fun IntArray.takeLast(n: Int): List<Int> {
     val list = ArrayList<Int>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -4059,7 +4059,7 @@ public fun LongArray.takeLast(n: Int): List<Long> {
     val list = ArrayList<Long>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -4073,7 +4073,7 @@ public fun FloatArray.takeLast(n: Int): List<Float> {
     val list = ArrayList<Float>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -4087,7 +4087,7 @@ public fun DoubleArray.takeLast(n: Int): List<Double> {
     val list = ArrayList<Double>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -4101,7 +4101,7 @@ public fun BooleanArray.takeLast(n: Int): List<Boolean> {
     val list = ArrayList<Boolean>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -4115,7 +4115,7 @@ public fun CharArray.takeLast(n: Int): List<Char> {
     val list = ArrayList<Char>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
diff --git a/libraries/stdlib/src/generated/_Collections.kt b/libraries/stdlib/src/generated/_Collections.kt
index 50eedadf033..7563880ec4d 100644
--- a/libraries/stdlib/src/generated/_Collections.kt
+++ b/libraries/stdlib/src/generated/_Collections.kt
@@ -553,7 +553,7 @@ public fun <T> Iterable<T>.drop(n: Int): List<T> {
                 for (item in listIterator(n))
                     list.add(item)
             }
-            return list
+            return list.optimizeReadOnlyList()
         }
     }
     else {
@@ -563,7 +563,7 @@ public fun <T> Iterable<T>.drop(n: Int): List<T> {
     for (item in this) {
         if (count++ >= n) list.add(item)
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -699,7 +699,7 @@ public fun <T> List<T>.slice(indices: IntRange): List<T> {
  */
 public fun <T> List<T>.slice(indices: Iterable<Int>): List<T> {
     val size = indices.collectionSizeOrDefault(10)
-    if (size == 0) return listOf()
+    if (size == 0) return emptyList()
     val list = ArrayList<T>(size)
     for (index in indices) {
         list.add(get(index))
@@ -721,7 +721,7 @@ public fun <T> Iterable<T>.take(n: Int): List<T> {
             break
         list.add(item)
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -740,7 +740,7 @@ public fun <T> List<T>.takeLast(n: Int): List<T> {
         for (item in listIterator(n))
             list.add(item)
     }
-    return list
+    return list.optimizeReadOnlyList()
 }
 
 /**
@@ -788,7 +788,7 @@ public fun <T> MutableList<T>.reverse(): Unit {
  * Returns a list with elements in reversed order.
  */
 public fun <T> Iterable<T>.reversed(): List<T> {
-    if (this is Collection && isEmpty()) return emptyList()
+    if (this is Collection && size <= 1) return toList()
     val list = toMutableList()
     Collections.reverse(list)
     return list
@@ -820,7 +820,7 @@ public fun <T : Comparable<T>> MutableList<T>.sortDescending(): Unit {
  */
 public fun <T : Comparable<T>> Iterable<T>.sorted(): List<T> {
     if (this is Collection) {
-        if (size <= 1) return this.toMutableList()
+        if (size <= 1) return this.toList()
         @Suppress("CAST_NEVER_SUCCEEDS")
         return (toTypedArray<Comparable<T>>() as Array<T>).apply { sort() }.asList()
     }
@@ -853,7 +853,7 @@ public fun <T : Comparable<T>> Iterable<T>.sortedDescending(): List<T> {
  */
 public fun <T> Iterable<T>.sortedWith(comparator: Comparator<in T>): List<T> {
     if (this is Collection) {
-       if (size <= 1) return this.toMutableList()
+       if (size <= 1) return this.toList()
        @Suppress("CAST_NEVER_SUCCEEDS")
        return (toTypedArray<Any?>() as Array<T>).apply { sortWith(comparator) }.asList()
     }
diff --git a/libraries/tools/kotlin-stdlib-gen/src/templates/Filtering.kt b/libraries/tools/kotlin-stdlib-gen/src/templates/Filtering.kt
index c8e0ce9e1a0..aa354e32013 100644
--- a/libraries/tools/kotlin-stdlib-gen/src/templates/Filtering.kt
+++ b/libraries/tools/kotlin-stdlib-gen/src/templates/Filtering.kt
@@ -56,7 +56,7 @@ fun filtering(): List<GenericFunction> {
                         for (item in listIterator(n))
                             list.add(item)
                     }
-                    return list
+                    return list.optimizeReadOnlyList()
                 }
             }
             else {
@@ -66,7 +66,7 @@ fun filtering(): List<GenericFunction> {
             for (item in this) {
                 if (count++ >= n) list.add(item)
             }
-            return list
+            return list.optimizeReadOnlyList()
             """
         }
 
@@ -105,7 +105,7 @@ fun filtering(): List<GenericFunction> {
             for (index in n..size - 1) {
                 list.add(this[index])
             }
-            return list
+            return list.optimizeReadOnlyList()
             """
         }
     }
@@ -126,7 +126,7 @@ fun filtering(): List<GenericFunction> {
                     break
                 list.add(item)
             }
-            return list
+            return list.optimizeReadOnlyList()
             """
         }
 
@@ -165,7 +165,7 @@ fun filtering(): List<GenericFunction> {
                     break;
                 list.add(item)
             }
-            return list
+            return list.optimizeReadOnlyList()
             """
         }
     }
@@ -215,7 +215,7 @@ fun filtering(): List<GenericFunction> {
             val list = ArrayList<T>(n)
             for (index in size - n .. size - 1)
                 list.add(this[index])
-            return list
+            return list.optimizeReadOnlyList()
             """
         }
         body(Lists) {
@@ -232,7 +232,7 @@ fun filtering(): List<GenericFunction> {
                 for (item in listIterator(n))
                     list.add(item)
             }
-            return list
+            return list.optimizeReadOnlyList()
             """
         }
     }
@@ -663,7 +663,7 @@ fun filtering(): List<GenericFunction> {
         body {
             """
             val size = indices.collectionSizeOrDefault(10)
-            if (size == 0) return listOf()
+            if (size == 0) return emptyList()
             val list = ArrayList<T>(size)
             for (index in indices) {
                 list.add(get(index))
diff --git a/libraries/tools/kotlin-stdlib-gen/src/templates/Ordering.kt b/libraries/tools/kotlin-stdlib-gen/src/templates/Ordering.kt
index e87b479d915..edee0847f21 100644
--- a/libraries/tools/kotlin-stdlib-gen/src/templates/Ordering.kt
+++ b/libraries/tools/kotlin-stdlib-gen/src/templates/Ordering.kt
@@ -31,7 +31,7 @@ fun ordering(): List<GenericFunction> {
         returns { "List<T>" }
         body {
             """
-            if (this is Collection && isEmpty()) return emptyList()
+            if (this is Collection && size <= 1) return toList()
             val list = toMutableList()
             Collections.reverse(list)
             return list
@@ -101,7 +101,7 @@ fun ordering(): List<GenericFunction> {
         body {
             """
                 if (this is Collection) {
-                    if (size <= 1) return this.toMutableList()
+                    if (size <= 1) return this.toList()
                     @Suppress("CAST_NEVER_SUCCEEDS")
                     return (toTypedArray<Comparable<T>>() as Array<T>).apply { sort() }.asList()
                 }
@@ -231,7 +231,7 @@ fun ordering(): List<GenericFunction> {
         body {
             """
              if (this is Collection) {
-                if (size <= 1) return this.toMutableList()
+                if (size <= 1) return this.toList()
                 @Suppress("CAST_NEVER_SUCCEEDS")
                 return (toTypedArray<Any?>() as Array<T>).apply { sortWith(comparator) }.asList()
             }