Handle special cases of drop, take, takeLast when it is known in advance that the result is a single-item list. #KT-9990

libraries/stdlib/src/generated/_Arrays.kt

@@ -2518,15 +2518,7 @@ public inline fun CharArray.singleOrNull(predicate: (Char) -> Boolean): Char? {
  */
 public fun <T> Array<out T>.drop(n: Int): List<T> {
     require(n >= 0) { "Requested element count $n is less than zero." }
-    if (n == 0)
-        return toList()
-    if (n >= size)
-        return emptyList()
-    val list = ArrayList<T>(size - n)
-    for (index in n..size - 1) {
-        list.add(this[index])
-    }
-    return list.optimizeReadOnlyList()
+    return takeLast((size - n).coerceAtLeast(0))
 }
 
 /**
@@ -2534,15 +2526,7 @@ public fun <T> Array<out T>.drop(n: Int): List<T> {
  */
 public fun ByteArray.drop(n: Int): List<Byte> {
     require(n >= 0) { "Requested element count $n is less than zero." }
-    if (n == 0)
-        return toList()
-    if (n >= size)
-        return emptyList()
-    val list = ArrayList<Byte>(size - n)
-    for (index in n..size - 1) {
-        list.add(this[index])
-    }
-    return list.optimizeReadOnlyList()
+    return takeLast((size - n).coerceAtLeast(0))
 }
 
 /**
@@ -2550,15 +2534,7 @@ public fun ByteArray.drop(n: Int): List<Byte> {
  */
 public fun ShortArray.drop(n: Int): List<Short> {
     require(n >= 0) { "Requested element count $n is less than zero." }
-    if (n == 0)
-        return toList()
-    if (n >= size)
-        return emptyList()
-    val list = ArrayList<Short>(size - n)
-    for (index in n..size - 1) {
-        list.add(this[index])
-    }
-    return list.optimizeReadOnlyList()
+    return takeLast((size - n).coerceAtLeast(0))
 }
 
 /**
@@ -2566,15 +2542,7 @@ public fun ShortArray.drop(n: Int): List<Short> {
  */
 public fun IntArray.drop(n: Int): List<Int> {
     require(n >= 0) { "Requested element count $n is less than zero." }
-    if (n == 0)
-        return toList()
-    if (n >= size)
-        return emptyList()
-    val list = ArrayList<Int>(size - n)
-    for (index in n..size - 1) {
-        list.add(this[index])
-    }
-    return list.optimizeReadOnlyList()
+    return takeLast((size - n).coerceAtLeast(0))
 }
 
 /**
@@ -2582,15 +2550,7 @@ public fun IntArray.drop(n: Int): List<Int> {
  */
 public fun LongArray.drop(n: Int): List<Long> {
     require(n >= 0) { "Requested element count $n is less than zero." }
-    if (n == 0)
-        return toList()
-    if (n >= size)
-        return emptyList()
-    val list = ArrayList<Long>(size - n)
-    for (index in n..size - 1) {
-        list.add(this[index])
-    }
-    return list.optimizeReadOnlyList()
+    return takeLast((size - n).coerceAtLeast(0))
 }
 
 /**
@@ -2598,15 +2558,7 @@ public fun LongArray.drop(n: Int): List<Long> {
  */
 public fun FloatArray.drop(n: Int): List<Float> {
     require(n >= 0) { "Requested element count $n is less than zero." }
-    if (n == 0)
-        return toList()
-    if (n >= size)
-        return emptyList()
-    val list = ArrayList<Float>(size - n)
-    for (index in n..size - 1) {
-        list.add(this[index])
-    }
-    return list.optimizeReadOnlyList()
+    return takeLast((size - n).coerceAtLeast(0))
 }
 
 /**
@@ -2614,15 +2566,7 @@ public fun FloatArray.drop(n: Int): List<Float> {
  */
 public fun DoubleArray.drop(n: Int): List<Double> {
     require(n >= 0) { "Requested element count $n is less than zero." }
-    if (n == 0)
-        return toList()
-    if (n >= size)
-        return emptyList()
-    val list = ArrayList<Double>(size - n)
-    for (index in n..size - 1) {
-        list.add(this[index])
-    }
-    return list.optimizeReadOnlyList()
+    return takeLast((size - n).coerceAtLeast(0))
 }
 
 /**
@@ -2630,15 +2574,7 @@ public fun DoubleArray.drop(n: Int): List<Double> {
  */
 public fun BooleanArray.drop(n: Int): List<Boolean> {
     require(n >= 0) { "Requested element count $n is less than zero." }
-    if (n == 0)
-        return toList()
-    if (n >= size)
-        return emptyList()
-    val list = ArrayList<Boolean>(size - n)
-    for (index in n..size - 1) {
-        list.add(this[index])
-    }
-    return list.optimizeReadOnlyList()
+    return takeLast((size - n).coerceAtLeast(0))
 }
 
 /**
@@ -2646,15 +2582,7 @@ public fun BooleanArray.drop(n: Int): List<Boolean> {
  */
 public fun CharArray.drop(n: Int): List<Char> {
     require(n >= 0) { "Requested element count $n is less than zero." }
-    if (n == 0)
-        return toList()
-    if (n >= size)
-        return emptyList()
-    val list = ArrayList<Char>(size - n)
-    for (index in n..size - 1) {
-        list.add(this[index])
-    }
-    return list.optimizeReadOnlyList()
+    return takeLast((size - n).coerceAtLeast(0))
 }
 
 /**
@@ -3846,6 +3774,7 @@ public fun <T> Array<out T>.take(n: Int): List<T> {
     require(n >= 0) { "Requested element count $n is less than zero." }
     if (n == 0) return emptyList()
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[0])
     var count = 0
     val list = ArrayList<T>(n)
     for (item in this) {
@@ -3853,7 +3782,7 @@ public fun <T> Array<out T>.take(n: Int): List<T> {
             break
         list.add(item)
     }
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -3863,6 +3792,7 @@ public fun ByteArray.take(n: Int): List<Byte> {
     require(n >= 0) { "Requested element count $n is less than zero." }
     if (n == 0) return emptyList()
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[0])
     var count = 0
     val list = ArrayList<Byte>(n)
     for (item in this) {
@@ -3870,7 +3800,7 @@ public fun ByteArray.take(n: Int): List<Byte> {
             break
         list.add(item)
     }
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -3880,6 +3810,7 @@ public fun ShortArray.take(n: Int): List<Short> {
     require(n >= 0) { "Requested element count $n is less than zero." }
     if (n == 0) return emptyList()
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[0])
     var count = 0
     val list = ArrayList<Short>(n)
     for (item in this) {
@@ -3887,7 +3818,7 @@ public fun ShortArray.take(n: Int): List<Short> {
             break
         list.add(item)
     }
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -3897,6 +3828,7 @@ public fun IntArray.take(n: Int): List<Int> {
     require(n >= 0) { "Requested element count $n is less than zero." }
     if (n == 0) return emptyList()
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[0])
     var count = 0
     val list = ArrayList<Int>(n)
     for (item in this) {
@@ -3904,7 +3836,7 @@ public fun IntArray.take(n: Int): List<Int> {
             break
         list.add(item)
     }
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -3914,6 +3846,7 @@ public fun LongArray.take(n: Int): List<Long> {
     require(n >= 0) { "Requested element count $n is less than zero." }
     if (n == 0) return emptyList()
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[0])
     var count = 0
     val list = ArrayList<Long>(n)
     for (item in this) {
@@ -3921,7 +3854,7 @@ public fun LongArray.take(n: Int): List<Long> {
             break
         list.add(item)
     }
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -3931,6 +3864,7 @@ public fun FloatArray.take(n: Int): List<Float> {
     require(n >= 0) { "Requested element count $n is less than zero." }
     if (n == 0) return emptyList()
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[0])
     var count = 0
     val list = ArrayList<Float>(n)
     for (item in this) {
@@ -3938,7 +3872,7 @@ public fun FloatArray.take(n: Int): List<Float> {
             break
         list.add(item)
     }
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -3948,6 +3882,7 @@ public fun DoubleArray.take(n: Int): List<Double> {
     require(n >= 0) { "Requested element count $n is less than zero." }
     if (n == 0) return emptyList()
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[0])
     var count = 0
     val list = ArrayList<Double>(n)
     for (item in this) {
@@ -3955,7 +3890,7 @@ public fun DoubleArray.take(n: Int): List<Double> {
             break
         list.add(item)
     }
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -3965,6 +3900,7 @@ public fun BooleanArray.take(n: Int): List<Boolean> {
     require(n >= 0) { "Requested element count $n is less than zero." }
     if (n == 0) return emptyList()
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[0])
     var count = 0
     val list = ArrayList<Boolean>(n)
     for (item in this) {
@@ -3972,7 +3908,7 @@ public fun BooleanArray.take(n: Int): List<Boolean> {
             break
         list.add(item)
     }
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -3982,6 +3918,7 @@ public fun CharArray.take(n: Int): List<Char> {
     require(n >= 0) { "Requested element count $n is less than zero." }
     if (n == 0) return emptyList()
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[0])
     var count = 0
     val list = ArrayList<Char>(n)
     for (item in this) {
@@ -3989,7 +3926,7 @@ public fun CharArray.take(n: Int): List<Char> {
             break
         list.add(item)
     }
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -4000,10 +3937,11 @@ public fun <T> Array<out T>.takeLast(n: Int): List<T> {
     if (n == 0) return emptyList()
     val size = size
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[size - 1])
     val list = ArrayList<T>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -4014,10 +3952,11 @@ public fun ByteArray.takeLast(n: Int): List<Byte> {
     if (n == 0) return emptyList()
     val size = size
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[size - 1])
     val list = ArrayList<Byte>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -4028,10 +3967,11 @@ public fun ShortArray.takeLast(n: Int): List<Short> {
     if (n == 0) return emptyList()
     val size = size
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[size - 1])
     val list = ArrayList<Short>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -4042,10 +3982,11 @@ public fun IntArray.takeLast(n: Int): List<Int> {
     if (n == 0) return emptyList()
     val size = size
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[size - 1])
     val list = ArrayList<Int>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -4056,10 +3997,11 @@ public fun LongArray.takeLast(n: Int): List<Long> {
     if (n == 0) return emptyList()
     val size = size
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[size - 1])
     val list = ArrayList<Long>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -4070,10 +4012,11 @@ public fun FloatArray.takeLast(n: Int): List<Float> {
     if (n == 0) return emptyList()
     val size = size
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[size - 1])
     val list = ArrayList<Float>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -4084,10 +4027,11 @@ public fun DoubleArray.takeLast(n: Int): List<Double> {
     if (n == 0) return emptyList()
     val size = size
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[size - 1])
     val list = ArrayList<Double>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -4098,10 +4042,11 @@ public fun BooleanArray.takeLast(n: Int): List<Boolean> {
     if (n == 0) return emptyList()
     val size = size
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[size - 1])
     val list = ArrayList<Boolean>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**
@@ -4112,10 +4057,11 @@ public fun CharArray.takeLast(n: Int): List<Char> {
     if (n == 0) return emptyList()
     val size = size
     if (n >= size) return toList()
+    if (n == 1) return listOf(this[size - 1])
     val list = ArrayList<Char>(n)
     for (index in size - n .. size - 1)
         list.add(this[index])
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**

libraries/stdlib/src/generated/_Collections.kt

@@ -544,6 +544,8 @@ public fun <T> Iterable<T>.drop(n: Int): List<T> {
         val resultSize = size - n
         if (resultSize <= 0)
             return emptyList()
+        if (resultSize == 1)
+            return listOf(last())
         list = ArrayList<T>(resultSize)
         if (this is List<T>) {
             if (this is RandomAccess) {
@@ -553,7 +555,7 @@ public fun <T> Iterable<T>.drop(n: Int): List<T> {
                 for (item in listIterator(n))
                     list.add(item)
             }
-            return list.optimizeReadOnlyList()
+            return list
         }
     }
     else {
@@ -713,7 +715,10 @@ public fun <T> List<T>.slice(indices: Iterable<Int>): List<T> {
 public fun <T> Iterable<T>.take(n: Int): List<T> {
     require(n >= 0) { "Requested element count $n is less than zero." }
     if (n == 0) return emptyList()
-    if (this is Collection<T> && n >= size) return toList()
+    if (this is Collection<T>) {
+        if (n >= size) return toList()
+        if (n == 1) return listOf(first())
+    }
     var count = 0
     val list = ArrayList<T>(n)
     for (item in this) {
@@ -732,6 +737,7 @@ public fun <T> List<T>.takeLast(n: Int): List<T> {
     if (n == 0) return emptyList()
     val size = size
     if (n >= size) return toList()
+    if (n == 1) return listOf(last())
     val list = ArrayList<T>(n)
     if (this is RandomAccess) {
         for (index in size - n .. size - 1)
@@ -740,7 +746,7 @@ public fun <T> List<T>.takeLast(n: Int): List<T> {
         for (item in listIterator(n))
             list.add(item)
     }
-    return list.optimizeReadOnlyList()
+    return list
 }
 
 /**