Optimize trivial cases of CharSequence.repeat. Provide CharSequence.repeat for JS.

#KT-3064 Fixed

libraries/stdlib/src/kotlin/text/StringsJVM.kt

@@ -457,14 +457,25 @@ public fun String.decapitalize(): String {
  * @sample test.text.StringJVMTest.repeat
  */
 public fun CharSequence.repeat(n: Int): String {
-    if (n < 0)
-        throw IllegalArgumentException("Value should be non-negative, but was $n")
+    require (n >= 0) { "Count 'n' must be non-negative, but was $n." }
 
-    val sb = StringBuilder(n * length)
-    for (i in 1..n) {
-        sb.append(this)
+    return when (n) {
+        0 -> ""
+        1 -> this.toString()
+        else -> {
+            when (length) {
+                0 -> ""
+                1 -> this[0].let { char -> String(CharArray(n) { char }) }
+                else -> {
+                    val sb = StringBuilder(n * length)
+                    for (i in 1..n) {
+                        sb.append(this)
+                    }
+                    sb.toString()
+                }
+            }
+        }
     }
-    return sb.toString()
 }
 
 

libraries/stdlib/test/text/StringJVMTest.kt

@@ -45,17 +45,6 @@ class StringJVMTest {
         }
     }
 
-    @test fun repeat() = withOneCharSequenceArg { arg1 ->
-        fun String.repeat(n: Int): String = arg1(this).repeat(n)
-
-        assertFails { "foo".repeat(-1) }
-        assertEquals("", "foo".repeat(0))
-        assertEquals("foo", "foo".repeat(1))
-        assertEquals("foofoo", "foo".repeat(2))
-        assertEquals("foofoofoo", "foo".repeat(3))
-    }
-
-
     @test fun sliceCharSequenceFails() = withOneCharSequenceArg { arg1 ->
         assertFails {
             arg1("abc").slice(1..4)

libraries/stdlib/test/text/StringTest.kt

@@ -334,6 +334,19 @@ class StringTest {
         assertEquals("xxx", s.replaceBeforeLast("=", "/new/path", "xxx"))
     }
 
+    @test fun repeat() = withOneCharSequenceArg { arg1 ->
+        fun String.repeat(n: Int): String = arg1(this).repeat(n)
+
+        assertFails { "foo".repeat(-1) }
+        assertEquals("", "foo".repeat(0))
+        assertEquals("foo", "foo".repeat(1))
+        assertEquals("foofoo", "foo".repeat(2))
+        assertEquals("foofoofoo", "foo".repeat(3))
+
+        assertEquals("", "".repeat(Int.MAX_VALUE))
+        assertEquals("aaaaaaaaaaaaa", "a".repeat(13))
+    }
+
     @test fun stringIterator() = withOneCharSequenceArg("239") { data ->
         var sum = 0
         for(c in data)