stdlib: Add StringBuilder.reverse method

backend.native/tests/build.gradle

@@ -1080,6 +1080,11 @@ task string_builder0(type: RunKonanTest) {
     source = "runtime/text/string_builder0.kt"
 }
 
+task string_builder_reverse(type: RunKonanTest) {
+    goldValue = "OK\n"
+    source = "runtime/text/string_builder_reverse.kt"
+}
+
 task string0(type: RunKonanTest) {
     goldValue = "true\ntrue\nПРИВЕТ\nпривет\nПока\ntrue\n"
     source = "runtime/text/string0.kt"

backend.native/tests/runtime/text/string_builder_reverse.kt

@@ -0,0 +1,90 @@
+fun assertEquals(builder: StringBuilder, content: String) {
+    if (builder.toString() != content) throw AssertionError("Test failed: $builder != $content")
+}
+
+fun assertTrue(condition: Boolean) {
+    if (!condition) throw AssertionError("The condition is false")
+}
+
+fun reverseTest(original: String, reversed: String, reversedBack: String) {
+    assertEquals(StringBuilder(original).reverse(), reversed)
+    assertEquals(StringBuilder(reversed).reverse(), reversedBack)
+}
+
+
+fun main(args: Array<String>) {
+    var builder = StringBuilder("123456")
+    assertTrue(builder === builder.reverse())
+    assertEquals(builder, "654321")
+
+    builder.length = 1
+    assertEquals(builder, "6")
+
+    builder.length = 0
+    assertEquals(builder, "")
+
+    var str: String = "a"
+    reverseTest(str, str, str)
+
+    str = "ab"
+    reverseTest(str, "ba", str)
+
+    str = "abcdef"
+    reverseTest(str, "fedcba", str)
+
+    str = "abcdefg"
+    reverseTest(str, "gfedcba", str)
+
+    str = "\ud800\udc00"
+    reverseTest(str, str, str)
+
+    str = "\udc00\ud800"
+    reverseTest(str, "\ud800\udc00", "\ud800\udc00")
+
+    str = "a\ud800\udc00"
+    reverseTest(str, "\ud800\udc00a", str)
+
+    str = "ab\ud800\udc00"
+    reverseTest(str, "\ud800\udc00ba", str)
+
+    str = "abc\ud800\udc00"
+    reverseTest(str, "\ud800\udc00cba", str)
+
+    str = "\ud800\udc00\udc01\ud801\ud802\udc02"
+    reverseTest(str, "\ud802\udc02\ud801\udc01\ud800\udc00",
+            "\ud800\udc00\ud801\udc01\ud802\udc02")
+
+    str = "\ud800\udc00\ud801\udc01\ud802\udc02"
+    reverseTest(str, "\ud802\udc02\ud801\udc01\ud800\udc00", str)
+
+    str = "\ud800\udc00\udc01\ud801a"
+    reverseTest(str, "a\ud801\udc01\ud800\udc00",
+            "\ud800\udc00\ud801\udc01a")
+
+    str = "a\ud800\udc00\ud801\udc01"
+    reverseTest(str, "\ud801\udc01\ud800\udc00a", str)
+
+    str = "\ud800\udc00\udc01\ud801ab"
+    reverseTest(str, "ba\ud801\udc01\ud800\udc00",
+            "\ud800\udc00\ud801\udc01ab")
+
+    str = "ab\ud800\udc00\ud801\udc01"
+    reverseTest(str, "\ud801\udc01\ud800\udc00ba", str)
+
+    str = "\ud800\udc00\ud801\udc01"
+    reverseTest(str, "\ud801\udc01\ud800\udc00", str)
+
+    str = "a\ud800\udc00z\ud801\udc01"
+    reverseTest(str, "\ud801\udc01z\ud800\udc00a", str)
+
+    str = "a\ud800\udc00bz\ud801\udc01"
+    reverseTest(str, "\ud801\udc01zb\ud800\udc00a", str)
+
+    str = "abc\ud802\udc02\ud801\udc01\ud800\udc00"
+    reverseTest(str, "\ud800\udc00\ud801\udc01\ud802\udc02cba", str)
+
+    str = "abcd\ud802\udc02\ud801\udc01\ud800\udc00"
+    reverseTest(str, "\ud800\udc00\ud801\udc01\ud802\udc02dcba", str)
+
+    println("OK")
+}

runtime/src/main/kotlin/kotlin/text/StringBuilder.kt

@@ -40,6 +40,10 @@ class StringBuilder private constructor (
         length = array.size
     }
 
+    constructor(sequence: CharSequence): this(sequence.length) {
+        append(sequence)
+    }
+
     override var length: Int = 0
         set(capacity) {
             ensureCapacity(capacity)
@@ -75,6 +79,37 @@ class StringBuilder private constructor (
         }
     }
 
+    fun reverse(): StringBuilder {
+        var front = 0
+        var end = array.lastIndex
+        var hasSurrogates = false
+        while (front < end) {
+            if (array[front].isSurrogate() || array[end].isSurrogate()) {
+                hasSurrogates = true
+            }
+            val tmp = array[front]
+            array[front] = array[end]
+            array[end] = tmp
+            front++
+            end--
+        }
+
+        // Reverse surrogate pairs back.
+        if (hasSurrogates) {
+            var i = 0
+            while (i < length - 1) {
+                if (array[i].isLowSurrogate() && array[i + 1].isHighSurrogate()) {
+                    val tmp = array[i]
+                    array[i] = array[i + 1]
+                    array[i + 1] = tmp
+                    i++
+                }
+                i++
+            }
+        }
+        return this
+    }
+
     // Of Appenable.
     override fun append(c: Char) : StringBuilder {
         ensureExtraCapacity(1)