Add String.subSequence implementation. (#61)

backend.native/tests/build.gradle

@@ -198,6 +198,11 @@ task tostring0(type: RunKonanTest) {
     source = "runtime/basic/tostring0.kt"
 }
 
+task tostring1(type: RunKonanTest) {
+    goldValue = "ello\n"
+    source = "runtime/basic/tostring1.kt"
+}
+
 task array0(type: RunKonanTest) {
     goldValue = "5\n6\n7\n8\n9\n10\n11\n12\n13\n"
     source = "runtime/basic/array0.kt"

backend.native/tests/runtime/basic/tostring1.kt

@@ -0,0 +1,4 @@
+fun main(args : Array<String>) {
+    val hello = "Hello world"
+    println(hello.subSequence(1, 5).toString())
+}

runtime/src/main/cpp/Natives.cpp

@@ -11,6 +11,9 @@
 
 extern "C" {
 
+// TODO: remove, once can generate empty string constant in compile time.
+KString theEmptyString = nullptr;
+
 // Any.kt
 KBoolean Kotlin_Any_equals(KConstRef thiz, KConstRef other) {
   return thiz == other;
@@ -123,9 +126,21 @@ KInt Kotlin_String_hashCode(KString thiz) {
   return CityHash64(ByteArrayAddressOfElementAt(thiz, 0), thiz->count_);
 }
 
-KRef Kotlin_String_subSequence(KString thiz, KInt startIndex, KInt endIndex) {
-  RuntimeAssert(false, "Unsupported operation");
-  return nullptr;
+KString Kotlin_String_subSequence(KString thiz, KInt startIndex, KInt endIndex) {
+  if (startIndex < 0 || endIndex >= thiz->count_ || startIndex > endIndex) {
+    // TODO: is it correct exception?
+    ThrowArrayIndexOutOfBoundsException();
+  }
+  if (startIndex == endIndex) {
+    return theEmptyString;
+  }
+  // TODO: support UTF-8.
+  KInt length = endIndex - startIndex;
+  ArrayHeader* result = ArrayContainer(theStringTypeInfo, length).GetPlace();
+  memcpy(ByteArrayAddressOfElementAt(result, 0),
+         ByteArrayAddressOfElementAt(thiz, startIndex),
+         length);
+  return result;
 }
 
 }  // extern "C"

runtime/src/main/cpp/Natives.h

@@ -51,6 +51,8 @@ inline const KRef* ArrayAddressOfElementAt(const ArrayHeader* obj, KInt index) {
 extern "C" {
 #endif
 
+extern KString theEmptyString;
+
 // Any.kt
 KBoolean Kotlin_Any_equals(KConstRef thiz, KConstRef other);
 KInt Kotlin_Any_hashCode(KConstRef thiz);
@@ -95,7 +97,7 @@ KChar Kotlin_String_get(KString thiz, KInt index);
 KString Kotlin_String_fromUtf8Array(const ArrayHeader* array);
 KString Kotlin_String_plusImpl(KString thiz, KString other);
 KInt Kotlin_String_getStringLength(KString thiz);
-KRef Kotlin_String_subSequence(KString thiz, KInt startIndex, KInt endIndex);
+KString Kotlin_String_subSequence(KString thiz, KInt startIndex, KInt endIndex);
 
 #ifdef __cplusplus
 }

runtime/src/main/kotlin/kotlin/String.kt

@@ -3,6 +3,10 @@ package kotlin
 @SymbolName("Kotlin_String_fromUtf8Array")
 external fun fromUtf8Array(array: ByteArray) : String
 
+// TODO: enable, once global variables implemented.
+// @SymbolName("theEmptyString")
+// val theEmptyString = ""
+
 @ExportTypeInfo("theStringTypeInfo")
 public final class String : Comparable<String>, CharSequence {
     @SymbolName("Kotlin_String_hashCode")