Fix KT-24370. Make indexOf consistent with Kotlin/JVM

backend.native/tests/build.gradle

@@ -1720,6 +1720,9 @@ task chars0(type: RunKonanTest) {
     expectedExitStatus = 0
 }
 
+task indexof(type: RunKonanTest) {
+    source = "runtime/text/indexof.kt"
+}
 
 task utf8(type: RunKonanTest) {
     expectedFail = (project.testTarget == 'wasm32') // Uses exceptions.

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

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package runtime.text.indexof
+
+import kotlin.test.*
+
+@Test fun runTest() {
+    var str = "Hello World!!"    // for indexOf String
+    var ch = 'a'                 // for indexOf Char
+
+    assertEquals(6, str.indexOf("World", 0))
+    assertEquals(6, str.indexOf("World", -1))
+
+    assertEquals(-1, str.indexOf(ch, 0))
+
+    str = "Kotlin/Native"
+    assertEquals(-1, str.indexOf("/", str.length + 1))
+    assertEquals(-1, str.indexOf("/", Int.MAX_VALUE))
+    assertEquals(str.length, str.indexOf("", Int.MAX_VALUE))
+    assertEquals(1, str.indexOf("", 1))
+
+    assertEquals(8, str.indexOf(ch, 1))
+    assertEquals(-1, str.indexOf(ch, str.length - 1))
+
+    str = ""
+    assertEquals(-1, str.indexOf("a", -3))
+    assertEquals(0, str.indexOf("", 0))
+
+    assertEquals(-1, str.indexOf(ch, -3))
+    assertEquals(-1, str.indexOf(ch, 10))
+
+    ch = 0.toChar()
+    assertEquals(-1, str.indexOf(ch, -3))
+    assertEquals(-1, str.indexOf(ch, 10))
+}

runtime/src/main/cpp/KString.cpp

@@ -1056,7 +1056,10 @@ KInt Kotlin_Char_digitOfChecked(KChar ch, KInt radix) {
 }
 
 KInt Kotlin_String_indexOfChar(KString thiz, KChar ch, KInt fromIndex) {
-  if (fromIndex < 0 || fromIndex > thiz->count_) {
+  if (fromIndex < 0) {
+    fromIndex = 0;
+  }
+  if (fromIndex > thiz->count_) {
     return -1;
   }
   KInt count = thiz->count_;
@@ -1087,8 +1090,13 @@ KInt Kotlin_String_lastIndexOfChar(KString thiz, KChar ch, KInt fromIndex) {
 
 // TODO: or code up Knuth-Moris-Pratt.
 KInt Kotlin_String_indexOfString(KString thiz, KString other, KInt fromIndex) {
-  if (fromIndex < 0 || fromIndex > thiz->count_ ||
+  if (fromIndex < 0) {
-      other->count_ > thiz->count_ - fromIndex) {
+    fromIndex = 0;
+  }
+  if (fromIndex >= thiz->count_) {
+    return (other->count_ == 0) ? thiz->count_ : -1;
+  }
+  if (other->count_ > thiz->count_ - fromIndex) {
     return -1;
   }
   // An empty string can be always found.