[WASM] Replace String storage type from CharArray to WasmCharArray

libraries/stdlib/wasm/builtins/kotlin/String.kt

@@ -12,20 +12,27 @@ import kotlin.math.min
  * The `String` class represents character strings. All string literals in Kotlin programs, such as `"abc"`, are
  * implemented as instances of this class.
  */
-public class String private constructor(internal val chars: CharArray) : Comparable<String>, CharSequence {
+public class String private constructor(internal val chars: WasmCharArray) : Comparable<String>, CharSequence {
     public companion object {
         // Note: doesn't copy the array, use with care.
-        internal fun unsafeFromCharArray(chars: CharArray) = String(chars)
+        internal fun unsafeFromCharArray(chars: WasmCharArray) = String(chars)
     }
 
     /**
      * Returns a string obtained by concatenating this string with the string representation of the given [other] object.
      */
-    public operator fun plus(other: Any?): String =
-        String(chars + other.toString().chars)
+    public operator fun plus(other: Any?): String {
+        val otherChars = if (other is String) other.chars else other.toString().chars
+        val newCharsLen = chars.len() + otherChars.len()
+        val newChars = WasmCharArray(newCharsLen)
+        newChars.fill(newCharsLen) { i ->
+            if (i < chars.len()) chars.get(i) else otherChars.get(i - chars.len())
+        }
+        return String(newChars)
+    }
 
     public override val length: Int
-        get() = chars.size
+        get() = chars.len()
 
     /**
      * Returns the character of this string at the specified [index].
@@ -33,10 +40,20 @@ public class String private constructor(internal val chars: CharArray) : Compara
      * If the [index] is out of bounds of this string, throws an [IndexOutOfBoundsException] except in Kotlin/JS
      * where the behavior is unspecified.
      */
-    public override fun get(index: Int): Char = chars[index]
+    public override fun get(index: Int): Char {
+        if (index < 0 || index >= chars.len()) throw IndexOutOfBoundsException()
+        return chars.get(index)
+    }
 
     public override fun subSequence(startIndex: Int, endIndex: Int): CharSequence {
-        return String(chars.sliceArray(startIndex until endIndex))
+        val actualStartIndex = startIndex.coerceAtLeast(0)
+        val actualEndIndex = endIndex.coerceAtMost(chars.len())
+        val newCharsLen = actualEndIndex - actualStartIndex
+        val newChars = WasmCharArray(newCharsLen)
+        newChars.fill(newCharsLen) { i ->
+            chars.get(actualStartIndex + i)
+        }
+        return String(newChars)
     }
 
     public override fun compareTo(other: String): Int {
@@ -62,12 +79,16 @@ public class String private constructor(internal val chars: CharArray) : Compara
     public override fun hashCode(): Int {
         if (_hashCode != 0 || this.isEmpty())
             return _hashCode
+
         var hash = 0
-        for (c in chars)
-            hash = 31 * hash + c.toInt()
+        var i = 0
+        while (i < chars.len()) {
+            hash = 31 * hash + chars.get(i).toInt()
+            i++
+        }
         _hashCode = hash
         return _hashCode
     }
 }
 
-internal fun stringLiteral(startAddr: Int, length: Int) = String.unsafeFromCharArray(unsafeRawMemoryToCharArray(startAddr, length))
+internal fun stringLiteral(startAddr: Int, length: Int) = String.unsafeFromCharArray(unsafeRawMemoryToWasmCharArray(startAddr, length))