[K/N] Experimental Char code point constants and functions

Internalize WASM Char code point functions.

As a part of efforts to stabilize Native stdlib.

kotlin-native/runtime/src/main/kotlin/generated/_StringLowercase.kt

@@ -77,6 +77,7 @@ internal fun Int.isCaseIgnorable(): Boolean {
     return index >= 0 && this <= caseIgnorableEnd[index]
 }
 
+@OptIn(kotlin.experimental.ExperimentalNativeApi::class)
 private fun String.codePointBefore(index: Int): Int {
     val low = this[index]
     if (low.isLowSurrogate() && index - 1 >= 0) {

kotlin-native/runtime/src/main/kotlin/generated/_StringUppercase.kt

@@ -10,6 +10,7 @@ package kotlin.text
 // See: https://github.com/JetBrains/kotlin/tree/master/libraries/stdlib
 //
 
+@OptIn(kotlin.experimental.ExperimentalNativeApi::class)
 internal fun String.codePointAt(index: Int): Int {
     val high = this[index]
     if (high.isHighSurrogate() && index + 1 < this.length) {
@@ -21,10 +22,10 @@ internal fun String.codePointAt(index: Int): Int {
     return high.code
 }
 
-internal fun Int.charCount(): Int = if (this >= Char.MIN_SUPPLEMENTARY_CODE_POINT) 2 else 1 
+internal fun Int.charCount(): Int = if (this > Char.MAX_VALUE.code) 2 else 1 
 
 internal fun StringBuilder.appendCodePoint(codePoint: Int) {
-    if (codePoint < Char.MIN_SUPPLEMENTARY_CODE_POINT) {
+    if (codePoint <= Char.MAX_VALUE.code) {
         append(codePoint.toChar())
     } else {
         append(Char.MIN_HIGH_SURROGATE + ((codePoint - 0x10000) shr 10))

kotlin-native/runtime/src/main/kotlin/kotlin/Char.kt

@@ -6,6 +6,7 @@
 
 package kotlin
 
+import kotlin.experimental.ExperimentalNativeApi
 import kotlin.native.internal.GCUnsafeCall
 import kotlin.native.internal.TypedIntrinsic
 import kotlin.native.internal.IntrinsicType
@@ -151,18 +152,30 @@ public class Char private constructor() : Comparable<Char> {
         public const val MAX_SURROGATE: Char = MAX_LOW_SURROGATE
 
         /**
-         * The minimum value of a supplementary code point, `\u0x10000`. Kotlin/Native specific.
+         * The minimum value of a supplementary code point, `\u0x10000`.
+         *
+         * Note that this constant is experimental.
+         * In the future it could be deprecated in favour of another constant of a `CodePoint` type.
          */
+        @ExperimentalNativeApi
         public const val MIN_SUPPLEMENTARY_CODE_POINT: Int = 0x10000
 
         /**
-         * The minimum value of a Unicode code point. Kotlin/Native specific.
+         * The minimum value of a Unicode code point.
+         *
+         * Note that this constant is experimental.
+         * In the future it could be deprecated in favour of another constant of a `CodePoint` type.
          */
+        @ExperimentalNativeApi
         public const val MIN_CODE_POINT = 0x000000
 
         /**
-         * The maximum value of a Unicode code point. Kotlin/Native specific.
+         * The maximum value of a Unicode code point.
+         *
+         * Note that this constant is experimental.
+         * In the future it could be deprecated in favour of another constant of a `CodePoint` type.
          */
+        @ExperimentalNativeApi
         public const val MAX_CODE_POINT = 0X10FFFF
 
         /**

kotlin-native/runtime/src/main/kotlin/kotlin/text/CharNative.kt

@@ -5,6 +5,7 @@
 
 package kotlin.text
 
+import kotlin.experimental.ExperimentalNativeApi
 import kotlin.native.internal.GCUnsafeCall
 
 /**
@@ -30,6 +31,58 @@ external public actual fun Char.isLowSurrogate(): Boolean
 @GCUnsafeCall("Kotlin_Char_isISOControl")
 external public actual fun Char.isISOControl(): Boolean
 
+/**
+ * Converts a surrogate pair to a unicode code point. Doesn't validate that the characters are a valid surrogate pair.
+ *
+ * Note that this function is unstable.
+ * In the future it could be deprecated in favour of an overload that would return a `CodePoint` type.
+ */
+@ExperimentalNativeApi
+// TODO: Consider removing from public API
+public fun Char.Companion.toCodePoint(high: Char, low: Char): Int =
+        (((high - MIN_HIGH_SURROGATE) shl 10) or (low - MIN_LOW_SURROGATE)) + 0x10000
+
+/**
+ * Checks if the codepoint specified is a supplementary codepoint or not.
+ *
+ * Note that this function is unstable.
+ * In the future it could be deprecated in favour of an overload that would accept a `CodePoint` type.
+ */
+@ExperimentalNativeApi
+// TODO: Consider removing from public API
+public fun Char.Companion.isSupplementaryCodePoint(codepoint: Int): Boolean =
+        codepoint in MIN_SUPPLEMENTARY_CODE_POINT..MAX_CODE_POINT
+
+/**
+ * Checks if the specified [high] and [low] chars are [Char.isHighSurrogate] and [Char.isLowSurrogate] correspondingly.
+ */
+@ExperimentalNativeApi
+// TODO: Consider removing from public API
+public fun Char.Companion.isSurrogatePair(high: Char, low: Char): Boolean = high.isHighSurrogate() && low.isLowSurrogate()
+
+/**
+ * Converts the codepoint specified to a char array. If the codepoint is not supplementary, the method will
+ * return an array with one element otherwise it will return an array A with a high surrogate in A[0] and
+ * a low surrogate in A[1].
+ *
+ *
+ * Note that this function is unstable.
+ * In the future it could be deprecated in favour of an overload that would accept a `CodePoint` type.
+ */
+@ExperimentalNativeApi
+// TODO: Consider removing from public API
+@Suppress("DEPRECATION")
+public fun Char.Companion.toChars(codePoint: Int): CharArray =
+        when {
+            codePoint in 0 until MIN_SUPPLEMENTARY_CODE_POINT -> charArrayOf(codePoint.toChar())
+            codePoint in MIN_SUPPLEMENTARY_CODE_POINT..MAX_CODE_POINT -> {
+                val low = ((codePoint - 0x10000) and 0x3FF) + MIN_LOW_SURROGATE.toInt()
+                val high = (((codePoint - 0x10000) ushr 10) and 0x3FF) + MIN_HIGH_SURROGATE.toInt()
+                charArrayOf(high.toChar(), low.toChar())
+            }
+            else -> throw IllegalArgumentException()
+        }
+
 @SharedImmutable
 private val digits = intArrayOf(
         0, 1, 2, 3, 4, 5, 6, 7, 8, 9,