Fix String.offsetByCodePoints for unpaired surrogates (scala-native#3471

)

Previously, String.offsetByCodePoints had an error
when the offsetByCodePoints ends with the unpaired higher surrogate.

This commit fix the bound check in the offsetByCodePoints not to
check the next index is the lower surrogate after the higher surrogate.

javalib/src/main/scala/java/lang/Character.scala

@@ -378,7 +378,7 @@ object Character {
         }
         if (isHighSurrogate(seq(i))) {
           val next = i + 1
-          if (next <= end && isLowSurrogate(seq(next))) {
+          if (next < end && isLowSurrogate(seq(next))) {
             i += 1
           }
         }
@@ -396,7 +396,7 @@ object Character {
         }
         if (isLowSurrogate(seq(i))) {
           val prev = i - 1
-          if (prev >= start && isHighSurrogate(seq(prev))) {
+          if (prev > start && isHighSurrogate(seq(prev))) {
             i -= 1
           }
         }

unit-tests/shared/src/test/scala/org/scalanative/testsuite/javalib/lang/StringTest.scala

@@ -127,6 +127,32 @@ class StringTest {
     )
   }
 
+  @Test def offsetByCodePoints(): Unit = {
+    assertTrue("abc".offsetByCodePoints(0, 3) == 3)
+    assertTrue("abc".offsetByCodePoints(1, 2) == 3)
+
+    assertTrue("abc".offsetByCodePoints(3, -3) == 0)
+    assertTrue("abc".offsetByCodePoints(3, -2) == 1)
+
+    assertTrue("\uD800\uDC00".offsetByCodePoints(0, 1) == 2)
+    assertTrue("\uD800\uDC00".offsetByCodePoints(1, -1) == 0)
+  }
+
+  @Test def offsetByCodePointsUnpairedSurrogates(): Unit = {
+    assertTrue("\uD800".offsetByCodePoints(0, 1) == 1)
+    assertTrue("\uDBFF".offsetByCodePoints(0, 1) == 1)
+    assertTrue("\uDC00".offsetByCodePoints(0, 1) == 1)
+    assertTrue("\uDFFF".offsetByCodePoints(0, 1) == 1)
+
+    assertTrue("\uD800".offsetByCodePoints(1, -1) == 0)
+    assertTrue("\uDBFF".offsetByCodePoints(1, -1) == 0)
+    assertTrue("\uDC00".offsetByCodePoints(1, -1) == 0)
+    assertTrue("\uDFFF".offsetByCodePoints(1, -1) == 0)
+
+    assertTrue("\uD800x".offsetByCodePoints(0, 2) == 2)
+    assertTrue("x\uD800".offsetByCodePoints(0, 2) == 2)
+  }
+
   @Test def compareTo(): Unit = {
     assertTrue("test".compareTo("utest") < 0)
     assertTrue("test".compareTo("test") == 0)