Surrogate pair index conversion in Java.

SurrogatePair.java

public class SurrogatePair {
    public static int convertToUTF16Index(String str, int index) {
        int result = 0;
        for (int i = 0; i < index && result < str.length(); i++, result++) {
            if (result + 1 < str.length() &&
                    Character.isSurrogatePair(str.charAt(result), str.charAt(result + 1))) {
                result++;
            }
        }
        return result;
    }

    public static int convertFromUTF16Index(String str, int index) {
        int result = 0;
        for (int i = 0; i < index && i < str.length(); i++, result++) {
            if (i + 1 < str.length() &&
                    Character.isSurrogatePair(str.charAt(i), str.charAt(i + 1))) {
                i++;
            }
        }
        return result;
    }
}

SurrogatePairTest.java

import org.junit.Test;
import static org.junit.Assert.assertEquals;

public class SurrogatePairTest {
    @Test
    public void test1() {
        String str = "アナと雪の女王";
        int len = str.length();
        assertEquals(0, SurrogatePair.convertToUTF16Index(str, -1));
        assertEquals(0, SurrogatePair.convertFromUTF16Index(str, -1));
        for (int i = 0; i <= len; i++) {
            assertEquals(i, SurrogatePair.convertToUTF16Index(str, i));
            assertEquals(i, SurrogatePair.convertFromUTF16Index(str, i));
        }
        assertEquals(len, SurrogatePair.convertToUTF16Index(str, len + 1));
        assertEquals(len, SurrogatePair.convertFromUTF16Index(str, len + 1));
    }

    @Test
    public void test2() {
        String str = "𩸽𩹉と𡢽の👰王";
        int len = str.length();
        assertEquals(11, len);

        assertEquals(0, SurrogatePair.convertToUTF16Index(str, 0));
        assertEquals(2, SurrogatePair.convertToUTF16Index(str, 1));
        assertEquals(4, SurrogatePair.convertToUTF16Index(str, 2));
        assertEquals(5, SurrogatePair.convertToUTF16Index(str, 3));
        assertEquals(7, SurrogatePair.convertToUTF16Index(str, 4));
        assertEquals(8, SurrogatePair.convertToUTF16Index(str, 5));
        assertEquals(10, SurrogatePair.convertToUTF16Index(str, 6));
        assertEquals(11, SurrogatePair.convertToUTF16Index(str, 7));
    }

    @Test
    public void test3() {
        String str = "𩸽𩹉と𡢽の👰王";
        int len = str.length();
        assertEquals(11, len);

        assertEquals(0, SurrogatePair.convertFromUTF16Index(str, 0));
        assertEquals(1, SurrogatePair.convertFromUTF16Index(str, 2));
        assertEquals(2, SurrogatePair.convertFromUTF16Index(str, 4));
        assertEquals(3, SurrogatePair.convertFromUTF16Index(str, 5));
        assertEquals(4, SurrogatePair.convertFromUTF16Index(str, 7));
        assertEquals(5, SurrogatePair.convertFromUTF16Index(str, 8));
        assertEquals(6, SurrogatePair.convertFromUTF16Index(str, 10));
        assertEquals(7, SurrogatePair.convertFromUTF16Index(str, 11));
    }
}