CodePointCounter.java

public final class CodePointCounter {
  private final String input;
  public int inputIndex = 0;
  public int codePointIndex = 0;
  
  public int advanceToIndex(int newCodePointIndex) {
    assert newCodePointIndex >= codePointIndex;
    while (codePointIndex < newCodePointOffset) {
        int codePoint = Character.codePointAt(input, inputIndex);
        inputIndex += Character.charCount(codePoint);
        codePointIndex++;
    }
    return inputIndex;
  }
}