getCharacterStartBounds function

int getCharacterStartBounds(String text, int endingCodePointIndex)

Returns the code point index for the code point that begins the visual character that ends at endingCodePointIndex.

A single visual character might be comprised of multiple code points. Each code point occupies a slot within a String, which means that an index into a String might refer to a piece of a single visual character.

endingCodePointIndex is the traditional String index for the trailing code point of a visual character.

This function starts at the given endingCodePointIndex and walks towards the beginning of text until it has accumulated an entire visual character. The String index of the initial code point for the given character is returned.

Implementation

int getCharacterStartBounds(String text, int endingCodePointIndex) {
  // This implementation was copied and adapted from text_editing_action_target
  // in the Flutter repo.
  assert(endingCodePointIndex >= 0 && endingCodePointIndex <= text.length);

  if (endingCodePointIndex == 0) {
    return 0;
  }

  final CharacterRange range = CharacterRange.at(text, 0, endingCodePointIndex);
  // If index is not on a character boundary, return the previous character
  // boundary.
  if (range.current.length != endingCodePointIndex) {
    range.dropLast();
    return range.current.length;
  }

  range.dropLast();
  return range.current.length;
}