If you make justa small change to theGraphemes::next method then this breaks the tests ingraphemes_test because theGraphemeCursor::next_boundary returnsGraphemeIncomplete::PreContext(1). As far as I understand, this indicates that the cursor tries to look back at preceding grapheme clusters that it has already processed. I don't know if this behavior is intentional or required to process UTF-8 correctly, but it seems unnecessary because the tests do pass if you just use a fresh cursor instead and treat your current position as the start of the string. Some performance improvements might be possible if you do it that way.