the index of the character in the original text that corresponds to this #hb_glyph_info_t, or whatever the client passes to harfbuzz.global.bufferAdd. More than one #hb_glyph_info_t can have the same @cluster value, if they resulted from the same character (e.g. one to many glyph substitution), and when more than one character gets merged in the same glyph (e.g. many to one glyph substitution) the #hb_glyph_info_t will have the smallest cluster value of them. By default some characters are merged into the same cluster (e.g. combining marks have the same cluster as their bases) even if they are separate glyphs, harfbuzz.global.bufferSetClusterLevel allow selecting more fine-grained cluster handling.
Set cluster field.