toUCSindex

fnsize_t toUCSindex(C)(const(C)[] str, size_t index) if (isSomeChar!C) @safe pure

Given index into str and assuming that index is at the start of a UTF sequence, toUCSindex determines the number of UCS characters up to index. So, index is the index of a code unit at the beginning of a code point, and the return value is how many code points into the string that that code point is.