findUtf8Start

pub fn findUtf8Start(text: []const u8, pos: usize) usize {
    var p = pos;
    p = @min(p, text.len);

    // find start of previous utf8 char
    var start = p -| 1;
    while (start > 0 and start < p and text[start] & 0xc0 == 0x80) {
        start -|= 1;
    }

    if (start < p) {
        const utf8_size = std.unicode.utf8ByteSequenceLength(text[start]) catch 0;
        if (utf8_size != (p - start)) {
            p = start;
        }
    }

    return p;
}