func mb_strpos(str, substr string) int {
    asciiPos := strings.Index(str, substr)
    if asciiPos == -1 || asciiPos == 0 {
        return asciiPos
    }
    pos := 0
    totalSize := 0
    reader := strings.NewReader(str)
    for {
        _, size, err := reader.ReadRune()
        if err != nil {
            return -1
        }
        totalSize += size
        pos++
        // got it
        if totalSize == asciiPos {
            return pos
        }
    }
}