Command-line rust notes (3)

Conflicting command-line arguments

In some versions of the wc program, the flags for counting bytes -c and for counting characters -m cannot be used together, or an error message will be displayed. The crate for parsing command line arguments clap has the option to set two command-line arguments as conflicting using the attribute conflicts_with:

#[derive(Parser, Debug)]
struct Arg {
    /// The number of words in each input file is written to the standard
    /// output
    #[arg(short = 'w')]
    words: bool,

    /// The number of bytes in each input file is written to the standard
    /// output. This will conflict with the usage of "-m" option
    #[arg(short = 'c')]
    #[arg(conflicts_with("chars"))]
    bytes: bool,

    /// The number of characters in each input file is written to the standard
    /// output. If the current locale does not support multibyte characters,
    /// then this is equivalent to the "-c" option. This will conflict with
    /// the usage of the "-c" option
    #[arg(short = 'm')]
    chars: bool,

    /// The number of lines in each input file is written to the standard
    /// output
    #[arg(short = 'l')]
    lines: bool,

    files: Vec<String>,
}

When the two conflicting arguments are used in the same call, the parser will throw an error that can then be propagated onto the top level main() function. The error message looks something like this:

error: the argument '-c' cannot be used with '-m'

Mock buffered reader

The std::io::Cursor struct “wraps an in-memory buffer” and implements the BufRead trait. As a result, it can be used to quickly mock up a buffered reader for testing functions that take BufRead.

In my wc implementation, I used it to test the instantiation of word count info from buffered readers:

#[cfg(test)]
mod tests {
    use super::*;
    use std::io::Cursor;

    #[test]
    fn create_word_cnt_info() {
        let test_str = "锟斤拷\n锘锘锘\n烫烫烫\n屯屯屯\n";
        let mut reader = Cursor::new(test_str);
        let wcinfo = WordCountInfo::from_read("", &mut reader).unwrap();

        assert_eq!(wcinfo.line_cnt, 4);
        assert_eq!(wcinfo.word_cnt, 4);
        assert_eq!(wcinfo.byte_cnt, 40);
        assert_eq!(wcinfo.char_cnt, 16);
    }
}