cmp argument parser rewrite

[GunterSchmidt]

Hello everyone,

I was checking the functionality of the cmp.rs and found some issues around not working options/arguments.

I did a lot of testing regarding the parser and found the following rules.

    /// Parses the command line arguments. \
    /// Since cmp is called from diffutils, the first argument must always be "cmp".
    ///
    /// The following checks require more extensive checks than a simple compare.
    /// These are all identical and make parsing extensive:
    /// - cmp file_1 file_2 -b -l -n 50
    /// - cmp file_1 file_2 -b -l -n50
    /// - cmp file_1 file_2 -bl -n50
    /// - cmp file_1 file_2 -bln 50
    /// - cmp file_1 file_2 -bln50
    /// - cmp file_1 file_2 --print-bytes --verbose --bytes 50
    /// - cmp file_1 file_2 --print-bytes --verbose --bytes=50
    /// - cmp file_1 file_2 --p --verb --by 50
    /// - cmp file_1 file_2 --p --verb --by=50
    /// 
    /// and many more rules.

A number of them are not working in the current cmp.rs version.

After I started to fix some of the issues, I started refactoring the whole parser.

• Separation of concerns:
  • I moved all code regarding parsing into to Params struct.
  • The parser now only returns enums (Params or InfoText for Version and Help) for the OK case or a specified Error.
  • All errors have been moved into an Error Enum ParamsParseError which handles all output in Display.
  • This allows a centralized maintenance of errors and error messages. The caller can work with the error enum more easily.
• Renaming of the Param properties.
  • from and to replaced with file_1 and file_2, as these are the parameters.
  • max_bytes with bytes_limit
  • quiet with silent as s is the short code for it
  • skip_a with ignore_initial_bytes_file_1, so there is a connection between options and the property name
• The bytes variables where size usize, thus limiting the readable bytes on 32-bit systems. GNU cmp is compliles with
  Large File Support and allows i64 values. I changed it to u64 with a Feature to change it to u128.
• bytes-limit: The tests set the bytes-limit to MAX:usize if a very large number is encountered. This is incorrect,
  GNU cmp gives an error message.
• Corrected parser, e.g. -bln50KiB is now allowed or --ig for --ignore-initial
• Correct output of error messages, Help text is missing. New error messages like in GNU cmp.
• Added text for --version and --help
• Tests
  • Added new tests for the above valid options.
  • Changed tests where bytes where set to MAX.
  • Tests not can be written with simple String "cmp foo bar -n 50"

Questions:

• Params executable: What is the reason to store this value? It is the argument 'cmp' which is used to start the program.
  I don't see how this can ever change and it requires a lot of extra code to pass it to the error messages.
  const APP_NAME: &str = 'cmp'; would be sufficient, or am I missing something here? I would like to remove it.
• GNU cmp behaves very odd when passing '--'. Try 'cmp -- file_1 --help'. Why whould --help be a file?
  I implemented an error message instead, as the usage is probably not intended.
  Is this an issue, as it would not behave 100% like GNU cmp. Try 'cmp -- file_1 --help'. Why whould --help be a file?
• The error messages are slightly changed, e.g.
  • 'option requires an argument -- 'n'' when only passing -n, but
  • 'option '--bytes' requires and argument', which is nicer to read.
    I do not differentiate here and print:
  • 'option '--bytes' ('-n') requires an argument'
• Params is generally public and could be used elsewhere, e.g. the number conversion. Should I keep it public?

Before submitting a PR I would like to have some feedback on this and the refactoring.

Check out the new version at my fork.