GH-48680: [GLib][Ruby] Add CSVWriter

[stenlarsson]

Rationale for this change

Using Arrow for writing CSVs could potentially give a lot better performance than using Ruby, but the CSV writer is not available.

What changes are included in this PR?

This adds a CSVWriter and options class to the GLib wrapper.

Are these changes tested?

Yes, with Ruby unit tests.

Are there any user-facing changes?

Yes, new classes.

• GitHub Issue: [GLib][Ruby] Missing CSV writer #48680

[github-actions]

⚠️ GitHub issue #48680 has been automatically assigned in GitHub to PR creator.

[kou]

Suggested change

	"",
	write_options.null_string.c_str(),

[stenlarsson]

I always find this difficult in C++, so I wonder, is that safe? The data for the string will be owned by the write_options variable, right? And when that goes out of scope, the string could potentially be gone? I guess it works because it is a constant somewhere.

[kou]

Ah. g_param_spec_string()'s documentation https://docs.gtk.org/gobject/func.param_spec_string.html#parameters says "The data is owned by the caller of the function." but it's copied in the function: https://gitlab.gnome.org/GNOME/glib/-/blob/main/gobject/gparamspecs.c?ref_type=heads#L2438

So, we can use c_str() here.

[kou]

Suggested change

	"\n",
	write_options.eol.c_str(),

[kou]

We can use String, right?

Suggested change

	@options.delimiter = ";".ord
	@options.delimiter = ";"

[stenlarsson]

No, this is implemented in the Ruby wrapper. It can't be used in the tests for the GLib wrapper, right?

[kou]

Ah, sorry. You're right.

[kou]

We can use

options = {
  include_header: false,
  delimiter: ";",
  quoting_style: :none,
  null_string: "NULL,
}

by defining Arrow::CSVWriteOptions.try_convert:

module Arrow
  class CSVWriteOptions
    class << self
      def try_convert(value)
        case value
        when Hash
          options = new
          value.each do |k, v|
            options.public_send("#{k}=", value)
          end
          options
        else
          nil
        end
      end
    end
  end
end

[stenlarsson]

This is good idea! I assume you meant to comment on the test for the Ruby wrapper, not GLib though.

[kou]

Oh, sorry. You're right. I confused the GLib layer with the Ruby layer...

[kou]

+1

[kou]

Suggested change