Remove pattern matching usages

[Brcrwilliams]

The PackageURL gem currently uses pattern matching, which is an unstable experimental feature and is subject to change in future Ruby releases. On Ruby 2.7.5, a warning it outputted when the gem is require'd.

/Users/bwilliams/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/packageurl-ruby-0.1.0/lib/package_url.rb:87: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!
/Users/bwilliams/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/packageurl-ruby-0.1.0/lib/package_url.rb:115: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!
/Users/bwilliams/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/packageurl-ruby-0.1.0/lib/package_url.rb:110: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!
/Users/bwilliams/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/packageurl-ruby-0.1.0/lib/package_url.rb:140: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!
/Users/bwilliams/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/packageurl-ruby-0.1.0/lib/package_url.rb:152: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!
/Users/bwilliams/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/packageurl-ruby-0.1.0/lib/package_url.rb:166: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!
/Users/bwilliams/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/packageurl-ruby-0.1.0/lib/package_url.rb:181: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!
/Users/bwilliams/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/packageurl-ruby-0.1.0/lib/package_url.rb:252: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!
/Users/bwilliams/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/packageurl-ruby-0.1.0/lib/package_url.rb:272: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!
/Users/bwilliams/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/packageurl-ruby-0.1.0/lib/package_url.rb:299: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!
/Users/bwilliams/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/packageurl-ruby-0.1.0/lib/package_url.rb:293: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!
/Users/bwilliams/.asdf/installs/ruby/2.7.5/lib/ruby/gems/2.7.0/gems/packageurl-ruby-0.1.0/lib/package_url.rb:325: warning: Pattern matching is experimental, and the behavior may change in future versions of Ruby!

This discourages usage of the gem, as it could potentially break when upgrading Ruby versions.
This PR refactors the code so that it no longer uses pattern matching, in favor of more stable language features.

[Brcrwilliams]

@mattt Could you review this PR please? 🙏 I would love to use this gem in GitLab (https://gitlab.com/gitlab-org/gitlab/-/merge_requests/100759#note_1132079448), but since pattern matching is experimental in Ruby I am worried that it won't be stable and could break when upgrading Ruby versions. I've done some refactoring to mitigate this.

[mattt]

Hi @Brcrwilliams. Thanks for opening this PR.

Pattern matching is now stable as of Ruby 3 (see https://rubyreferences.github.io/rubychanges/3.0.html#pattern-matching). Do you know of a way to silence these warnings for anyone using this library with Ruby 2.7?

[Brcrwilliams]

I thought that the subpath and the namespace were parsed the same way. However, after studying https://github.com/package-url/purl-spec/blob/0b1559f76b79829e789c4f20e6d832c7314762c5/PURL-SPECIFICATION.rst#how-to-build-purl-string-from-its-components more carefully, it seems that the subpath should discard . and .. but the namespace can contain them. Is this right?

[Brcrwilliams]

I've updated the PR to allow . and .. to be present in the namespace, to maintain the existing behavior.

[Brcrwilliams]

@mattt These can be silenced with Warning[:experimental] = false, but there are some caveats:

1. This has to be done before the first time the gem is required
2. This will also hide other experimental warnings, so there is a risk of other experimental features being used in the codebase or in other libraries without any warning appearing for them. I would prefer not to silence the warnings because of this.

Overall, I feel this PR still provides a net benefit even with pattern matching being promoted to stable.

1. The cognitive complexity of the parse and to_s methods has been reduced
2. Additional test coverage has been added
3. Code for parsing / encoding components of the PURL has been de-duplicated.

[Brcrwilliams]

@mattt Is this PR something you might accept? If not, I'll need to figure out some sort of workaround for the warnings until we are able to upgrade to Ruby 3.

[Brcrwilliams]

I have a different approach in #6 which may be better than this one.

[mattt]

@Brcrwilliams Thank you for taking the time to look into this and for your patience in my following up on this.

The approach I took for this implementation was to minimize indirection between the implementation and specification. I started with the verbatim text of the PURL spec and did my best to translate it into Ruby code as literally as possible (kind of like Literate Programming). It's subjective, but I found Ruby's new pattern matching feature to provide the most direct mapping between the spec and executable code. The code may not be as DRY or modular as it could be if it were decomposed into smaller helper functions, but that's a trade-off I'm happy to make for an implementation that can be read linearly, stanza by stanza next to the spec.

As for the warnings generated by Ruby 2.7, I agree that the ergonomics aren't great and apologize for the inconvenience there. Because Ruby 2.7.6 will be EOL on March 31, 2023, my preference is to keep the existing implementation as-is rather than cater to a version of Ruby that won't be supported in a few months.

If you're deploying this library no Ruby 2.7 or earlier, I'd recommend either suppressing the warning or using your fork for the time being. I'll update the README to provide instructions for suppressing that warning in Ruby 2.7.