Create GitHub reporter (#188)

Hello, we want to use Slim Lint in our GitHub Actions CI pipeline so I
would like to add a GitHub reporter. The name of it is taken from the
[Rubocop GitHub Actions
formatter](https://docs.rubocop.org/rubocop/1.66/formatters.html#github-actions-formatter)
which does the same thing for Rubocop.

I'm also adding a sample GH Actions workflow snippet that follows the
new [Rails GH CI
convention](https://github.com/rails/rails/blob/6df3358abf5fb99ede40d45ef7a643b97e1a802d/railties/lib/rails/generators/rails/app/templates/github/ci.yml.tt#L46)
to the README. This setup creates Pull Request annotations on Lint
failures that look like this:


![image](https://github.com/user-attachments/assets/a6818d73-4bae-498f-9194-77e27efd4bba)

README.md

@@ -21,6 +21,7 @@ your [SCM hooks](https://github.com/sds/overcommit).
 * [Linters](#linters)
 * [Editor Integration](#editor-integration)
 * [Git Integration](#git-integration)
+* [GitHub Integration](#github-integration)
 * [Rake Integration](#rake-integration)
 * [Contributing](#contributing)
 * [Changelog](#changelog)
@@ -181,6 +182,35 @@ If you'd like to integrate `slim-lint` into your Git workflow, check out
 [overcommit](https://github.com/sds/overcommit), a powerful and flexible
 Git hook manager.
 
+## Github Integration
+
+To run `slim-lint` in your [GitHub Actions](https://docs.github.com/en/actions) CI pipeline,
+use the `github` reporter, for example:
+
+```yml
+on:
+  pull_request:
+  push:
+    branches: [ main ]
+
+jobs:
+  lint:
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: .ruby-version
+          bundler-cache: true
+
+      - name: Lint Slim templates for consistent style
+        run: bundle exec slim-lint -r github app/views
+```
+
+On lint failures, this setup will create annotations in your pull requests on GitHub.
+
 ## Rake Integration
 
 To execute `slim-lint` via a [Rake](https://github.com/ruby/rake) task, make

lib/slim_lint/reporter/github_reporter.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module SlimLint
+  # Outputs lints in a format suitable for GitHub Actions.
+  # See https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions/.
+  class Reporter::GithubReporter < Reporter
+    def display_report(report)
+      sorted_lints = report.lints.sort_by { |l| [l.filename, l.line] }
+
+      sorted_lints.each do |lint|
+        print_type(lint)
+        print_location(lint)
+        print_message(lint)
+      end
+    end
+
+    private
+
+    def print_type(lint)
+      if lint.error?
+        log.log '::error ', false
+      else
+        log.log '::warning ', false
+      end
+    end
+
+    def print_location(lint)
+      log.log "file=#{lint.filename},line=#{lint.line},", false
+    end
+
+    def print_message(lint)
+      log.log 'title=Slim Lint', false
+      log.log "::#{lint.message}"
+    end
+  end
+end

spec/slim_lint/reporter/github_reporter_spec.rb

@@ -0,0 +1,105 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe SlimLint::Reporter::GithubReporter do
+  describe '#display_report' do
+    let(:io) { StringIO.new }
+    let(:output) { io.string }
+    let(:logger) { SlimLint::Logger.new(io) }
+    let(:report) { SlimLint::Report.new(lints, []) }
+    let(:reporter) { described_class.new(logger) }
+
+    subject { reporter.display_report(report) }
+
+    context 'when there are no lints' do
+      let(:lints) { [] }
+
+      it 'prints nothing' do
+        subject
+        output.should be_empty
+      end
+    end
+
+    context 'when there are lints' do
+      let(:filenames)    { ['some-filename.slim', 'other-filename.slim'] }
+      let(:lines)        { [502, 724] }
+      let(:descriptions) { ['Description of lint 1', 'Description of lint 2'] }
+      let(:severities)   { [:warning] * 2 }
+      let(:linter)       { double(name: 'SomeLinter') }
+
+      let(:lints) do
+        filenames.each_with_index.map do |filename, index|
+          SlimLint::Lint.new(linter, filename, lines[index], descriptions[index], severities[index])
+        end
+      end
+
+      it 'prints each lint on its own line' do
+        subject
+        output.count("\n").should == 2
+      end
+
+      it 'prints a trailing newline' do
+        subject
+        output[-1].should == "\n"
+      end
+
+      it 'prints the filename in the "file" parameter for each lint' do
+        subject
+        filenames.each do |filename|
+          output.scan(/file=#{filename}/).count.should == 1
+        end
+      end
+
+      it 'prints the line number in the "line" parameter for each lint' do
+        subject
+        lines.each do |line|
+          output.scan(/line=#{line}/).count.should == 1
+        end
+      end
+
+      it 'prints a "Slim Lint" annotation title for each lint' do
+        subject
+        output.scan(/title=Slim Lint/).count.should == 2
+      end
+
+      it 'prints the description for each lint at the end of the line' do
+        subject
+        descriptions.each do |description|
+          output.scan(/::#{description}$/).count.should == 1
+        end
+      end
+
+      context 'when lints are warnings' do
+        it 'prints the warning severity annotation at the beginning of each line' do
+          subject
+          output.split("\n").each do |line|
+            line.scan(/^::warning /).count.should == 1
+          end
+        end
+      end
+
+      context 'when lints are errors' do
+        let(:severities) { [:error] * 2 }
+
+        it 'prints the error severity annotation at the beginning of each line' do
+          subject
+          output.split("\n").each do |line|
+            line.scan(/^::error /).count.should == 1
+          end
+        end
+      end
+
+      context 'when lint has no associated linter' do
+        let(:linter) { nil }
+
+        it 'prints the description for each lint' do
+          subject
+          descriptions.each do |description|
+            output.scan(/#{description}/).count.should == 1
+          end
+        end
+      end
+    end
+  end
+end