Add Git blob include processor and preprocessor

gems/lib/asciidoctor/foodogsquared-extensions.rb

@@ -13,6 +13,7 @@ require_relative 'github-raw-content-include-processor/extension'
 require_relative 'gitlab-link-inline-macro/extension'
 require_relative 'gitlab-raw-content-include-processor/extension'
 require_relative 'chat-block-processor/extension'
+require_relative 'git-blob-include-processor/extension'
 require_relative 'wikipedia-inline-macro/extension'
 
 Asciidoctor::Extensions.register do
@@ -28,5 +29,8 @@ Asciidoctor::Extensions.register do
   inline_macro GitLabLinkInlineMacro
   include_processor GitLabRawIncludeProcessor
 
+  include_processor GitBlobIncludeProcessor
+  preprocessor GitContentBranchAttributePreprocessor
+
   inline_macro WikipediaInlineMacro
 end

gems/lib/asciidoctor/git-blob-include-processor/README.adoc

@@ -0,0 +1,58 @@
+= Git blob include processor
+:toc:
+
+
+An include processor that includes content from the current Git repo.
+The use case for this is specifically for creating dedicated branches for certain content.
+
+
+== Synopsis
+
+[source, asciidoc]
+----
+include::git:$REVSPEC[]
+----
+
+Where `$REVSPEC` is a revision as specified in link:https://manpages.debian.org/gitrevisions.7[gitrevisions.7].
+Take note this include processor only accepts a blob object's content to be extracted.
+This is meant to be used with other attributes.
+See <<Attributes>> for more details.
+
+If the resulting operation ends in an error (i.e., non-existing revision, file, something went wrong), it should log a warning and transclude an error.
+
+
+== Attributes
+
+- `path` is the filepath to be retrieved from the revision.
+
+- `context-lines` is the number of surrounding lines for the diff.
+This is only effective if `diff` option is given.
+
+There's also a couple of link:https://docs.asciidoctor.org/asciidoc/latest/attributes/options/[options] for this component.
+You can give the following options through `opts` attribute (i.e., `opts="diff,reverse"`).
+
+- `diff` will show the difference in link:https://en.wikipedia.org/wiki/Diff[diff] format.
+
+- `reverse` reverses the sides to be compared.
+
+
+== Extra notes
+
+The extension is composed of two parts: an include processor and a preprocessor.
+
+The preprocessor add a document attribute, `doccontentref`, which it gives the Git reference for that content.
+`doccontentref` is the difference between the process working directory (`Dir.pwd`) and the base directory of the content to be converted.
+In case the process working directory is not entirely consistent for several reasons, you could give the attribute `rootdir` as the basis for the comparison.
+
+The include processor also accepts an empty target with the prefix (i.e., `include::git:[]`) as a shorthand for `include::git:{doccontentref}`.
+
+
+== Example usage
+
+The following examples assume that `doccontentref` points to `content/posts/sample`.
+
+- `include::git:HEAD[path=shell.nix]` should transclude the current iteration of `shell.nix`.
+
+- `include::git:{doccontentref}[opts="diff", path=Gemfile]` should include a diff of the Gemfile from `content/posts/sample` branch (assuming that Gemfile has changed).
+
+- `include::git:non-existing-rev[opts="diff"]` should result in a warning with the non-existing revision.

gems/lib/asciidoctor/git-blob-include-processor/extension.rb

@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+require 'rugged'
+
+class GitBlobIncludeProcessor < Asciidoctor::Extensions::IncludeProcessor
+  def handles?(target)
+    target.start_with? 'git:'
+  end
+
+  def process(doc, reader, target, attrs)
+    repo = Rugged::Repository.discover(__dir__)
+
+    git_object_ref = target.delete_prefix 'git:'
+    git_object_ref = doc.attributes['doccontentref'] if git_object_ref.empty?
+
+    begin
+      git_object = repo.rev_parse git_object_ref
+
+      if attrs.key? 'diff-option'
+        options = {}
+
+        options[:paths] = [attrs['path']] if attrs.key? 'path'
+        options[:context_lines] = attrs['context-lines'] if attrs.key? 'context-lines'
+        options[:reverse] = true if attrs.key? 'reverse-option'
+
+        reader.push_include git_object.diff(**options).patch
+      else
+        inner_entry = case git_object.type
+                      when :blob
+                        git_object
+                      when :commit
+                        git_object.tree.path attrs['path']
+                      when :tree
+                        git_object.path attrs['path']
+                      when :tag
+                        git_object.target.tree.path attrs['path']
+                      end
+
+        reader.push_include repo.lookup(inner_entry[:oid]).content
+      end
+    rescue StandardError => e
+      reader.push_include "Unresolved directive for '#{target}' with the following error:\n#{e}"
+      warn e
+    end
+
+    reader
+  end
+end
+
+class GitContentBranchAttributePreprocessor < Asciidoctor::Extensions::Preprocessor
+  def process(document, reader)
+    base_dir = Pathname.new(document.base_dir)
+    rootdir = if document.attributes['rootdir'].nil?
+                Dir.pwd
+              else
+                document.attributes['rootdir']
+              end
+
+    document.attributes['doccontentref'] = base_dir.relative_path_from(rootdir).to_s
+    reader
+  end
+end