Make rugged optional, default to pure-Ruby git backend

Gemfile

@@ -11,6 +11,7 @@ gem 'rake', '~> 13.0'
 gem 'rspec', '~> 3.0'
 gem 'rubocop', '~> 1.84.0'
 gem 'ruby-lsp-rspec', require: false
+gem 'rugged', '>= 0.27', '< 1.10'
 gem 'simplecov'
 gem 'simplecov-html'
 gem 'simplecov_json_formatter'

README.md

@@ -35,6 +35,24 @@ Or install it yourself as:
 
     $ gem install undercover
 
+### Git backend
+
+Undercover reads diffs through one of two interchangeable backends:
+
+- **`git` gem** (default) — pure Ruby, no native extensions. Shells out to the system `git` binary, which must be on `PATH`.
+- **`rugged`** — libgit2-based, faster on large repos. Optional, no system `git` required at runtime.
+
+The `git` gem requirement on a system `git` CLI is satisfied automatically in most environments (developer machines, CI runners, standard container images), but minimal production images such as distroless or `slim` Docker bases may need `git` added explicitly. If your deployment cannot install `git`, opt into `rugged` instead.
+
+To opt into `rugged`, add it to your Gemfile alongside `undercover`:
+
+```ruby
+gem 'undercover'
+gem 'rugged'
+```
+
+When `rugged` is loadable it is picked automatically; otherwise the `git` gem is used. No configuration is needed to switch — install or remove `rugged` from your bundle.
+
 ## Setting up coverage reporting
 
 To make your specs or tests compatible with `undercover`, please add `undercover` to your gemfile to use the undercover formatter the test helper.

lib/undercover/changeset.rb

@@ -1,24 +1,28 @@
 # frozen_string_literal: true
 
-require 'rugged'
 require 'time'
 
 module Undercover
   # Base class for different kinds of input
   class Changeset
     T_ZERO = Time.strptime('0', '%s').freeze
 
-    def initialize(dir, compare_base = nil, filter_set = nil)
-      @dir = dir
-      @repo = Rugged::Repository.new(dir)
-      @repo.workdir = Pathname.new(dir).dirname.to_s # TODO: can replace?
-      @compare_base = compare_base
+    def self.default_adapter_class
+      require 'undercover/changeset/rugged_adapter'
+      RuggedAdapter
+    rescue LoadError
+      require 'undercover/changeset/git_adapter'
+      GitAdapter
+    end
+
+    def initialize(dir, compare_base = nil, filter_set = nil, adapter: nil)
+      @adapter = adapter || self.class.default_adapter_class.new(dir, compare_base)
       @filter_set = filter_set
     end
 
     def last_modified
       mod = file_paths.map do |f|
-        path = File.join(repo.workdir, f)
+        path = File.join(@adapter.workdir, f)
         next T_ZERO unless File.exist?(path)
 
         File.mtime(path)
@@ -27,24 +31,19 @@ def last_modified
     end
 
     def file_paths
-      full_diff.deltas.map { |d| d.new_file[:path] }.sort
+      @adapter.changed_files
     end
 
     def each_changed_line
-      full_diff.each_patch do |patch|
-        filepath = patch.delta.new_file[:path]
+      @adapter.each_added_line do |filepath, lineno|
         next if filter_set && !filter_set.include?(filepath)
 
-        patch.each_hunk do |hunk|
-          hunk.lines.select(&:addition?).each do |line|
-            yield filepath, line.new_lineno
-          end
-        end
+        yield filepath, lineno
       end
     end
 
     def validate(lcov_report_path)
-      return :no_changes if full_diff.deltas.empty?
+      return :no_changes if @adapter.empty?
 
       :stale_coverage if last_modified > File.mtime(lcov_report_path)
     end
@@ -55,25 +54,6 @@ def filter_with(filter_set)
 
     private
 
-    # Diffs `head` or `head` + `compare_base` (if exists),
-    # as it makes sense to run Undercover with the most recent file versions
-    def full_diff
-      base = compare_base_obj || head
-      @full_diff ||= base.diff(repo.index).merge!(repo.diff_workdir(head))
-    end
-
-    def compare_base_obj
-      return nil unless compare_base
-
-      merge_base = repo.merge_base(compare_base.to_s, head)
-      # merge_base may be nil with --depth 1, compare two refs directly
-      merge_base ? repo.lookup(merge_base) : repo.rev_parse(compare_base)
-    end
-
-    def head
-      repo.head.target
-    end
-
-    attr_reader :repo, :compare_base, :filter_set
+    attr_reader :filter_set
   end
 end

lib/undercover/changeset/git_adapter.rb

@@ -0,0 +1,81 @@
+# frozen_string_literal: true
+
+require 'git'
+require 'pathname'
+
+module Undercover
+  class Changeset
+    class GitAdapter
+      HUNK_HEADER = /^@@ -\d+(?:,\d+)? \+(\d+)(?:,\d+)? @@/
+
+      def initialize(dir, compare_base = nil)
+        git_dir = File.expand_path(dir)
+        @workdir = Pathname.new(git_dir).dirname.to_s
+        @repo = Git.open(@workdir, repository: git_dir)
+        @compare_base = compare_base
+      end
+
+      attr_reader :workdir
+
+      def changed_files
+        diff_files.map(&:path).sort
+      end
+
+      def each_added_line
+        diff_files.each do |fd|
+          patch = fd.patch
+          next if patch.nil? || patch.empty?
+
+          parse_added_lines(patch) do |lineno|
+            yield fd.path, lineno
+          end
+        end
+      end
+
+      def empty?
+        diff_files.none?
+      end
+
+      private
+
+      attr_reader :repo, :compare_base
+
+      def diff_files
+        @diff_files ||= repo.diff(diff_base).to_a
+      end
+
+      def diff_base
+        return 'HEAD' unless compare_base
+
+        merge_base_sha || compare_base.to_s
+      end
+
+      def merge_base_sha
+        commits = repo.merge_base(compare_base.to_s, 'HEAD')
+        commits.first&.sha
+      rescue Git::FailedError, ArgumentError
+        nil
+      end
+
+      def parse_added_lines(patch)
+        new_lineno = nil
+        patch.each_line do |line|
+          if (m = line.match(HUNK_HEADER))
+            new_lineno = m[1].to_i
+          elsif new_lineno
+            yield new_lineno if added_line?(line)
+            new_lineno += 1 if added_line?(line) || context_line?(line)
+          end
+        end
+      end
+
+      def added_line?(line)
+        line.start_with?('+') && !line.start_with?('+++')
+      end
+
+      def context_line?(line)
+        line.start_with?(' ')
+      end
+    end
+  end
+end

lib/undercover/changeset/rugged_adapter.rb

@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+require 'rugged'
+require 'pathname'
+
+module Undercover
+  class Changeset
+    class RuggedAdapter
+      def initialize(dir, compare_base = nil)
+        @repo = Rugged::Repository.new(dir)
+        @repo.workdir = Pathname.new(dir).dirname.to_s
+        @compare_base = compare_base
+      end
+
+      def workdir
+        @repo.workdir
+      end
+
+      def changed_files
+        full_diff.deltas.map { |d| d.new_file[:path] }.sort
+      end
+
+      def each_added_line
+        full_diff.each_patch do |patch|
+          filepath = patch.delta.new_file[:path]
+          patch.each_hunk do |hunk|
+            hunk.lines.select(&:addition?).each do |line|
+              yield filepath, line.new_lineno
+            end
+          end
+        end
+      end
+
+      def empty?
+        full_diff.deltas.empty?
+      end
+
+      private
+
+      attr_reader :repo, :compare_base
+
+      def full_diff
+        base = compare_base_obj || head
+        @full_diff ||= base.diff(repo.index).merge!(repo.diff_workdir(head))
+      end
+
+      def compare_base_obj
+        return nil unless compare_base
+
+        merge_base = repo.merge_base(compare_base.to_s, head)
+        merge_base ? repo.lookup(merge_base) : repo.rev_parse(compare_base)
+      end
+
+      def head
+        repo.head.target
+      end
+    end
+  end
+end