ruby-git · tarcinil · Oct 2, 2018 · Oct 1, 2018
diff --git a/README.md b/README.md
@@ -216,6 +216,8 @@ And here are the operations that will need to write to your git repository.
      g.merge(g.branch('master'))
      g.merge([branch1, branch2])
 
+     g.merge_base('branch1', 'branch2')
+
      r = g.add_remote(name, uri)  # Git::Remote
      r = g.add_remote(name, Git::Base)  # Git::Remote
 

diff --git a/lib/git/base/factory.rb b/lib/git/base/factory.rb
@@ -68,6 +68,14 @@ def tag(tag_name)
         Git::Object.new(self, tag_name, 'tag', true)
       end
 
+      # Find as good common ancestors as possible for a merge
+      # example: g.merge_base('master', 'some_branch', 'some_sha', octopus: true)
+      # returns Array<Git::Object::Commit>
+      def merge_base(*args)
+        shas = self.lib.merge_base(*args)
+        shas.map { |sha| gcommit(sha) }
+      end
+
     end
 
   end

diff --git a/lib/git/lib.rb b/lib/git/lib.rb
@@ -666,6 +666,21 @@ def merge(branch, message = nil)
       command('merge', arr_opts)
     end
 
+    def merge_base(*args)
+      opts = args.last.is_a?(Hash) ? args.pop : {}
+
+      arg_opts = []
+
+      arg_opts << '--octopus' if opts[:octopus]
+      arg_opts << '--independent' if opts[:independent]
+      arg_opts << '--fork-point' if opts[:fork_point]
+      arg_opts << '--all' if opts[:all]
+
+      arg_opts += args
+
+      command('merge-base', arg_opts).lines.map(&:strip)
+    end
+
     def unmerged
       unmerged = []
       command_lines('diff', ["--cached"]).each do |line|

diff --git a/tests/units/test_merge_base.rb b/tests/units/test_merge_base.rb
@@ -0,0 +1,144 @@
+#!/usr/bin/env ruby
+
+require File.dirname(__FILE__) + '/../test_helper'
+
+class TestMergeBase < Test::Unit::TestCase
+  def setup
+    set_file_paths
+  end
+
+  def test_branch_and_master_merge_base
+    in_temp_dir do |_path|
+      repo = Git.clone(@wbare, 'branch_merge_test')
+      Dir.chdir('branch_merge_test') do
+        true_ancestor_sha = repo.gcommit('master').sha
+
+        add_commit(repo, 'new_branch')
+        add_commit(repo, 'master')
+
+        ancestors = repo.merge_base('master', 'new_branch')
+        assert_equal(ancestors.size, 1) # there is only one true ancestor
+        assert_equal(ancestors.first.sha, true_ancestor_sha) # proper common ancestor
+      end
+    end
+  end
+
+  def test_branch_and_master_independent_merge_base
+    in_temp_dir do |_path|
+      repo = Git.clone(@wbare, 'branch_merge_test')
+      Dir.chdir('branch_merge_test') do
+        true_ancestor_sha = repo.gcommit('master').sha
+
+        add_commit(repo, 'new_branch')
+        add_commit(repo, 'master')
+
+        independent_commits = repo.merge_base(true_ancestor_sha, 'master', 'new_branch', independent: true)
+        assert_equal(independent_commits.size, 2) # both new master and a branch are unreachable from each other
+        true_independent_commits_shas = [repo.gcommit('master').sha, repo.gcommit('new_branch').sha]
+        assert_equal(independent_commits.map(&:sha).sort, true_independent_commits_shas.sort)
+      end
+    end
+  end
+
+  def test_branch_and_master_fork_point_merge_base
+    in_temp_dir do |_path|
+      repo = Git.clone(@wbare, 'branch_merge_test')
+      Dir.chdir('branch_merge_test') do
+        add_commit(repo, 'master')
+
+        true_ancestor_sha = repo.gcommit('master').sha
+
+        add_commit(repo, 'new_branch')
+
+        repo.reset_hard(repo.gcommit('HEAD^'))
+
+        add_commit(repo, 'master')
+
+        ancestors = repo.merge_base('master', 'new_branch', fork_point: true)
+        assert_equal(ancestors.size, 1) # there is only one true ancestor
+        assert_equal(ancestors.first.sha, true_ancestor_sha) # proper common ancestor
+      end
+    end
+  end
+
+  def test_branch_and_master_all_merge_base
+    in_temp_dir do |_path|
+      repo = Git.clone(@wbare, 'branch_merge_test')
+      Dir.chdir('branch_merge_test') do
+        add_commit(repo, 'new_branch_1')
+
+        first_commit_sha = repo.gcommit('new_branch_1').sha
+
+        add_commit(repo, 'new_branch_2')
+
+        second_commit_sha = repo.gcommit('new_branch_2').sha
+
+        repo.branch('new_branch_1').merge('new_branch_2')
+        repo.branch('new_branch_2').merge('new_branch_1^')
+
+        add_commit(repo, 'new_branch_1')
+        add_commit(repo, 'new_branch_2')
+
+        true_ancestors_shas = [first_commit_sha, second_commit_sha]
+
+        ancestors = repo.merge_base('new_branch_1', 'new_branch_2')
+        assert_equal(ancestors.size, 1) # default behavior returns only one ancestor
+        assert(true_ancestors_shas.include?(ancestors.first.sha))
+
+        all_ancestors = repo.merge_base('new_branch_1', 'new_branch_2', all: true)
+        assert_equal(all_ancestors.size, 2) # there are two best ancestors in such case
+        assert_equal(all_ancestors.map(&:sha).sort, true_ancestors_shas.sort)
+      end
+    end
+  end
+
+  def test_branches_and_master_merge_base
+    in_temp_dir do |_path|
+      repo = Git.clone(@wbare, 'branch_merge_test')
+      Dir.chdir('branch_merge_test') do
+        add_commit(repo, 'new_branch_1')
+        add_commit(repo, 'master')
+
+        non_octopus_ancestor_sha = repo.gcommit('master').sha
+
+        add_commit(repo, 'new_branch_2')
+        add_commit(repo, 'master')
+
+        ancestors = repo.merge_base('master', 'new_branch_1', 'new_branch_2')
+        assert_equal(ancestors.size, 1) # there is only one true ancestor
+        assert_equal(ancestors.first.sha, non_octopus_ancestor_sha) # proper common ancestor
+      end
+    end
+  end
+
+  def test_branches_and_master_octopus_merge_base
+    in_temp_dir do |_path|
+      repo = Git.clone(@wbare, 'branch_merge_test')
+      Dir.chdir('branch_merge_test') do
+        true_ancestor_sha = repo.gcommit('master').sha
+
+        add_commit(repo, 'new_branch_1')
+        add_commit(repo, 'master')
+        add_commit(repo, 'new_branch_2')
+        add_commit(repo, 'master')
+
+        ancestors = repo.merge_base('master', 'new_branch_1', 'new_branch_2', octopus: true)
+        assert_equal(ancestors.size, 1) # there is only one true ancestor
+        assert_equal(ancestors.first.sha, true_ancestor_sha) # proper common ancestor
+      end
+    end
+  end
+
+  private
+
+  def add_commit(repo, branch_name)
+    @commit_number ||= 0
+    @commit_number += 1
+
+    repo.branch(branch_name).in_branch("test commit #{@commit_number}") do
+      new_file("new_file_#{@commit_number}", 'hello')
+      repo.add
+      true
+    end
+  end
+end