Use git to check for empty index.

lib/format-staged/io.rb

@@ -17,6 +17,17 @@ module FormatStaged
       output
     end
 
+    def get_status(*args)
+      puts "> #{args.join(' ')}" if @verbose
+      result = system(*args)
+
+      raise 'Failed to run command' if result.nil?
+
+      puts "? #{$CHILD_STATUS.exitstatus}" if @verbose
+
+      $CHILD_STATUS.exitstatus
+    end
+
     def pipe_command(*args, source: nil)
       puts (source.nil? ? '> ' : '| ') + args.join(' ') if @verbose
       r, w = IO.pipe

lib/format-staged/job.rb

@@ -36,7 +36,10 @@ module FormatStaged
 
       formatted = files.filter { |file| format_file file }
 
-      !formatted.empty?
+      return false unless formatted.size == files.size
+
+      quiet = @verbose ? [] : ['--quiet']
+      return get_status('git', 'diff-index', '--cached', '--exit-code', *quiet, 'HEAD') != 0
     end
 
     def repo_root
@@ -76,7 +79,7 @@ module FormatStaged
 
       if new_hash == file.src_hash
         info "File #{file.src_path} equal to comitted"
-        return false
+        return true
       end
 
       true

spec/test_spec.rb

@@ -57,7 +57,7 @@ describe FormatStaged do
     expect(repo.get_content('test.other')).to eq('x=y')
   end
 
-  it 'fails if files are changed to already comitted version' do
+  it 'fails if all files are changed to already comitted version' do
     repo.file_in_tree 'test.test', 'x = y'
     repo.set_content 'test.test', 'x=y'
     repo.stage 'test.test'
@@ -68,7 +68,55 @@ describe FormatStaged do
     expect(repo.get_content('test.test')).to eq('x = y')
   end
 
+  it 'succeeds if there are excluded files to commit' do
+    repo.file_in_tree 'test.test', 'x = y'
+    repo.set_content 'test.test', 'x=y'
+    repo.stage 'test.test'
+    repo.set_content 'test.other', 'abc'
+    repo.stage 'test.other'
+
+    success = repo.run_formatter
+
+    expect(success).to be_truthy
+    expect(repo.get_content('test.test')).to eq('x = y')
+  end
+
   it 'succeeds if there are no staged files' do
     expect(repo.run_formatter).to be_truthy
   end
+
+  it 'succeeds if only excluded files are changed' do
+    repo.set_content 'test.other', 'abc'
+    repo.stage 'test.other'
+
+    expect(repo.run_formatter).to be_truthy
+  end
+
+  it 'succeeds if one file is changed' do
+    repo.file_in_tree 'test.test', 'x = y'
+    repo.set_content 'test.test', 'x=y'
+    repo.stage 'test.test'
+    repo.set_content 'other.test', 'a=b'
+    repo.stage 'other.test'
+
+    success = repo.run_formatter
+
+    expect(success).to be_truthy
+    expect(repo.get_content('test.test')).to eq('x = y')
+    expect(repo.get_content('other.test')).to eq('a = b')
+  end
+
+  it 'fails if a single file becomes empty' do
+    repo.file_in_tree 'test.test', 'x = y'
+    repo.set_content 'test.test', '#clear'
+    repo.stage 'test.test'
+    repo.set_content 'other.test', 'a=b'
+    repo.stage 'other.test'
+
+    success = repo.run_formatter
+
+    expect(success).to be_falsy
+    expect(repo.get_content('test.test')).to eq('#clear')
+    expect(repo.get_content('other.test')).to eq('a = b')
+  end
 end