Allow formatting code with a different base level of indentation

lib/syntax_tree.rb

@@ -44,6 +44,10 @@ module SyntaxTree
   # It shouldn't really be changed except in very niche circumstances.
   DEFAULT_RUBY_VERSION = Formatter::SemanticVersion.new(RUBY_VERSION).freeze
 
+  # The default indentation level for formatting. We allow changing this so
+  # that Syntax Tree can format arbitrary parts of a document.
+  DEFAULT_INDENTATION = 0
+
   # This is a hook provided so that plugins can register themselves as the
   # handler for a particular file type.
   def self.register_handler(extension, handler)
@@ -61,12 +65,13 @@ def self.parse(source)
   def self.format(
     source,
     maxwidth = DEFAULT_PRINT_WIDTH,
+    base_indentation = DEFAULT_INDENTATION,
     options: Formatter::Options.new
   )
     formatter = Formatter.new(source, [], maxwidth, options: options)
     parse(source).format(formatter)
 
-    formatter.flush
+    formatter.flush(base_indentation)
     formatter.output.join
   end
 

lib/syntax_tree/formatter.rb

@@ -84,10 +84,10 @@ def initialize(source, *args, options: Options.new)
       @target_ruby_version = options.target_ruby_version
     end
 
-    def self.format(source, node)
+    def self.format(source, node, base_indentation = 0)
       q = new(source, [])
       q.format(node)
-      q.flush
+      q.flush(base_indentation)
       q.output.join
     end
 

test/formatting_test.rb

@@ -27,5 +27,37 @@ def test_stree_ignore
 
       assert_equal(source, SyntaxTree.format(source))
     end
+
+    def test_formatting_with_different_indentation_level
+      source = <<~SOURCE
+        def foo
+          puts "a"
+        end
+      SOURCE
+
+      # Default indentation
+      assert_equal(source, SyntaxTree.format(source))
+
+      # Level 2
+      assert_equal(<<-EXPECTED.chomp, SyntaxTree.format(source, 80, 2).rstrip)
+  def foo
+    puts "a"
+  end
+      EXPECTED
+
+      # Level 4
+      assert_equal(<<-EXPECTED.chomp, SyntaxTree.format(source, 80, 4).rstrip)
+    def foo
+      puts "a"
+    end
+      EXPECTED
+
+      # Level 6
+      assert_equal(<<-EXPECTED.chomp, SyntaxTree.format(source, 80, 6).rstrip)
+      def foo
+        puts "a"
+      end
+      EXPECTED
+    end
   end
 end