1 files changed, 89 insertions, 0 deletions
diff --git a/lib/psych/tree_builder.rb b/lib/psych/tree_builder.rb
new file mode 100644
index 0000000000..6437a73771
--- /dev/null
+++ b/lib/psych/tree_builder.rb
@@ -0,0 +1,89 @@
+require 'psych/handler'
+
+module Psych
+  ###
+  # This class works in conjunction with Psych::Parser to build an in-memory
+  # parse tree tree that represents a YAML document.
+  #
+  # == Example
+  #
+  #   parser = Psych::Parser.new Psych::TreeBuilder.new
+  #   parser.parse('--- foo')
+  #   tree = parser.handler.root
+  #
+  # See Psych::Handler for documentation on the event methods used in this
+  # class.
+  class TreeBuilder < Psych::Handler
+    # Create a new TreeBuilder instance
+    def initialize
+      @stack = []
+      @last  = nil
+    end
+
+    # Returns the root node for the built tree
+    def root
+      @stack.first
+    end
+
+    %w{
+      Sequence
+      Mapping
+    }.each do |node|
+      class_eval %{
+        def start_#{node.downcase}(anchor, tag, implicit, style)
+          n = Nodes::#{node}.new(anchor, tag, implicit, style)
+          @last.children << n
+          push n
+        end
+
+        def end_#{node.downcase}
+          pop
+        end
+      }
+    end
+
+    ###
+    # Handles start_document events with +version+, +tag_directives+,
+    # and +implicit+ styling.
+    #
+    # See Psych::Handler#start_document
+    def start_document version, tag_directives, implicit
+      n = Nodes::Document.new(version, tag_directives, implicit)
+      @last.children << n
+      push n
+    end
+
+    ###
+    # Handles end_document events with +version+, +tag_directives+,
+    # and +implicit+ styling.
+    #
+    # See Psych::Handler#start_document
+    def end_document implicit_end
+      @last.implicit_end = implicit_end
+      pop
+    end
+
+    def start_stream encoding
+      push Nodes::Stream.new(encoding)
+    end
+
+    def scalar value, anchor, tag, plain, quoted, style
+      @last.children << Nodes::Scalar.new(value,anchor,tag,plain,quoted,style)
+    end
+
+    def alias anchor
+      @last.children << Nodes::Alias.new(anchor)
+    end
+
+    private
+    def push value
+      @stack.push value
+      @last = value
+    end
+
+    def pop
+      @stack.pop
+      @last = @stack.last
+    end
+  end
+end