1 files changed, 71 insertions, 0 deletions
diff --git a/lib/arel/algebra/header.rb b/lib/arel/algebra/header.rb
new file mode 100644
index 0000000000..ec45488dbf
--- /dev/null
+++ b/lib/arel/algebra/header.rb
@@ -0,0 +1,71 @@
+module Arel
+  class Header
+    include Enumerable
+
+    def initialize(attrs = [])
+      @attributes = attrs.to_ary
+      @names = Hash.new do |h,k|
+        h[k] = @attributes.detect { |a| a.named?(k) }
+      end
+    end
+
+    def each(&block)
+      to_ary.each(&block)
+      self
+    end
+
+    def [](key)
+      case key
+      when String, Symbol then find_by_name(key)
+      when Attribute      then find_by_attribute(key)
+      end
+    end
+
+    def ==(other)
+      to_set == other.to_set
+    end
+
+    def union(other)
+      new(to_ary | other)
+    end
+
+    alias | union
+
+    def to_ary
+      @attributes
+    end
+
+    def bind(relation)
+      Header.new(map { |a| a.bind(relation) })
+    end
+
+    # TMP
+    def index(i)
+      to_ary.index(i)
+    end
+
+  private
+
+    def new(attrs)
+      self.class.new(attrs)
+    end
+
+    def matching(attribute)
+      # (@matching_attributes ||= attributes.inject({}) do |hash, a|
+      #   (hash[a.is_a?(Value) ? a.value : a.root] ||= []) << a
+      #   hash
+      # end)[attribute.root] || []
+      select { |a| !a.is_a?(Value) && a.root == attribute.root }
+    end
+
+    def find_by_name(name)
+      @names[name.to_sym]
+    end
+
+    def find_by_attribute(attr)
+      matching(attr).max do |a, b|
+        (a.original_attribute / attr) <=> (b.original_attribute / attr)
+      end
+    end
+  end
+end
+\ No newline at end of file