Add method to return offset of specified struct attribute

lib/binary_struct.rb

@@ -126,6 +126,21 @@ def each(&block)
     self.definition.each_slice(2, &block)
   end
 
+  def offset(attr)
+    offset = 0
+    self.definition.each_slice(2) do |format, name|
+      return offset if name == attr
+
+      type,count = format[0,1], format[1..-1]
+      count = count.empty? ? 1 : count.to_i
+      size = BinaryStruct::SIZES[type]
+
+      offset += BinaryStruct::STRING_FORMATS.include?(type) ?
+                size * count : size
+    end
+    offset
+  end
+
   #
   # Methods to handle the old style of calling
   #

spec/binary_struct_spec.rb

@@ -109,6 +109,10 @@
     expect(dup_def).to eq(STRUCT_DEF)
   end
 
+  it "#offset to specified attribute" do
+    expect(BinaryStruct.new(STRUCT_DEF).offset(:short)).to eq(12)
+  end
+
   context "old style methods" do
     after(:each) { BinaryStruct.clear_structs_by_definition_cache }