Add "offsetof" to Struct classes (#83)

* Add "offsetof" to Struct classes

I need to get the offset of a member inside a struct without allocating
the struct.  This patch adds an "offsetof" class method to structs that
are generated.

The usage is like this:

```ruby
MyStruct = struct [
  "int64_t i",
  "char c",
]

MyStruct.offsetof("i") # => 0
MyStruct.offsetof("c") # => 8
```

* Update test/fiddle/test_c_struct_builder.rb

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

lib/fiddle/struct.rb

@@ -13,6 +13,30 @@ def CStruct.entity_class
       CStructEntity
     end
 
+    def self.offsetof(name, members, types) # :nodoc:
+      offset = 0
+      index = 0
+      member_index = members.index(name)
+
+      types.each { |type, count = 1|
+        orig_offset = offset
+        if type.respond_to?(:entity_class)
+          align = type.alignment
+          type_size = type.size
+        else
+          align = PackInfo::ALIGN_MAP[type]
+          type_size = PackInfo::SIZE_MAP[type]
+        end
+        offset = PackInfo.align(orig_offset, align)
+
+        return offset if index == member_index
+
+        offset += (type_size * count)
+        index += 1
+      }
+      nil
+    end
+
     def each
       return enum_for(__function__) unless block_given?
 
@@ -75,6 +99,10 @@ class CUnion
     def CUnion.entity_class
       CUnionEntity
     end
+
+    def self.offsetof(name, members, types) # :nodoc:
+      0
+    end
   end
 
   # Wrapper for arrays within a struct
@@ -172,6 +200,21 @@ def create(klass, types, members)
         define_method(:to_i){ @entity.to_i }
         define_singleton_method(:types) { types }
         define_singleton_method(:members) { members }
+
+        # Return the offset of a struct member given its name.
+        # For example:
+        #
+        #     MyStruct = struct [
+        #       "int64_t i",
+        #       "char c",
+        #     ]
+        #
+        #     MyStruct.offsetof("i") # => 0
+        #     MyStruct.offsetof("c") # => 8
+        #
+        define_singleton_method(:offsetof) { |name|
+          klass.offsetof(name, members, types)
+        }
         members.each{|name|
           name = name[0] if name.is_a?(Array) # name is a nested struct
           next if method_defined?(name)

test/fiddle/test_c_struct_builder.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+begin
+  require_relative 'helper'
+  require 'fiddle/struct'
+  require 'fiddle/cparser'
+rescue LoadError
+end
+
+module Fiddle
+  class TestCStructBuilder < TestCase
+    include Fiddle::CParser
+
+    def test_offsetof
+      types, members = parse_struct_signature(['int64_t i','char c'])
+      my_struct = Fiddle::CStructBuilder.create(Fiddle::CStruct, types, members)
+      assert_equal 0, my_struct.offsetof("i")
+      assert_equal Fiddle::SIZEOF_INT64_T, my_struct.offsetof("c")
+    end
+
+    def test_offset_with_gap
+      types, members = parse_struct_signature(['void *p', 'char c', 'long x'])
+      my_struct = Fiddle::CStructBuilder.create(Fiddle::CStruct, types, members)
+
+      assert_equal PackInfo.align(0, ALIGN_VOIDP), my_struct.offsetof("p")
+      assert_equal PackInfo.align(SIZEOF_VOIDP, ALIGN_CHAR), my_struct.offsetof("c")
+      assert_equal SIZEOF_VOIDP + PackInfo.align(SIZEOF_CHAR, ALIGN_LONG), my_struct.offsetof("x")
+    end
+
+    def test_union_offsetof
+      types, members = parse_struct_signature(['int64_t i','char c'])
+      my_struct = Fiddle::CStructBuilder.create(Fiddle::CUnion, types, members)
+      assert_equal 0, my_struct.offsetof("i")
+      assert_equal 0, my_struct.offsetof("c")
+    end
+  end
+end if defined?(Fiddle)