[ruby/fiddle] Add "offsetof" to Struct classes (https://github.com/ruby/fiddle/pull/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>

https://github.com/ruby/fiddle/commit/4e3b60c5b6

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

1 files changed, 43 insertions, 0 deletions

diff --git a/ext/fiddle/lib/fiddle/struct.rb b/ext/fiddle/lib/fiddle/struct.rb
index a766eba83b..2353edcc94 100644
--- a/ext/fiddle/lib/fiddle/struct.rb
+++ b/ext/fiddle/lib/fiddle/struct.rb
@@ -13,6 +13,30 @@ module Fiddle
       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 @@ module Fiddle
     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 @@ module Fiddle
         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)