[ruby/fiddle] Fix assignment to array within struct (#26)

* Allow access to a struct's underlying memory with `struct[offset, length]`.

https://github.com/ruby/fiddle/commit/24083690a6

ext/fiddle/lib/fiddle/struct.rb

@@ -20,6 +20,33 @@ module Fiddle
     end
   end
 
+  # Wrapper for arrays within a struct
+  class StructArray < Array
+    include ValueUtil
+
+    def initialize(ptr, type, initial_values)
+      @ptr = ptr
+      @type = type
+      @align = PackInfo::ALIGN_MAP[type]
+      @size = Fiddle::PackInfo::SIZE_MAP[type]
+      @pack_format = Fiddle::PackInfo::PACK_MAP[type]
+      super(initial_values.collect { |v| unsigned_value(v, type) })
+    end
+
+    def to_ptr
+      @ptr
+    end
+
+    def []=(index, value)
+      if index < 0 || index >= size
+        raise IndexError, 'index %d outside of array bounds 0...%d' % [index, size]
+      end
+
+      to_ptr[index * @size, @size] = [value].pack(@pack_format)
+      super(index, value)
+    end
+  end
+
   # Used to construct C classes (CUnion, CStruct, etc)
   #
   # Fiddle::Importer#struct and Fiddle::Importer#union wrap this functionality in an
@@ -191,7 +218,7 @@ module Fiddle
       if( ty.is_a?(Integer) && (ty < 0) )
         return unsigned_value(val, ty)
       elsif( ty.is_a?(Array) && (ty[0] < 0) )
-        return val.collect{|v| unsigned_value(v,ty[0])}
+        return StructArray.new(self + @offset[idx], ty[0], val)
       else
         return val
       end

test/fiddle/test_import.rb

@@ -129,6 +129,20 @@ module Fiddle
       assert_equal([0,1,2], ary.value)
     end
 
+    def test_struct_array_assignment()
+      instance = Fiddle::Importer.struct(["unsigned int stages[3]"]).malloc
+      instance.stages[0] = 1024
+      instance.stages[1] = 10
+      instance.stages[2] = 100
+      assert_equal 1024, instance.stages[0]
+      assert_equal 10, instance.stages[1]
+      assert_equal 100, instance.stages[2]
+      assert_equal [1024, 10, 100].pack(Fiddle::PackInfo::PACK_MAP[-Fiddle::TYPE_INT] * 3),
+                   instance.to_ptr[0, 3 * Fiddle::SIZEOF_INT]
+      assert_raise(IndexError) { instance.stages[-1] = 5 }
+      assert_raise(IndexError) { instance.stages[3] = 5 }
+    end
+
     def test_struct()
       s = LIBC::MyStruct.malloc()
       s.num = [0,1,2,3,4]