Add convenience methods for getting a struct from a ruby object.

If we are trying to get the struct from something other than self, then we
should make sure to check the class of the object.  This util functions
make this easier.

ext/liquid/block.c

@@ -1,6 +1,5 @@
 #include "liquid_ext.h"
 
-extern VALUE mLiquid, cLiquidTokenizer, cLiquidTag, cLiquidTemplate, cLiquidVariable;
 VALUE cLiquidBlock;
 ID intern_assert_missing_delimitation, intern_block_delimiter, intern_is_blank, intern_new,
    intern_new_with_options, intern_tags, intern_unknown_tag, intern_unterminated_tag,
@@ -46,14 +45,7 @@ static bool parse_tag(struct liquid_tag *tag, char *token, long token_length)
 
 static VALUE rb_parse_body(VALUE self, VALUE tokenizerObj)
 {
-    Check_Type(tokenizerObj, T_DATA);
-    if (RBASIC_CLASS(tokenizerObj) != cLiquidTokenizer) {
-        rb_raise(rb_eTypeError, "wrong argument type %s (expected %s)",
-                 rb_obj_classname(tokenizerObj),
-                 rb_class2name(cLiquidTokenizer));
-    }
-    struct liquid_tokenizer *tokenizer;
-    Data_Get_Struct(tokenizerObj, struct liquid_tokenizer, tokenizer);
+    struct liquid_tokenizer *tokenizer = LIQUID_TOKENIZER_GET_STRUCT(tokenizerObj);
 
     bool blank = true;
     VALUE nodelist = rb_iv_get(self, "@nodelist");

ext/liquid/block.h

@@ -3,4 +3,6 @@
 
 void init_liquid_block();
 
+extern VALUE cLiquidBlock;
+
 #endif

ext/liquid/liquid_ext.h

@@ -7,5 +7,9 @@
 
 #include "tokenizer.h"
 #include "block.h"
+#include "utils.h"
+
+extern VALUE mLiquid;
+extern VALUE cLiquidTemplate, cLiquidTag, cLiquidVariable;
 
 #endif

ext/liquid/tokenizer.c

@@ -1,7 +1,6 @@
 #include "liquid_ext.h"
 
 VALUE cLiquidTokenizer;
-extern VALUE mLiquid;
 
 static void free_tokenizer(void *ptr)
 {

ext/liquid/tokenizer.h

@@ -1,6 +1,8 @@
 #ifndef LIQUID_TOKENIZER_H
 #define LIQUID_TOKENIZER_H
 
+extern VALUE cLiquidTokenizer;
+
 enum token_type {
     TOKEN_NONE,
     TOKEN_INVALID,
@@ -23,4 +25,6 @@ struct liquid_tokenizer {
 void init_liquid_tokenizer();
 void liquid_tokenizer_next(struct liquid_tokenizer *tokenizer, struct token *token);
 
+#define LIQUID_TOKENIZER_GET_STRUCT(obj) ((struct liquid_tokenizer *)obj_get_data_ptr(obj, cLiquidTokenizer))
+
 #endif

ext/liquid/utils.c

@@ -0,0 +1,21 @@
+#include <ruby.h>
+
+void raise_type_error(VALUE expected, VALUE got)
+{
+    rb_raise(rb_eTypeError, "wrong argument type %s (expected %s)",
+                             rb_class2name(got), rb_class2name(expected));
+}
+
+void check_class(VALUE obj, int type, VALUE klass)
+{
+    Check_Type(obj, type);
+    VALUE obj_klass = RBASIC_CLASS(obj);
+    if (obj_klass != klass)
+        raise_type_error(klass, obj_klass);
+}
+
+void *obj_get_data_ptr(VALUE obj, VALUE klass)
+{
+    check_class(obj, T_DATA, klass);
+    return DATA_PTR(obj);
+}

ext/liquid/utils.h

@@ -0,0 +1,8 @@
+#ifndef LIQUID_UTILS_H
+#define LIQUID_UTILS_H
+
+void raise_type_error(VALUE expected, VALUE got);
+void check_class(VALUE klass);
+void *obj_get_data_ptr(VALUE obj, VALUE klass);
+
+#endif