add mrbgems/mruby-string-ext, and method: String#getbyte

mrbgems/mruby-string-ext/src/string.c

@@ -0,0 +1,30 @@
+#include "mruby.h"
+#include "mruby/string.h"
+
+static mrb_value
+mrb_str_getbyte(mrb_state *mrb, mrb_value str)
+{
+  mrb_int pos;
+  mrb_get_args(mrb, "i", &pos);
+
+  if (pos < 0)
+    pos += RSTRING_LEN(str);
+  if (pos < 0 ||  RSTRING_LEN(str) <= pos)
+    return mrb_nil_value();
+
+  return mrb_fixnum_value((unsigned char)RSTRING_PTR(str)[pos]);
+}
+
+
+void
+mrb_mruby_string_ext_gem_init(mrb_state* mrb)
+{
+  struct RClass * s = mrb->string_class;
+
+  mrb_define_method(mrb, s, "getbyte", mrb_str_getbyte, ARGS_REQ(1));
+}
+
+void
+mrb_mruby_string_ext_gem_final(mrb_state* mrb)
+{
+}