diff --git a/src/FontPreprocessor.cc b/src/FontPreprocessor.cc
new file mode 100644
index 0000000..1ee1d35
--- /dev/null
+++ b/src/FontPreprocessor.cc
@@ -0,0 +1,61 @@
+/*
+ * FontPreprocessor.h
+ *
+ * Check used codes for each font
+ *
+ * by WangLu
+ * 2012.09.07
+ */
+
+#include <cstring>
+
+#include <GfxState.h>
+#include <GfxFont.h>
+
+#include "FontPreprocessor.h"
+#include "util.h"
+
+FontPreprocessor::FontPreprocessor(void)
+    : cur_font_id(0)
+    , cur_code_map(nullptr)
+{ }
+
+FontPreprocessor::~FontPreprocessor(void)
+{
+    for(auto & p : code_maps)
+        delete [] p.second;
+}
+
+void FontPreprocessor::drawChar(GfxState *state, double x, double y,
+      double dx, double dy,
+      double originX, double originY,
+      CharCode code, int nBytes, Unicode *u, int uLen)
+{
+    GfxFont * font = state->getFont();
+    if(!font) return;
+
+    long long fn_id = hash_ref(font->getID());
+
+    if(fn_id != cur_font_id)
+    {
+        cur_font_id = fn_id;
+        auto p = code_maps.insert(std::make_pair(cur_font_id, nullptr));
+        if(p.second)
+        {
+            // this is a new font
+            int len = font->isCIDFont() ? 0x10000 : 0x100;
+            p.first->second = new char [len];
+            memset(p.first->second, 0, len * sizeof(char));
+        }
+
+        cur_code_map = p.first->second;
+    }
+
+    cur_code_map[code] = 1;
+}
+
+const char * FontPreprocessor::get_code_map (long long font_id) const
+{
+    auto iter = code_maps.find(font_id);
+    return (iter == code_maps.end()) ? nullptr : (iter->second);
+}
diff --git a/src/FontPreprocessor.h b/src/FontPreprocessor.h
new file mode 100644
index 0000000..96bc8e5
--- /dev/null
+++ b/src/FontPreprocessor.h
@@ -0,0 +1,42 @@
+/*
+ * FontPreprocessor.h
+ *
+ * Check used codes for each font
+ *
+ * by WangLu
+ * 2012.09.07
+ */
+
+
+#ifndef FONTPREPROCESSOR_H__
+#define FONTPREPROCESSOR_H__
+
+#include <unordered_map>
+
+#include <OutputDev.h>
+
+class FontPreprocessor : public OutputDev {
+public:
+    FontPreprocessor(void);
+    virtual ~FontPreprocessor(void);
+
+    virtual GBool upsideDown() { return gFalse; }
+    virtual GBool useDrawChar() { return gTrue; }
+    virtual GBool interpretType3Chars() { return gFalse; }
+    virtual GBool needNonText() { return gFalse; }
+    virtual void drawChar(GfxState *state, double x, double y,
+      double dx, double dy,
+      double originX, double originY,
+      CharCode code, int nBytes, Unicode *u, int uLen);
+
+    const char * get_code_map (long long font_id) const;
+
+protected:
+    long long cur_font_id;
+    char * cur_code_map;
+
+    std::unordered_map<long long, char*> code_maps;
+};
+
+
+#endif //FONTPREPROCESSOR_H__