diff --git a/native/src/additional_proximity_chars.cpp b/native/src/additional_proximity_chars.cpp
index 74c96ae..401c85a 100644
--- a/native/src/additional_proximity_chars.cpp
+++ b/native/src/additional_proximity_chars.cpp
@@ -19,23 +19,23 @@
 namespace latinime {
 const std::string AdditionalProximityChars::LOCALE_EN_US("en");
 
-const uint16_t AdditionalProximityChars::EN_US_ADDITIONAL_A[EN_US_ADDITIONAL_A_SIZE] = {
+const uint32_t AdditionalProximityChars::EN_US_ADDITIONAL_A[EN_US_ADDITIONAL_A_SIZE] = {
     'e', 'i', 'o', 'u'
 };
 
-const uint16_t AdditionalProximityChars::EN_US_ADDITIONAL_E[EN_US_ADDITIONAL_E_SIZE] = {
+const uint32_t AdditionalProximityChars::EN_US_ADDITIONAL_E[EN_US_ADDITIONAL_E_SIZE] = {
     'a', 'i', 'o', 'u'
 };
 
-const uint16_t AdditionalProximityChars::EN_US_ADDITIONAL_I[EN_US_ADDITIONAL_I_SIZE] = {
+const uint32_t AdditionalProximityChars::EN_US_ADDITIONAL_I[EN_US_ADDITIONAL_I_SIZE] = {
     'a', 'e', 'o', 'u'
 };
 
-const uint16_t AdditionalProximityChars::EN_US_ADDITIONAL_O[EN_US_ADDITIONAL_O_SIZE] = {
+const uint32_t AdditionalProximityChars::EN_US_ADDITIONAL_O[EN_US_ADDITIONAL_O_SIZE] = {
     'a', 'e', 'i', 'u'
 };
 
-const uint16_t AdditionalProximityChars::EN_US_ADDITIONAL_U[EN_US_ADDITIONAL_U_SIZE] = {
+const uint32_t AdditionalProximityChars::EN_US_ADDITIONAL_U[EN_US_ADDITIONAL_U_SIZE] = {
     'a', 'e', 'i', 'o'
 };
 }
diff --git a/native/src/additional_proximity_chars.h b/native/src/additional_proximity_chars.h
index b76726c..e0049d9 100644
--- a/native/src/additional_proximity_chars.h
+++ b/native/src/additional_proximity_chars.h
@@ -26,23 +26,23 @@
  private:
     static const std::string LOCALE_EN_US;
     static const int EN_US_ADDITIONAL_A_SIZE = 4;
-    static const uint16_t EN_US_ADDITIONAL_A[];
+    static const uint32_t EN_US_ADDITIONAL_A[];
     static const int EN_US_ADDITIONAL_E_SIZE = 4;
-    static const uint16_t EN_US_ADDITIONAL_E[];
+    static const uint32_t EN_US_ADDITIONAL_E[];
     static const int EN_US_ADDITIONAL_I_SIZE = 4;
-    static const uint16_t EN_US_ADDITIONAL_I[];
+    static const uint32_t EN_US_ADDITIONAL_I[];
     static const int EN_US_ADDITIONAL_O_SIZE = 4;
-    static const uint16_t EN_US_ADDITIONAL_O[];
+    static const uint32_t EN_US_ADDITIONAL_O[];
     static const int EN_US_ADDITIONAL_U_SIZE = 4;
-    static const uint16_t EN_US_ADDITIONAL_U[];
+    static const uint32_t EN_US_ADDITIONAL_U[];
 
-    static bool isEnLocale(std::string* locale_str) {
+    static bool isEnLocale(const std::string* locale_str) {
         return locale_str && locale_str->size() >= LOCALE_EN_US.size()
                 && locale_str->compare(0, LOCALE_EN_US.size(), LOCALE_EN_US);
     }
 
  public:
-    static int getAdditionalCharsSize(std::string* locale_str, uint16_t c) {
+    static int getAdditionalCharsSize(const std::string* locale_str, const uint16_t c) {
         if (!isEnLocale(locale_str)) {
             return 0;
         }
@@ -62,7 +62,7 @@
         }
     }
 
-    static const uint16_t* getAdditionalChars(std::string* locale_str, uint16_t c) {
+    static const uint32_t* getAdditionalChars(const std::string* locale_str, const uint32_t c) {
         if (!isEnLocale(locale_str)) {
             return 0;
         }
@@ -82,7 +82,7 @@
         }
     }
 
-    static bool hasAdditionalChars(std::string* locale_str, uint16_t c) {
+    static bool hasAdditionalChars(const std::string* locale_str, const uint32_t c) {
         return getAdditionalCharsSize(locale_str, c) > 0;
     }
 };
diff --git a/native/src/proximity_info.cpp b/native/src/proximity_info.cpp
index 561fffa..e590740 100644
--- a/native/src/proximity_info.cpp
+++ b/native/src/proximity_info.cpp
@@ -50,6 +50,7 @@
           HAS_TOUCH_POSITION_CORRECTION_DATA(keyCount > 0 && keyXCoordinates && keyYCoordinates
                   && keyWidths && keyHeights && keyCharCodes && sweetSpotCenterXs
                   && sweetSpotCenterYs && sweetSpotRadii),
+          mLocaleStr(localeStr),
           mInputXCoordinates(0), mInputYCoordinates(0),
           mTouchPositionCorrectionEnabled(false) {
     const int proximityGridLength = GRID_WIDTH * GRID_HEIGHT * MAX_PROXIMITY_CHARS_SIZE;
@@ -164,6 +165,30 @@
             }
         }
     }
+    const int existingProximitySize = insertPos;
+    for (int i = 0; i < existingProximitySize; ++i) {
+        const uint32_t c = inputCodes[i];
+        const int additionalProximitySize =
+                AdditionalProximityChars::hasAdditionalChars(&mLocaleStr, c);
+        if (additionalProximitySize <= 0) {
+            continue;
+        }
+        const uint32_t* additionalProximityChars =
+                AdditionalProximityChars::getAdditionalChars(&mLocaleStr, c);
+        for (int j = 0; j < additionalProximitySize; ++j) {
+            const uint32_t ac = additionalProximityChars[j];
+            int k = 0;
+            for (; k < insertPos; ++k) {
+                if ((int)ac == inputCodes[k]) {
+                    break;
+                }
+            }
+            if (k < insertPos) {
+                continue;
+            }
+            inputCodes[insertPos++] = ac;
+        }
+    }
     // TODO: calculate additional chars
 }
 
diff --git a/native/src/proximity_info.h b/native/src/proximity_info.h
index bfcef63..8c342e0 100644
--- a/native/src/proximity_info.h
+++ b/native/src/proximity_info.h
@@ -104,6 +104,7 @@
     const int CELL_HEIGHT;
     const int KEY_COUNT;
     const bool HAS_TOUCH_POSITION_CORRECTION_DATA;
+    const std::string mLocaleStr;
     const int *mInputCodes;
     const int *mInputXCoordinates;
     const int *mInputYCoordinates;