diff --git a/klotski/short_code.h b/klotski/short_code.h
index 0c0da18..7487f21 100644
--- a/klotski/short_code.h
+++ b/klotski/short_code.h
@@ -5,6 +5,7 @@
 #include <cstdint>
 #include <unordered_map>
 
+/// ok
 const char SHORT_CODE_TABLE[32] = {
     '1', '2', '3', '4', '5', '6', '7', '8', '9', // skip `0`
     'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', // skip `I`
@@ -13,6 +14,7 @@ const char SHORT_CODE_TABLE[32] = {
     'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
 };
 
+/// ok
 const char SHORT_CODE_TABLE_REV[42] = {
 //  00  01  02  03  04  05  06  07  08
      0,  1,  2,  3,  4,  5,  6,  7,  8,
@@ -28,9 +30,10 @@ const char SHORT_CODE_TABLE_REV[42] = {
 
 class ShortCode {
 public:
-    // ok
+    /// ok
     enum Mode {NORMAL, FAST};
 
+    /// ok
     ShortCode() = default;
     void speed_up(enum Mode mode);
     explicit ShortCode(enum Mode mode);
@@ -39,19 +42,20 @@ public:
     uint32_t zip_short_code(uint64_t common_code);
     uint64_t unzip_short_code(uint32_t short_code);
 
+    /// ok
     static std::string code_to_string(uint32_t short_code);
     static uint32_t code_from_string(const std::string &short_code);
 
 private:
-    // ok
+    /// ok
     static const uint32_t SHORT_CODE_LIMIT = 29334498;
 
-    // ok
+    /// ok
     std::vector<uint32_t> basic_ranges;
     std::vector<uint64_t> all_cases_list; // short_code -> common_code
     std::unordered_map<uint64_t, uint32_t> all_cases_dict; // common_code -> short_code
 
-    // ok
+    /// ok
     void build_mappings();
     enum Mode check_mode();
     void build_base_ranges();
diff --git a/src/main.cc b/src/main.cc
index 14dadc2..ca0b360 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -100,6 +100,8 @@ int main() {
     auto s = ShortCode(14323231);
     std::cout << s.unwrap() << std::endl;
     std::cout << s.to_string() << std::endl;
+    std::cout << ShortCode("EP4HZ").unwrap() << std::endl;
+    std::cout << ShortCode("eP4hZ").to_string() << std::endl;
 
     return 0;
 }
diff --git a/src/short_code/short_code.cc b/src/short_code/short_code.cc
index 24c2f13..65422b8 100644
--- a/src/short_code/short_code.cc
+++ b/src/short_code/short_code.cc
@@ -1,4 +1,5 @@
 #include "short_code.h"
+#include "short_code_chars.h"
 
 uint32_t ShortCode::unwrap() const {
     return code; // get raw uint32_t code
@@ -15,13 +16,39 @@ ShortCode::ShortCode(uint32_t short_code) {
     code = short_code;
 }
 
-#include <iostream>
-
-std::string ShortCode::to_string() const {
+ShortCode::ShortCode(const std::string &short_code_str) { // 5-bits string decode
+    if (short_code_str.length() != 5) { // check string length
+        throw std::invalid_argument("short code format error");
+    }
 
-    // this->code ==> std::string
+    uint64_t short_code = 0;
+    for (auto bit : short_code_str) {
+        short_code *= 32;
+        if (bit >= 'a' && bit <= 'z') {
+            bit -= 32; // convert to uppercase
+        }
+        if (bit >= '1' && bit <= 'Z') {
+            short_code += (bit = SHORT_CODE_TABLE_REV[bit - 49]); // table convert
+            if (bit != -1) {
+                continue; // pass check
+            }
+        }
+        throw std::invalid_argument("short code format error"); // unknown characters
+    }
 
-    std::cout << "short code: " << code << std::endl;
+    if (!ShortCode::check(short_code)) { // check converted short code
+        throw std::invalid_argument("invalid short code");
+    }
+    code = short_code;
+}
 
-    return "";
+std::string ShortCode::to_string() const { // encode as 5-bits string
+    uint32_t short_code = code;
+    std::string result(5, '\0'); // short code length 5
+    for (int n = 0; n < 5; ++n) {
+        uint8_t bit = short_code % 32;
+        short_code = (short_code - bit) / 32;
+        result[4 - n] = SHORT_CODE_TABLE[bit];
+    }
+    return result;
 }
diff --git a/src/short_code/short_code.h b/src/short_code/short_code.h
index 583eb0c..9c70989 100644
--- a/src/short_code/short_code.h
+++ b/src/short_code/short_code.h
@@ -20,7 +20,7 @@ public:
     std::string to_string() const;
 
     explicit ShortCode(uint32_t short_code);
-//    explicit CommonCode(const std::string &common_code_str);
+    explicit ShortCode(const std::string &short_code_str);
 
 
 
diff --git a/src/short_code/short_code_chars.h b/src/short_code/short_code_chars.h
new file mode 100644
index 0000000..cb4b748
--- /dev/null
+++ b/src/short_code/short_code_chars.h
@@ -0,0 +1,22 @@
+#pragma once
+
+const char SHORT_CODE_TABLE[32] = {
+    '1', '2', '3', '4', '5', '6', '7', '8', '9', // skip `0`
+    'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', // skip `I`
+    'J', 'K', // skip `L`
+    'M', 'N', // skip `O`
+    'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
+};
+
+const char SHORT_CODE_TABLE_REV[42] = {
+//  00  01  02  03  04  05  06  07  08
+     0,  1,  2,  3,  4,  5,  6,  7,  8,
+//  09  10  11  12  13  14  15
+    -1, -1, -1, -1, -1, -1, -1,
+//  16  17  18  19  20  21  22  23  24  25
+     9, 10, 11, 12, 13, 14, 15, 16, -1, 17,
+//  26  27  28  29  30  31  32  33  34  35
+    18, -1, 19, 20, -1, 21, 22, 23, 24, 25,
+//  36  37  38  39  40  41
+    26, 27, 28, 29, 30, 31,
+};