diff --git a/src/fast_cal/cal_core.cc b/src/fast_cal/cal_core.cc
index 2388c4d..8753f6b 100644
--- a/src/fast_cal/cal_core.cc
+++ b/src/fast_cal/cal_core.cc
@@ -37,7 +37,7 @@ void FastCal::new_case(uint64_t code, uint64_t mask) {
 }
 
 /// found first matched target
-RawCode FastCal::target(RawCode code, const match_t &match) {
+RawCode FastCal::target(const RawCode &code, const match_t &match) {
     auto core = init((uint64_t)code);
     /// start bfs search
     while (!cache.empty()) {
@@ -51,7 +51,7 @@ RawCode FastCal::target(RawCode code, const match_t &match) {
 }
 
 /// found multi-targets matched in first same layer
-std::vector<RawCode> FastCal::target_multi(RawCode code, const match_t &match) {
+std::vector<RawCode> FastCal::target_multi(const RawCode &code, const match_t &match) {
     auto core = init((uint64_t)code);
     auto layer_end = cache.back();
     std::vector<RawCode> matched; // matched list
@@ -73,7 +73,7 @@ std::vector<RawCode> FastCal::target_multi(RawCode code, const match_t &match) {
 }
 
 /// found all of the furthest cases
-std::vector<RawCode> FastCal::furthest(RawCode code) {
+std::vector<RawCode> FastCal::furthest(const RawCode &code) {
     auto core = init((uint64_t)code);
     auto layer_end = cache.back();
     std::vector<RawCode> layer_cases;
diff --git a/src/fast_cal/fast_cal.cc b/src/fast_cal/fast_cal.cc
index 9f4cf42..9598786 100644
--- a/src/fast_cal/fast_cal.cc
+++ b/src/fast_cal/fast_cal.cc
@@ -4,21 +4,29 @@
 #include "fast_cal.h"
 #include "raw_code.h"
 
+/// klotski resolved -> 2x2 block at address 13 (aka 0xD)
 auto resolved = [](uint64_t code) {
     return ((code >> (3 * 0xD)) & 0b111) == B_2x2; // check 2x2 block address
 };
 
-/// klotski resolved -> 2x2 block at address 13 (aka 0xD)
-RawCode FastCal::solve(RawCode code) {
+RawCode FastCal::solve(const RawCode &code) {
     return FastCal::target(code, resolved);
 }
 
-std::vector<RawCode> FastCal::solve_multi(RawCode code) {
+std::vector<RawCode> FastCal::solve_multi(const RawCode &code) {
     return FastCal::target_multi(code, resolved);
 }
 
+std::vector<RawCode> FastCal::resolve(const RawCode &start) {
+    return FastCal::search(start, resolved);
+}
+
+std::vector<std::vector<RawCode>> FastCal::resolve_multi(const RawCode &start) {
+    return FastCal::search_multi(start, resolved);
+}
+
 /// backtrack of FastCal tree
-int FastCal::step_num(RawCode code) {
+int FastCal::step_num(const RawCode &code) {
     auto tmp = cases.find((uint64_t)code);
     if (tmp == cases.end()) {
         return -1; // code not exist
@@ -46,15 +54,8 @@ std::vector<RawCode> FastCal::backtrack(const RawCode &code) {
     return path;
 }
 
-std::vector<RawCode> FastCal::resolve(RawCode start) {
-    return FastCal::search(start, resolved);
-}
-
-std::vector<std::vector<RawCode>> FastCal::resolve_multi(RawCode start) {
-    return FastCal::search_multi(start, resolved);
-}
-
-std::vector<std::vector<RawCode>> FastCal::to_furthest(RawCode start) {
+/// static BFS search functions
+std::vector<std::vector<RawCode>> FastCal::to_furthest(const RawCode &start) {
     auto fc = FastCal();
     std::vector<std::vector<RawCode>> result;
     for (const auto &furthest : fc.furthest(start)) {
@@ -63,7 +64,7 @@ std::vector<std::vector<RawCode>> FastCal::to_furthest(RawCode start) {
     return result;
 }
 
-std::vector<RawCode> FastCal::search(RawCode start, const match_t &match) {
+std::vector<RawCode> FastCal::search(const RawCode &start, const match_t &match) {
     auto fc = FastCal();
     auto result = fc.target(start, match);
     if (result == FC_NOT_FOUND) {
@@ -72,7 +73,7 @@ std::vector<RawCode> FastCal::search(RawCode start, const match_t &match) {
     return fc.backtrack(result); // backtrack target path
 }
 
-std::vector<std::vector<RawCode>> FastCal::search_multi(RawCode start, const match_t &match) {
+std::vector<std::vector<RawCode>> FastCal::search_multi(const RawCode &start, const match_t &match) {
     auto fc = FastCal();
     std::vector<std::vector<RawCode>> result;
     for (const auto &target : fc.target_multi(start, match)) {
diff --git a/src/fast_cal/fast_cal.h b/src/fast_cal/fast_cal.h
index ad51b49..6fbb844 100644
--- a/src/fast_cal/fast_cal.h
+++ b/src/fast_cal/fast_cal.h
@@ -16,25 +16,23 @@ public:
     typedef std::function<bool(uint64_t)> match_t;
 
     /// backtrack functions
-    int step_num(RawCode code);
+    int step_num(const RawCode &code);
     std::vector<RawCode> backtrack(const RawCode &code);
 
     /// BFS search functions
     // TODO: build function with void return -> build total tree
-    RawCode solve(RawCode code);
-    std::vector<RawCode> furthest(RawCode code);
-    std::vector<RawCode> solve_multi(RawCode code);
-    RawCode target(RawCode code, const match_t &match);
-    std::vector<RawCode> target_multi(RawCode code, const match_t &match);
+    RawCode solve(const RawCode &code);
+    std::vector<RawCode> furthest(const RawCode &code);
+    std::vector<RawCode> solve_multi(const RawCode &code);
+    RawCode target(const RawCode &code, const match_t &match);
+    std::vector<RawCode> target_multi(const RawCode &code, const match_t &match);
 
     /// static BFS search functions
-    static std::vector<RawCode> resolve(RawCode start);
-    static std::vector<std::vector<RawCode>> to_furthest(RawCode start);
-    static std::vector<std::vector<RawCode>> resolve_multi(RawCode start);
-    static std::vector<RawCode> search(RawCode start, const match_t &match);
-    static std::vector<std::vector<RawCode>> search_multi(RawCode start, const match_t &match);
-
-    // TODO: (RawCode code) -> (const RawCode &code)
+    static std::vector<RawCode> resolve(const RawCode &start);
+    static std::vector<std::vector<RawCode>> to_furthest(const RawCode &start);
+    static std::vector<std::vector<RawCode>> resolve_multi(const RawCode &start);
+    static std::vector<RawCode> search(const RawCode &start, const match_t &match);
+    static std::vector<std::vector<RawCode>> search_multi(const RawCode &start, const match_t &match);
 
 private:
     struct fast_cal_t {
diff --git a/src/main.cc b/src/main.cc
index 6fd91d9..354915a 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -26,7 +26,6 @@ int main() {
 //    AllCases::build();
 
 //    auto f = FastCal();
-//    auto ret = f.solve((uint64_t)RawCode::from_common_code("1a9bf0c"));
 //    auto ret = f.solve(RawCode::from_common_code("1a9bf0c"));
 
 //    auto ret = f.target(RawCode::from_common_code("1a9bf0c"), [](uint64_t code) {
@@ -45,17 +44,17 @@ int main() {
 //        std::cout << "step number: " << f.step_num(ret) << std::endl;
 //    }
 
-//    for (const auto &c : FastCal::resolve(RawCode::from_common_code("1a9bf0c"))) {
+    for (const auto &c : FastCal::resolve(RawCode::from_common_code("1a9bf0c"))) {
 //        std::cout << c << std::endl;
-//    }
-
-    for (const auto &s : FastCal::to_furthest(RawCode::from_common_code("1a9bf0c"))) {
-        for (const auto &c : s) {
-            std::cout << c << std::endl;
-        }
-        std::cout << "--------------------------------------------" << std::endl;
     }
 
+//    for (const auto &s : FastCal::to_furthest(RawCode::from_common_code("1a9bf0c"))) {
+//        for (const auto &c : s) {
+//            std::cout << c << std::endl;
+//        }
+//        std::cout << "--------------------------------------------" << std::endl;
+//    }
+
 //    std::cout << f.step_num(ret) << std::endl;
 
 //    auto f = FastCal(RawCode::from_common_code("1a9bf0c").unwrap());