diff --git a/src/fast_cal/fast_cal.cc b/src/fast_cal/fast_cal.cc
index dbea396..9c128b6 100644
--- a/src/fast_cal/fast_cal.cc
+++ b/src/fast_cal/fast_cal.cc
@@ -7,7 +7,15 @@
 #include <iostream>
 #include <algorithm>
 
-Core FastCal::import_core() {
+Core FastCal::init() { // initialization process
+    /// clear working data
+    cases.clear();
+    std::queue<fast_cal_t*>{}.swap(cache);
+
+    // TODO: test the speed without hashmap reserve
+    cases.reserve(65536);
+
+    /// import klotski core
     return Core(
         [this](auto &&code, auto &&mask) { // lambda as function pointer
             new_case(std::forward<decltype(code)>(code), std::forward<decltype(mask)>(mask));
@@ -16,31 +24,18 @@ Core FastCal::import_core() {
 }
 
 void FastCal::new_case(uint64_t code, uint64_t mask) { // callback function for new case
-
     auto current = cases.find(code);
     if (current != cases.end()) { // find existed case
         current->second.mask |= mask; // update mask info
         return;
     }
-
-    cache.emplace(&cases.emplace(code, fast_cal_t {
+    cache.emplace(&cases.emplace(code, fast_cal_t { // record new case
         .code = code,
         .mask = mask,
-        .last = cache.front(),
+        .last = cache.front(), // link parent case
     }).first->second);
-
-}
-
-uint64_t FastCal::solve() {
-
-    auto resolved = [](uint64_t code) {
-        return ((code >> (3 * 0xD)) & 0b111) == B_2x2;
-    };
-    return target(resolved);
-
 }
 
-// TODO: single backtrack function
 std::vector<uint64_t> FastCal::backtrack(uint64_t code) {
     std::vector<uint64_t> path;
 
@@ -57,44 +52,27 @@ std::vector<uint64_t> FastCal::backtrack(uint64_t code) {
 
 }
 
-uint64_t FastCal::target(const std::function<bool(uint64_t)> &match) {
-
-    auto core = import_core();
-
-    // clear data
-    cases.clear();
-
-    std::queue<fast_cal_t*>{}.swap(cache);
-
-//    auto empty = std::queue<fast_cal_t*>{};
-//    std::swap(empty, cache);
+uint64_t FastCal::solve(uint64_t code) {
+    return FastCal::target(code, [](uint64_t code) {
+        return ((code >> (3 * 0xD)) & 0b111) == B_2x2; // check 2x2 block address
+    });
+}
 
-    cases.reserve(65536);
+uint64_t FastCal::target(uint64_t code, const check_t &match) {
+    auto core = init();
 
-    cache.emplace(&cases.emplace(root, fast_cal_t {
-        .code = root,
+    cache.emplace(&cases.emplace(code, fast_cal_t {
+        .code = code,
         .mask = 0,
-        .last = nullptr,
+        .last = nullptr, // without parent node
     }).first->second);
 
     while (!cache.empty()) {
-
         if (match(cache.front()->code)) {
-            break;
+            return cache.front()->code; // match target
         }
-
         core.next_cases(cache.front()->code, cache.front()->mask);
         cache.pop();
     }
-
-    std::cout << "size: " << cases.size() << std::endl;
-
-    // TODO: cache may empty -> never found
-    if (!cache.empty()) {
-
-        return cache.front()->code;
-
-    }
-    return FastCal::NOT_FOUND;
-
+    return FastCal::NOT_FOUND; // target not found
 }
diff --git a/src/fast_cal/fast_cal.h b/src/fast_cal/fast_cal.h
index 670eb8b..1bf2d3c 100644
--- a/src/fast_cal/fast_cal.h
+++ b/src/fast_cal/fast_cal.h
@@ -9,7 +9,9 @@
 class FastCal {
 public:
 
-    explicit FastCal(uint64_t code) : root(code) {}
+    typedef std::function<bool(uint64_t)> check_t;
+
+//    explicit FastCal(uint64_t code) : root(code) {}
 
     // search resolve
     // search all min-step resolve
@@ -19,17 +21,13 @@ public:
 
     /// xxx_multi only search until same layer
 
-//    std::vector<uint64_t> solve(uint64_t code);
-//    std::vector<uint64_t> solve();
-    uint64_t solve();
 //    solve_multi
 
-//    std::vector<uint64_t> target(const std::function<bool(uint64_t)> &match);
-    uint64_t target(const std::function<bool(uint64_t)> &match);
+    uint64_t solve(uint64_t code);
 
-    std::vector<uint64_t> backtrack(uint64_t code);
+    uint64_t target(uint64_t code, const check_t &match);
 
-    // TODO: continue search process? -> reuse exist data (ensure working code not changed)
+    std::vector<uint64_t> backtrack(uint64_t code);
 
     // TODO: static search functions
 
@@ -43,12 +41,12 @@ private:
     };
 
 
-    uint64_t root;
+//    uint64_t root;
 
     std::queue<fast_cal_t*> cache;
     std::unordered_map<uint64_t, fast_cal_t> cases;
 
-    Core import_core();
+    Core init();
 
     void new_case(uint64_t code, uint64_t mask);
 
diff --git a/src/main.cc b/src/main.cc
index 592c8ab..d7cbf3c 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -25,11 +25,11 @@ int main() {
 
 //    AllCases::build();
 
-//    auto f = FastCal();
-//    auto ret = f.solve(RawCode::from_common_code("1a9bf0c").unwrap());
+    auto f = FastCal();
+    auto ret = f.solve(RawCode::from_common_code("1a9bf0c").unwrap());
 
-    auto f = FastCal(RawCode::from_common_code("1a9bf0c").unwrap());
-    auto ret = f.solve();
+//    auto f = FastCal(RawCode::from_common_code("1a9bf0c").unwrap());
+//    auto ret = f.solve();
     std::cout << RawCode(ret) << std::endl;
 
 //    for (const auto &c : ret) {