diff --git a/src/core/CMakeLists.txt b/src/core/CMakeLists.txt
index 51d599a..50268c9 100644
--- a/src/core/CMakeLists.txt
+++ b/src/core/CMakeLists.txt
@@ -37,7 +37,7 @@ set(KLOTSKI_CORE_SRC
 add_library(klotski_core STATIC ${KLOTSKI_CORE_SRC})
 target_compile_options(klotski_core PRIVATE -fno-rtti -fno-exceptions)  # option for `-fvisibility=hidden`
 target_include_directories(klotski_core PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
-target_link_libraries(klotski_core PRIVATE absl::flat_hash_map)
+target_link_libraries(klotski_core PRIVATE absl::flat_hash_map phmap)
 add_library(klotski::core ALIAS klotski_core)
 
 # TODO: just for dev testing
diff --git a/src/core/fast_cal/internal/demo.cc b/src/core/fast_cal/internal/demo.cc
index 528f97f..8f20020 100644
--- a/src/core/fast_cal/internal/demo.cc
+++ b/src/core/fast_cal/internal/demo.cc
@@ -4,7 +4,15 @@
 
 #include <queue>
 
-#include <absl/container/flat_hash_map.h>
+//#include <absl/container/flat_hash_map.h>
+
+//#include <absl/hash/hash.h>
+//#define PHMAP_USE_ABSL_HASH
+
+//#include <robin_hood.h>
+//#include <ankerl/unordered_dense.h>
+//#include <tsl/robin_map.h>
+#include <parallel_hashmap/phmap.h>
 
 #include "mover/mover.h"
 #include "group/group.h"
@@ -23,46 +31,91 @@ struct data_t {
 
 // TODO: try wrapper as custom `std::queue`
 
+template <typename T>
+class MyQueue {
+public:
+    explicit MyQueue(size_t reserve) {
+//        vec_.reserve(reserve);
+        vec_.resize(reserve);
+    }
+
+    void emplace_back(T item) {
+//        vec_.emplace_back(item);
+        vec_[iter_] = item;
+        ++iter_;
+    }
+
+    T front() {
+        return vec_[offset_];
+    }
+
+    void pop() {
+        ++offset_;
+    }
+
+    bool empty() {
+        return offset_ == iter_;
+    }
+
+private:
+    size_t iter_ {0};
+    size_t offset_ {0};
+    std::vector<T> vec_ {};
+};
+
 class FCDemo {
 public:
-    explicit FCDemo(RawCode raw_code) {
+    explicit FCDemo(RawCode raw_code) : codes_(GroupUnion::from_raw_code(raw_code).max_group_size()) {
         auto reserve = GroupUnion::from_raw_code(raw_code).max_group_size();
-        codes_.reserve(reserve);
+//        codes_.reserve(reserve);
         cases_.reserve(reserve);
-        codes_.emplace_back(raw_code);
+        codes_.emplace_back(raw_code.unwrap());
         cases_.emplace(raw_code, data_t {0, 0}); // without mask
     }
 
     RawCode DoCal() {
-//        bool stop_flag = false;
         uint64_t result = 0;
         auto core = MaskMover([this, &result](uint64_t code, uint64_t mask) {
             if (const auto match = cases_.find(code); match != cases_.end()) {
                 match->second.mask |= mask; // update mask
+//                match.value().mask |= mask; // update mask
                 return;
             }
             cases_.emplace(code, data_t {
                 .mask = mask,
-                .back = codes_[offset_].unwrap(),
+                .back = codes_.front(),
+//                .back = codes_[offset_].unwrap(),
             });
-            codes_.emplace_back(RawCode::unsafe_create(code));
+            codes_.emplace_back(code);
 
             if (((code >> 39) & 0b111) == 0b100) {
-//                stop_flag = true;
                 result = code;
             }
         });
 
-        while (offset_ != codes_.size()) {
-            auto curr = codes_[offset_].unwrap();
+        while (!codes_.empty()) {
+            auto curr = codes_.front();
             core.next_cases(curr, cases_.find(curr)->second.mask);
-            ++offset_;
+            codes_.pop();
 
             if (result != 0) {
-                break;
+                return RawCode::unsafe_create(result);
             }
+
         }
 
+        return RawCode::unsafe_create(0);
+
+//        while (offset_ != codes_.size()) {
+//            auto curr = codes_[offset_].unwrap();
+//            core.next_cases(curr, cases_.find(curr)->second.mask);
+//            ++offset_;
+//
+//            if (result != 0) {
+//                break;
+//            }
+//        }
+
 //        std::vector<RawCode> path;
 //        auto code = result;
 //        while (true) {
@@ -78,89 +131,20 @@ public:
 //        }
 //        std::cout << path.size() << std::endl;
 
-	return codes_[offset_];
+//	    return codes_[offset_];
     }
 
 private:
-    uint64_t offset_ {0};
-    std::vector<RawCode> codes_;
-    absl::flat_hash_map<uint64_t, data_t> cases_; // <code, mask>
+//    uint64_t offset_ {0};
+    MyQueue<uint64_t> codes_;
+//    absl::flat_hash_map<uint64_t, data_t> cases_; // <code, mask>
+//    robin_hood::unordered_map<uint64_t, data_t> cases_;
+//    ankerl::unordered_dense::map<uint64_t, data_t> cases_;
+//    tsl::robin_map<uint64_t, data_t> cases_;
+    phmap::flat_hash_map<uint64_t, data_t> cases_;
 };
 
 RawCode FastCal_demo(RawCode raw_code) {
-
     FCDemo fc {raw_code};
     return fc.DoCal();
-
-//    std::queue<RawCode> cache;
-    std::vector<RawCode> codes;
-    absl::flat_hash_map<uint64_t, data_t> cases; // <code, mask>
-    auto reserve = GroupUnion::from_raw_code(raw_code).max_group_size();
-    codes.reserve(reserve);
-    cases.reserve(reserve);
-
-    uint64_t offset = 0;
-    bool stop_flag = false;
-    auto core = MaskMover([&codes, &cases, &offset, &stop_flag](uint64_t code, uint64_t mask) {
-        if (const auto match = cases.find(code); match != cases.end()) {
-            match->second.mask |= mask; // update mask
-            return;
-        }
-        cases.emplace(code, data_t {
-            .mask = mask,
-            .back = codes[offset].unwrap(),
-//            .back = cache.front().unwrap(),
-        });
-        codes.emplace_back(RawCode::unsafe_create(code));
-//        cache.emplace(RawCode::unsafe_create(code));
-        if (((code >> 39) & 0b111) == 0b100) {
-            stop_flag = true;
-        }
-    });
-
-    codes.emplace_back(raw_code);
-//    cache.emplace(raw_code);
-    cases.emplace(raw_code, data_t {0, 0}); // without mask
-
-    while (offset != codes.size()) {
-        auto curr = codes[offset].unwrap();
-        core.next_cases(curr, cases.find(curr)->second.mask);
-        ++offset;
-
-        if (stop_flag) {
-            break;
-        }
-
-    }
-
-//    while (!cache.empty()) {
-//        if (((cache.front().unwrap() >> 39) & 0b111) == 0b100) {
-//            break;
-//        }
-//        uint64_t curr = cache.front().unwrap();
-//        core.next_cases(curr, cases.find(curr)->second.mask);
-//        cache.pop();
-//        if (stop_flag) {
-//            break;
-//        }
-//    }
-
-//    std::vector<RawCode> path;
-//    auto code = cache.front().unwrap();
-//    while (true) {
-//        if (code == 0) {
-//            break;
-//        }
-//        path.emplace_back(RawCode::unsafe_create(code));
-//        code = cases.find(code)->second.back;
-//    }
-//    std::reverse(path.begin(), path.end());
-//    for (auto step : path) {
-//        std::cout << step << std::endl;
-//    }
-//    std::cout << path.size() << std::endl;
-
-    return codes[offset];
-//    return cache.front();
-
 }