diff --git a/src/fast_cal/fast_cal.cc b/src/fast_cal/fast_cal.cc
index 866dd14..6a01d04 100644
--- a/src/fast_cal/fast_cal.cc
+++ b/src/fast_cal/fast_cal.cc
@@ -135,6 +135,56 @@ std::vector<uint64_t> FastCal::target_multi(uint64_t code, const FastCal::check_
 
 }
 
+std::vector<uint64_t> FastCal::furthest(uint64_t code) {
+
+    auto core = init();
+
+    cache.emplace(&cases.emplace(code, fast_cal_t {
+        .code = code,
+        .mask = 0,
+        .last = nullptr, // without parent node
+    }).first->second);
+
+    auto layer_end = cache.back();
+
+    int layer_num = 0;
+
+    std::vector<uint64_t> layer_cases;
+
+    while (!cache.empty()) {
+
+        core.next_cases(cache.front()->code, cache.front()->mask);
+
+        layer_cases.emplace_back(cache.front()->code);
+
+        if (cache.front() == layer_end) {
+
+            std::cout << "layer size -> " << layer_cases.size() << std::endl;
+            std::cout << "reach layer " << layer_num << " ending -> " << cache.size() << std::endl;
+
+            if (cache.size() == 1) { // almost exit -> last layer
+                break;
+//                return layer_cases;
+
+            }
+
+            layer_cases.clear();
+
+            ++layer_num;
+
+            layer_end = cache.back();
+
+        }
+
+        cache.pop();
+
+    }
+
+
+    return layer_cases;
+
+}
+
 
 
 
diff --git a/src/fast_cal/fast_cal.h b/src/fast_cal/fast_cal.h
index a04ac0c..2f2a027 100644
--- a/src/fast_cal/fast_cal.h
+++ b/src/fast_cal/fast_cal.h
@@ -11,6 +11,8 @@ public:
 
     typedef std::function<bool(uint64_t)> check_t;
 
+    const static auto NOT_FOUND = (uint64_t)0;
+
 //    explicit FastCal(uint64_t code) : root(code) {}
 
     // search resolve
@@ -23,18 +25,22 @@ public:
 
 //    solve_multi
 
+    // TODO: shall we using RawCode instead of uint64_t?
+
     uint64_t solve(uint64_t code);
 
     uint64_t target(uint64_t code, const check_t &match);
 
     std::vector<uint64_t> solve_multi(uint64_t code);
+
     std::vector<uint64_t> target_multi(uint64_t code, const check_t &match);
 
+    std::vector<uint64_t> furthest(uint64_t code);
+
     std::vector<uint64_t> backtrack(uint64_t code);
 
     // TODO: static search functions
 
-    const static auto NOT_FOUND = (uint64_t)0;
 
 private:
     struct fast_cal_t {
diff --git a/src/main.cc b/src/main.cc
index 01daf99..08f4b81 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -27,7 +27,7 @@ int main() {
 
     auto f = FastCal();
 //    auto ret = f.solve((uint64_t)RawCode::from_common_code("1a9bf0c"));
-    auto ret = f.solve_multi((uint64_t)RawCode::from_common_code("1a9bf0c"));
+    auto ret = f.furthest((uint64_t)RawCode::from_common_code("1a9bf0c"));
 
 //    auto f = FastCal(RawCode::from_common_code("1a9bf0c").unwrap());
 //    auto ret = f.solve();