diff --git a/test/CMakeLists.txt b/test/CMakeLists.txt
index 4885750..1c03aa4 100644
--- a/test/CMakeLists.txt
+++ b/test/CMakeLists.txt
@@ -57,7 +57,7 @@ add_test(NAME codec COMMAND test_codec)
 
 set(TEST_CORE_SRC core/core.cc)
 add_executable(test_core ${TEST_CORE_SRC})
-target_link_libraries(test_core ${TEST_DEPS})
+target_link_libraries(test_core ${TEST_DEPS} absl::flat_hash_map)
 add_test(NAME core COMMAND test_core)
 
 ################################################################################
diff --git a/test/core/core.cc b/test/core/core.cc
index 054e471..f7c7016 100644
--- a/test/core/core.cc
+++ b/test/core/core.cc
@@ -1,40 +1,35 @@
 #include <unordered_set>
 #include "core.h"
+#include "group.h"
 #include "all_cases.h"
 #include "gtest/gtest.h"
 
+using klotski::Group;
 using klotski::RawCode;
 using klotski::AllCases;
 
-TEST(Core, demo) {
-    auto raw_codes = std::unordered_set<uint64_t>();
-//    auto raw_codes_ = std::vector<uint64_t>();
+using klotski::TYPE_ID_LIMIT;
 
-    raw_codes.reserve(klotski::ALL_CASES_SIZE_SUM);
-//    raw_codes_.reserve(klotski::ALL_CASES_SIZE_SUM);
-
-    for (auto &&common_code : AllCases::release()) { // load all raw codes
-
-//        EXPECT_NE(common_code.to_raw_code(), 0);
-
-        raw_codes.emplace(common_code.to_raw_code().unwrap());
-//        raw_codes_.emplace_back(common_code.to_raw_code().unwrap());
-    }
-
-    std::cout << "init ok" << std::endl;
-
-    auto core = klotski::Core(
-        [&raw_codes](auto &&code, auto &&mask) {
-            EXPECT_NE(raw_codes.find(code), raw_codes.end());
-//            new_case(std::forward<decltype(code)>(code), std::forward<decltype(mask)>(mask));
+TEST(Core, next_cases) {
+    auto test = [](uint32_t type_id) {
+        auto raw_codes = std::unordered_set<uint64_t>();
+        raw_codes.reserve(klotski::ALL_CASES_SIZE_SUM);
+        for (auto &&common_code: Group::all_cases(type_id)) { // load all cases in a type
+            raw_codes.emplace(common_code.to_raw_code().unwrap());
         }
-    );
-
 
-    for (auto &&raw_code : raw_codes) {
-        core.next_cases(raw_code, 0);
+        auto core = klotski::Core(
+            [&raw_codes](auto &&code, auto &&mask) {
+                EXPECT_NE(raw_codes.find(code), raw_codes.end()); // verify next case
+                EXPECT_EQ(mask >> __builtin_ctzll(mask), 0b111); // verify mask
+            }
+        );
+        for (auto &&raw_code: raw_codes) {
+            core.next_cases(raw_code, 0); // search next cases
+        }
+    };
+    // TODO: using multi-threads
+    for (uint32_t type_id = 0; type_id < TYPE_ID_LIMIT; ++type_id) {
+        test(type_id);
     }
-
-
-//    EXPECT_EQ(raw_codes.size(), 29334498);
 }