diff --git a/legacy/main-old.cc b/legacy/main-old.cc
index 14a817c..93d475c 100644
--- a/legacy/main-old.cc
+++ b/legacy/main-old.cc
@@ -2,51 +2,11 @@
 
 /*
 
-  uint64_t -> 0000 + [xxx] * 20
-
-  2x2  2x1  1x2  1x1
-  # #   #   # #  #
-  # #   #
-
-  00 01 02 03
-  04 05 06 07
-  08 09 10 11
-  12 13 14 15
-  16 17 18 19
-
   x1 (%4) => 0 1 2 3
   x3 (%4) => 0 3 2 1
 
 */
 
-/*
-======================================================
-
- 1x1 -> 011 000 000 000 -> 0000 0000 0011 -> 0x3
-
- 1x2 -> 001 111 000 000 -> 0000 0011 1001 -> 0x39
-
- 2x1 -> 010 000 000 000 -> 0000 0000 0010 -> 0x7002
-        111 000 000 000 -> 0000 0000 0111
-
- 2x2 -> 100 111 000 000 -> 0000 0011 1100 -> 0x3F03C
-        111 111 000 000 -> 0000 0011 1111
-
-======================================================
-
- 1x1 -> 111 000 000 000 -> 0000 0000 0111 -> 0x7
-
- 1x2 -> 111 111 000 000 -> 0000 0011 1111 -> 0x3F
-
- 2x1 -> 111 000 000 000 -> 0000 0000 0111 -> 0x7007
-        111 000 000 000 -> 0000 0000 0111
-
- 2x2 -> 111 111 000 000 -> 0000 0011 1111 -> 0x3F03F
-        111 111 000 000 -> 0000 0011 1111
-
-======================================================
-*/
-
 int main() {
     printf("Klotski engine\n");
 
diff --git a/src/klotski/utils/common.cc b/src/klotski/utils/common.cc
index 25f780f..6c12042 100644
--- a/src/klotski/utils/common.cc
+++ b/src/klotski/utils/common.cc
@@ -7,7 +7,7 @@ uint32_t Common::range_reverse(uint32_t bin) { // reverse binary every 2-bits
     return ((bin << 2) & 0xCCCCCCCC) | ((bin >> 2) & 0x33333333);
 }
 
-/// NOTE: don't check unknown ranges
+/// WARN: don't check unknown data -> may cause infinite loop
 uint8_t Common::check_range(uint32_t head, uint32_t range) { // check generated range
     ///   M_1x1     M_1x2     M_2x1     M_2x2
     ///  1 0 0 0   1 1 0 0   1 0 0 0   1 1 0 0
@@ -18,25 +18,24 @@ uint8_t Common::check_range(uint32_t head, uint32_t range) { // check generated
     constexpr uint32_t M_2x1 = 0b10001;
     constexpr uint32_t M_2x2 = 0b110011;
 
-    int block_offset = 1;
+    uint8_t block_offset = 1;
     uint32_t cache = M_2x2 << head; // fill 2x2 block
     for (int addr = 0; range; range >>= 2, ++block_offset) { // traverse every 2-bits
-        // TODO: will addr >= 32 in some special cases?
         while ((cache >> addr) & 0b1) {
             ++addr; // search next unfilled block
         }
         switch (range & 0b11) {
-            case 0b00: // space
-            case 0b11: // 1x1 block
+            case 0b00: /// space
+            case 0b11: /// 1x1 block
                 cache |= M_1x1 << addr; // fill space or 1x1 block
                 break;
-            case 0b10: // 2x1 block
+            case 0b10: /// 2x1 block
                 if (addr > 15 || cache >> (addr + 4) & 0b1) { // invalid address
                     return block_offset; // broken block number
                 }
                 cache |= M_2x1 << addr; // fill 2x1 block
                 break;
-            case 0b01: // 1x2 block
+            case 0b01: /// 1x2 block
                 if ((addr & 0b11) == 0b11 || cache >> (addr + 1) & 0b1) { // invalid address
                     return block_offset; // broken block number
                 }
diff --git a/src/klotski/utils/common.h b/src/klotski/utils/common.h
index 2b641a2..5b4006d 100644
--- a/src/klotski/utils/common.h
+++ b/src/klotski/utils/common.h
@@ -9,6 +9,10 @@
 ///
 ///   3. `2x2` block must have and only one, `2x1` `1x2` `1x1` are not required in number.
 ///
+///  NOTE:  2x2   2x1   1x2   1x1
+///         # #    #    # #    #
+///         # #    #
+///
 /// After statistics, there are a total of 29334498 cases that meet the above requirements.
 
 #include <cstdint>