diff --git a/src/core/ranges/internal/check.inl b/src/core/ranges/internal/check.inl
index c04beb2..f151db5 100644
--- a/src/core/ranges/internal/check.inl
+++ b/src/core/ranges/internal/check.inl
@@ -4,11 +4,12 @@ namespace klotski::cases {
 
 KLSK_INLINE_CE int Ranges::check(const int head, uint32_t range) {
     KLSK_ASSUME(head >= 0 && head < 16 && head % 4 != 3);
+    const int limit = 16 - (std::countl_zero(range) >> 1);
     uint32_t flags = 0b110011 << head; // fill 2x2 block
-    for (int addr = 0, offset = 1; range; range >>= 2, ++offset) { // traverse every 2-bit
-        const auto num = std::countr_one(flags);
-        addr += num; // next unfilled block
-        flags >>= num;
+
+    for (int offset = 0, addr = 0; offset < limit; ++offset, range >>= 2) { // traverse every 2-bit
+        addr += std::countr_one(flags); // next unfilled block
+        flags >>= std::countr_one(flags);
         switch (range & 0b11) {
         case 0b00: // space
         case 0b11: // 1x1 block
@@ -16,15 +17,18 @@ KLSK_INLINE_CE int Ranges::check(const int head, uint32_t range) {
             continue;
         case 0b01: // 1x2 block
             if (flags & 0b10 || addr % 4 == 3) { // invalid case
-                return offset; // broken offset
+                return offset + 1; // broken offset
             }
             flags |= 0b11;
             continue;
         case 0b10: // 2x1 block
             if (flags & 0b10000 || addr > 15) { // invalid case
-                return offset; // broken offset
+                return offset + 1; // broken offset
             }
             flags |= 0b10001;
+            continue;
+        default:
+            std::unreachable();
         }
     }
     return 0; // pass check