diff --git a/1_longest_substring_with_k_distinct_characters.cpp b/1_longest_substring_with_k_distinct_characters.cpp
index c5bb10c..28c733b 100644
--- a/1_longest_substring_with_k_distinct_characters.cpp
+++ b/1_longest_substring_with_k_distinct_characters.cpp
@@ -49,12 +49,12 @@ public:
   unsigned long longest_start = 0;
   unsigned long longest_end = 0;
   bool match_found = false;
-  bool operator==(const result r);
+  bool operator==(const result r) const;
   friend std::istream &operator>>(std::istream &in, result &r);
   friend std::ostream &operator<<(std::ostream &out, const result &r);
 };
 
-bool result::operator==(const result r) {
+bool result::operator==(const result r) const {
   return longest_start == r.longest_start && longest_end == r.longest_end &&
          match_found == r.match_found;
 }
@@ -151,6 +151,20 @@ bool K_verify(const test_case &t) {
   return unique_chars.size() == t.i.k;
 }
 
+bool thorough_test(const test_case &t) {
+  unsigned long org_window_size = t.r.longest_end - t.r.longest_start + 1;
+  if (org_window_size >= t.i.s.length())
+    return true;
+  for (unsigned long i = 0, n_tests = t.i.s.length() - org_window_size;
+       i < n_tests; i++) {
+    result r = {i, i + org_window_size, true};
+    test_case mock_test = {t.i, r};
+    if (K_verify(mock_test))
+      return false;
+  }
+  return true;
+}
+
 int main(int argc, char *argv[]) {
   std::cout << "Processing static test cases from input file (if any)\n";
   semicolon_is_space delimeter;
@@ -164,41 +178,47 @@ int main(int argc, char *argv[]) {
         test(t);
       }
     }
+  }
 
-    std::cout
-        << "\n\n\nPerforming metamorphic tests from randomly generated input "
-           "(string and K values)"
-        << std::endl;
-    random_number_generator<unsigned int> random_ascii_char(97, 122); // a-z
-    random_number_generator<unsigned int> rng;
-    for (unsigned int i = 0; i < 10; i++) {
-      test_case t;
-      t.i.s += (char)random_ascii_char();
-      t.r = find(t.i.s, t.i.k);
-      std::cout << t;
-      for (unsigned int j = 0; j < 10; j++) {
-        for (unsigned int k = 0, increase_str_len_by = rng(100, 1000);
-             k < increase_str_len_by; k++) {
-          t.i.s += (char)random_ascii_char();
-          result r = find(t.i.s, t.i.k);
-          if (r.match_found && t.r.match_found)
-            assert(r.longest_end - r.longest_start >=
-                   t.r.longest_end - t.r.longest_start);
-          t.r = r;
-          std::cout << t;
-          assert(K_verify(t));
-        }
-        while (t.r.match_found) {
-          result r = find(t.i.s, ++t.i.k);
-          if (r.match_found)
-            assert(r.longest_end - r.longest_start >=
-                   t.r.longest_end - t.r.longest_start);
-          t.r = r;
-          std::cout << t;
-          assert(K_verify(t));
-        }
-        t.i.k = rng(1, --t.i.k);
+  // Thorough test (100% verifiable correct) is performaned randomly in ~1%
+  // cases
+  std::cout << "\n\n\nPerforming metamorphic tests from randomly generated "
+               "input (string and K values).\nThorough test (100\% verifiable "
+               "correct) is performaned randomly in ~1\% cases."
+            << std::endl;
+  random_number_generator<unsigned int> random_ascii_char(97, 122); // a-z
+  random_number_generator<unsigned int> rng;
+  for (unsigned int i = 0; i < 10; i++) {
+    test_case t;
+    t.i.s += (char)random_ascii_char();
+    t.r = find(t.i.s, t.i.k);
+    std::cout << t;
+    for (unsigned int j = 0; j < 10; j++) {
+      for (unsigned int k = 0, increase_str_len_by = rng(100, 1000);
+           k < increase_str_len_by; k++) {
+        t.i.s += (char)random_ascii_char();
+        result r = find(t.i.s, t.i.k);
+        if (r.match_found && t.r.match_found)
+          assert(r.longest_end - r.longest_start >=
+                 t.r.longest_end - t.r.longest_start);
+        t.r = r;
+        std::cout << t;
+        assert(K_verify(t));
+        if (!rng(0, 99))
+          assert(thorough_test(t));
+      }
+      while (t.r.match_found) {
+        result r = find(t.i.s, ++t.i.k);
+        if (r.match_found)
+          assert(r.longest_end - r.longest_start >=
+                 t.r.longest_end - t.r.longest_start);
+        t.r = r;
+        std::cout << t;
+        assert(K_verify(t));
+        if (!rng(0, 99))
+          assert(thorough_test(t));
       }
+      t.i.k = rng(1, --t.i.k);
     }
   }
   return 0;