diff --git a/strings/z_function.cpp b/strings/z_function.cpp
index 6056dd5ae..8493e8a9c 100644
--- a/strings/z_function.cpp
+++ b/strings/z_function.cpp
@@ -26,11 +26,11 @@
  * \param[in] pattern text on which to apply the Z-function
  * \returns the Z-function output as a vector array
  */
-std::vector<int> Z_function(const std::string &pattern) {
-    int pattern_length = pattern.size();
-    std::vector<int> z(pattern_length, 0);
+std::vector<uint64_t> Z_function(const std::string &pattern) {
+    uint64_t pattern_length = pattern.size();
+    std::vector<uint64_t> z(pattern_length, 0);
 
-    for (int i = 1, l = 0, r = 0; i < pattern_length; i++) {
+    for (uint64_t i = 1, l = 0, r = 0; i < pattern_length; i++) {
         if (i <= r)
             z[i] = std::min(r - i + 1, z[i - l]);
         while (i + z[i] < pattern_length && pattern[z[i]] == pattern[i + z[i]])
@@ -47,13 +47,13 @@ std::vector<int> Z_function(const std::string &pattern) {
  * \param[in] text text in which to search
  * \returns a vector of starting indexes where pattern is found in the text
  */
-std::vector<int> find_pat_in_text(const std::string &pattern,
+std::vector<uint64_t> find_pat_in_text(const std::string &pattern,
                                   const std::string &text) {
-    int text_length = text.size(), pattern_length = pattern.size();
-    std::vector<int> z = Z_function(pattern + '#' + text);
-    std::vector<int> matching_indexes;
+    uint64_t text_length = text.size(), pattern_length = pattern.size();
+    std::vector<uint64_t> z = Z_function(pattern + '#' + text);
+    std::vector<uint64_t> matching_indexes;
 
-    for (int i = 0; i < text_length; i++) {
+    for (uint64_t i = 0; i < text_length; i++) {
         if (z[i + pattern_length + 1] == pattern_length)
             matching_indexes.push_back(i);
     }
@@ -69,15 +69,15 @@ static void test() {
     std::string text1 = "alskfjaldsabc1abc1abcbksbcdnsdabcabc";
     std::string pattern1 = "abc";
 
-    std::vector<int> matching_indexes1 = find_pat_in_text(pattern1, text1);
-    assert((matching_indexes1 == std::vector<int>{10, 14, 18, 30, 33}));
+    std::vector<uint64_t> matching_indexes1 = find_pat_in_text(pattern1, text1);
+    assert((matching_indexes1 == std::vector<uint64_t>{10, 14, 18, 30, 33}));
 
     // corner case
     std::string text2 = "greengrass";
     std::string pattern2 = "abc";
 
-    std::vector<int> matching_indexes2 = find_pat_in_text(pattern2, text2);
-    assert((matching_indexes2 == std::vector<int>{}));
+    std::vector<uint64_t> matching_indexes2 = find_pat_in_text(pattern2, text2);
+    assert((matching_indexes2 == std::vector<uint64_t>{}));
 }
 
 /**