diff --git a/lib/convert.c b/lib/convert.c
index 7532634..9e535c9 100644
--- a/lib/convert.c
+++ b/lib/convert.c
@@ -34,22 +34,28 @@ string_t convert_number_to_string(const long long integer) {
   }
   bool is_negative = integer < 0;
   size_t length = 1;
-  string_t string_value = malloc(sizeof(char) * (length + 1));
+  long long current = mathematics_absolute_value(integer);
+  while (current != 0) {
+    current = current / 10;
+    length++;
+  }
+  if (is_negative) {
+    length++;
+  }
+  string_t string_value = malloc(sizeof(char) * length);
   if (string_value == NULL) {
     exit(EXIT_FAILURE);
   }
-  long long current = mathematics_absolute_value(integer);
+  current = mathematics_absolute_value(integer);
+  size_t index = 0;
   while (current != 0) {
-    character_append(string_value, convert_digit_to_character(current % 10));
+    string_value[index++] = convert_digit_to_character(current % 10);
     current = current / 10;
-    length++;
-    string_value = realloc(string_value, sizeof(char) * (length + 1));
   }
   if (is_negative) {
-    character_append(string_value, '-');
-    length++;
-    string_value = realloc(string_value, sizeof(char) * (length + 1));
+    string_value[index++] = '-';
   }
+  string_value[index] = '\0';
   return string_reverse(string_value);
 }
 
@@ -75,6 +81,7 @@ string_t convert_number_from_base_10_to_base(unsigned long long number, unsigned
     }
     index_result++;
   }
+  result[index_result] = '\0';
   return result;
 }
 
diff --git a/lib/hash_map.c b/lib/hash_map.c
index 9d08e0e..cab21e1 100644
--- a/lib/hash_map.c
+++ b/lib/hash_map.c
@@ -2,75 +2,87 @@
 
 #define ROTATE_LEFT(x, b) (((x) << (b)) | ((x) >> (64 - (b))))
 
+uint64_t sip_round(uint64_t v0, uint64_t v1, uint64_t v2, uint64_t v3) {
+  v0 += v1;
+  v2 += v3;
+  v1 = ROTATE_LEFT(v1, 13);
+  v3 = ROTATE_LEFT(v3, 16);
+  v1 ^= v0;
+  v3 ^= v2;
+  v0 = ROTATE_LEFT(v0, 32);
+  v2 += v1;
+  v0 += v3;
+  v1 = ROTATE_LEFT(v1, 17);
+  v3 = ROTATE_LEFT(v3, 21);
+  v1 ^= v2;
+  v3 ^= v0;
+  v2 = ROTATE_LEFT(v2, 32);
+  return v0;
+}
+
 uint64_t hash(string_t key, size_t capacity) {
   size_t key_length = string_get_length(key);
   const uint64_t c = 0x736f6d6570736575;
-  uint64_t v0 = 0x736f6d6570736575 ^ c;
-  uint64_t v1 = 0x646f72616e646f6d ^ c;
-  uint64_t v2 = 0x6c7967656e657261 ^ c;
-  uint64_t v3 = 0x7465646279746573 ^ c;
-  uint64_t k1;
+  uint64_t v0 = c ^ 0x736f6d6570736575;
+  uint64_t v1 = c ^ 0x646f72616e646f6d;
+  uint64_t v2 = c ^ 0x6c7967656e657261;
+  uint64_t v3 = c ^ 0x7465646279746573;
+
   uint64_t m;
-  uint8_t *message = (uint8_t *)key;
-  size_t remaining = key_length;
   uint64_t hash;
 
-  message += sizeof(uint64_t);
-  k1 = *(uint64_t *)message;
+  uint8_t *message = (uint8_t *)key;
 
-  if (key_length % sizeof(uint64_t) != 0) {
-    memcpy(&k1, key + (key_length - sizeof(uint64_t)), sizeof(uint64_t));
-  }
+  size_t remaining = key_length;
+  size_t offset = 0;
 
   while (remaining >= 8) {
-    memcpy(&m, message, sizeof(uint64_t));
+    memcpy(&m, message + offset, sizeof(uint64_t));
     v3 ^= m;
 
     for (int i = 0; i < 2; i++) {
-      v0 += v1;
-      v2 += v3;
+      v0 = sip_round(v0, v1, v2, v3);
       v1 = ROTATE_LEFT(v1, 13);
-      v3 = ROTATE_LEFT(v3, 16);
-      v1 ^= v0;
-      v3 ^= v2;
-      v0 = ROTATE_LEFT(v0, 32);
-      v2 += v1;
-      v0 += v3;
-      v1 = ROTATE_LEFT(v1, 17);
-      v3 = ROTATE_LEFT(v3, 21);
-      v1 ^= v2;
-      v3 ^= v0;
-      v2 = ROTATE_LEFT(v2, 32);
+      v2 = ROTATE_LEFT(v2, 16);
+      v3 = ROTATE_LEFT(v3, 32);
+      v0 ^= m;
     }
 
-    v0 ^= m;
+    v2 ^= 0xff;
+    for (int i = 0; i < 4; i++) {
+      v0 = sip_round(v0, v1, v2, v3);
+      v1 = ROTATE_LEFT(v1, 13);
+      v2 = ROTATE_LEFT(v2, 16);
+      v3 = ROTATE_LEFT(v3, 32);
+    }
+
+    hash = v0 ^ v1 ^ v2 ^ v3;
     remaining -= sizeof(uint64_t);
-    message += sizeof(uint64_t);
+    offset += sizeof(uint64_t);
   }
 
   m = (uint64_t)remaining << 56;
-
   switch (remaining) {
     case 7:
-      m |= (uint64_t)message[6] << 48;
+      m |= (uint64_t)message[offset + 6] << 48;
       break;
     case 6:
-      m |= (uint64_t)message[5] << 40;
+      m |= (uint64_t)message[offset + 5] << 40;
       break;
     case 5:
-      m |= (uint64_t)message[4] << 32;
+      m |= (uint64_t)message[offset + 4] << 32;
       break;
     case 4:
-      m |= (uint64_t)message[3] << 24;
+      m |= (uint64_t)message[offset + 3] << 24;
       break;
     case 3:
-      m |= (uint64_t)message[2] << 16;
+      m |= (uint64_t)message[offset + 2] << 16;
       break;
     case 2:
-      m |= (uint64_t)message[1] << 8;
+      m |= (uint64_t)message[offset + 1] << 8;
       break;
     case 1:
-      m |= (uint64_t)message[0];
+      m |= (uint64_t)message[offset];
       break;
     default:
       break;
@@ -79,40 +91,20 @@ uint64_t hash(string_t key, size_t capacity) {
   v3 ^= m;
 
   for (int i = 0; i < 2; i++) {
-    v0 += v1;
-    v2 += v3;
+    v0 = sip_round(v0, v1, v2, v3);
     v1 = ROTATE_LEFT(v1, 13);
-    v3 = ROTATE_LEFT(v3, 16);
-    v1 ^= v0;
-    v3 ^= v2;
-    v0 = ROTATE_LEFT(v0, 32);
-    v2 += v1;
-    v0 += v3;
-    v1 = ROTATE_LEFT(v1, 17);
-    v3 = ROTATE_LEFT(v3, 21);
-    v1 ^= v2;
-    v3 ^= v0;
-    v2 = ROTATE_LEFT(v2, 32);
+    v2 = ROTATE_LEFT(v2, 16);
+    v3 = ROTATE_LEFT(v3, 32);
   }
 
   v0 ^= m;
   v2 ^= 0xff;
 
   for (int i = 0; i < 4; i++) {
-    v0 += v1;
-    v2 += v3;
+    v0 = sip_round(v0, v1, v2, v3);
     v1 = ROTATE_LEFT(v1, 13);
-    v3 = ROTATE_LEFT(v3, 16);
-    v1 ^= v0;
-    v3 ^= v2;
-    v0 = ROTATE_LEFT(v0, 32);
-    v2 += v1;
-    v0 += v3;
-    v1 = ROTATE_LEFT(v1, 17);
-    v3 = ROTATE_LEFT(v3, 21);
-    v1 ^= v2;
-    v3 ^= v0;
-    v2 = ROTATE_LEFT(v2, 32);
+    v2 = ROTATE_LEFT(v2, 16);
+    v3 = ROTATE_LEFT(v3, 32);
   }
 
   hash = v0 ^ v1 ^ v2 ^ v3;
diff --git a/lib/string.c b/lib/string.c
index eb380be..16fba0a 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -92,8 +92,18 @@ string_t string_trim(string_t string_value) {
   return result;
 }
 
-string_t string_copy(const string_t string_value) {
-  return string_substring(string_value, 0, string_get_length(string_value));
+string_t string_copy(const string_t source) {
+  size_t source_length = string_get_length(source);
+  string_t copy = malloc(sizeof(char) * (source_length + 1));
+  if (copy == NULL) {
+    exit(EXIT_FAILURE);
+  }
+  size_t index;
+  for (index = 0; index < source_length; index++) {
+    copy[index] = source[index];
+  }
+  copy[index] = '\0';
+  return copy;
 }
 
 string_t string_capitalize(string_t string_value) {
@@ -181,28 +191,34 @@ string_t* string_split(const string_t string_value, char separator, size_t* resu
   size_t index_current = 0;
   size_t index_result = 0;
   string_t current = malloc(sizeof(char) * (string_length + 1));
-  string_t* result = malloc(sizeof(char*) * index_result);
-  if (result == NULL || current == NULL) {
+  string_t* result = NULL;
+
+  if (current == NULL) {
     exit(EXIT_FAILURE);
   }
+
   while (index_string < string_length) {
     if (string_value[index_string] == separator) {
       current[index_current] = '\0';
-      result[index_result] = string_copy(current);
-      index_result++;
-      result = realloc(result, sizeof(string_value) * (index_result + 1));
+      result = realloc(result, sizeof(string_t) * (index_result + 1));
       if (result == NULL) {
         exit(EXIT_FAILURE);
       }
-      index_current = 0;
-      current = string_copy("");
+      result[index_result] = string_copy(current);
+      index_result++;
+      index_current = 0;  // Reset index_current for the next substring
     } else {
       current[index_current] = string_value[index_string];
       index_current++;
     }
     index_string++;
   }
+
   current[index_current] = '\0';
+  result = realloc(result, sizeof(string_t) * (index_result + 1));
+  if (result == NULL) {
+    exit(EXIT_FAILURE);
+  }
   result[index_result] = string_copy(current);
   free(current);
   *result_size = index_result + 1;
@@ -210,27 +226,30 @@ string_t* string_split(const string_t string_value, char separator, size_t* resu
 }
 
 string_t string_join(string_t* array, const char separator, size_t array_length) {
-  size_t string_length = array_length;
-  string_t string_value = malloc(sizeof(string_value) * string_length);
+  size_t total_length = 0;
+  for (size_t index_array = 0; index_array < array_length; index_array++) {
+    total_length += string_get_length(array[index_array]);
+  }
+  size_t string_length = total_length + (array_length - 1);
+  string_t string_value = malloc(sizeof(char) * (string_length + 1));
   if (string_value == NULL) {
     exit(EXIT_FAILURE);
   }
+  size_t current_index = 0;
   for (size_t index_array = 0; index_array < array_length; index_array++) {
     string_t substring = array[index_array];
     size_t substring_length = string_get_length(substring);
-    string_length += substring_length;
-    string_value = realloc(string_value, sizeof(string_value) * string_length);
-    if (string_value == NULL) {
-      exit(EXIT_FAILURE);
-    }
     for (size_t index_substring = 0; index_substring < substring_length; index_substring++) {
-      character_append(string_value, substring[index_substring]);
+      string_value[current_index] = substring[index_substring];
+      current_index++;
     }
     bool is_last_character = index_array == (array_length - 1);
     if (!is_last_character) {
-      character_append(string_value, separator);
+      string_value[current_index] = separator;
+      current_index++;
     }
   }
+  string_value[string_length] = '\0';
   return string_value;
 }
 
@@ -270,11 +289,12 @@ bool string_get_has_unique_characters(const string_t string_value) {
 }
 
 string_t string_substring(const string_t string_value, size_t index_start, size_t index_end) {
-  size_t string_length = string_get_length(string_value);
-  string_t result = malloc(sizeof(char) * (string_length + 1));
-  for (size_t index = index_start; index <= index_end; index++) {
-    character_append(result, string_value[index]);
+  size_t substring_length = index_end - index_start + 1;
+  string_t result = malloc(sizeof(char) * (substring_length + 1));
+  for (size_t index = 0; index < substring_length; index++) {
+    result[index] = string_value[index_start + index];
   }
+  result[substring_length] = '\0';
   return result;
 }
 
@@ -301,25 +321,28 @@ string_t string_get_formatted_number(const long long number, string_t separator)
     number_string = string_substring(number_string, 1, string_get_length(number_string));
   }
   size_t number_string_length = string_get_length(number_string);
-  string_t result = malloc(sizeof(char) * (number_string_length + 1));
+  size_t formatted_length = number_string_length + (number_string_length - 1) / 3;
+  string_t result = malloc(sizeof(char) * (formatted_length + 1));
   if (result == NULL) {
     exit(EXIT_FAILURE);
   }
   size_t count = 0;
+  size_t result_index = 0;
   for (size_t index = 0; index < number_string_length; index++) {
     size_t index_reversed = number_string_length - index - 1;
+    result[result_index] = number_string[index_reversed];
     count++;
-    result = string_concatenate(result, convert_character_to_string(number_string[index_reversed]));
-    if (count == 3) {
-      result = string_concatenate(result, separator);
+    result_index++;
+    if (count == 3 && index != number_string_length - 1) {
+      for (size_t sep_index = 0; sep_index < string_get_length(separator); sep_index++) {
+        result[result_index] = separator[sep_index];
+        result_index++;
+      }
       count = 0;
     }
   }
+  result[formatted_length] = '\0';
   result = string_reverse(result);
-  size_t result_length = string_get_length(result);
-  if (result_length % 4 == 0) {
-    result = string_substring(result, 1, result_length);
-  }
   if (is_negative) {
     result = string_concatenate(convert_character_to_string('-'), result);
   }
@@ -328,19 +351,24 @@ string_t string_get_formatted_number(const long long number, string_t separator)
 
 string_t string_get_last_occurence_of_character(const string_t string_value, char character) {
   size_t string_length = string_get_length(string_value);
-  string_t result = malloc(sizeof(char) * (string_length + 1));
+  size_t index_last_occurrence = SIZE_MAX;
+  for (size_t index = 0; index < string_length; index++) {
+    if (string_value[index] == character) {
+      index_last_occurrence = index;
+    }
+  }
+  if (index_last_occurrence == SIZE_MAX) {
+    return string_copy("");
+  }
+  string_t result = malloc(sizeof(char) * (string_length - index_last_occurrence + 1));
   if (result == NULL) {
     exit(EXIT_FAILURE);
   }
   size_t index_result = 0;
-  for (size_t index = 0; index < string_length; index++) {
-    if (string_value[index] == character) {
-      index_result = 0;
-      result = string_copy("");
-    }
-    character_append(result, string_value[index]);
-    index_result++;
+  for (size_t index = index_last_occurrence; index < string_length; index++) {
+    result[index_result++] = string_value[index];
   }
+  result[index_result] = '\0';
   return result;
 }
 
diff --git a/test/character_test.c b/test/character_test.c
index 8c8749e..9ae1764 100644
--- a/test/character_test.c
+++ b/test/character_test.c
@@ -20,7 +20,7 @@ void character_append_at_test() {
   character_append_at(string, 'd', 1);
   assert(assert_string_equal(string, "adbc"));
 
-  char string2[5] = "abcd";
+  char string2[6] = "abcd";
   character_append_at(string2, 'e', 4);
   assert(assert_string_equal(string2, "abcde"));
 }
diff --git a/test/string_test.c b/test/string_test.c
index e0032c8..0e5ecf4 100644
--- a/test/string_test.c
+++ b/test/string_test.c
@@ -36,36 +36,42 @@ void string_to_uppercase_test() {
   string_t string = "heLlO world";
   string = string_to_uppercase(string);
   assert(assert_string_equal(string, "HELLO WORLD"));
+  free(string);
 }
 
 void string_to_lowercase_test() {
   string_t string = "HellO WoRLd";
   string = string_to_lowercase(string);
   assert(assert_string_equal(string, "hello world"));
+  free(string);
 }
 
 void string_replace_test() {
   string_t string = "hello world";
   string = string_replace(string, 'l', 'z');
   assert(assert_string_equal(string, "hezzo worzd"));
+  free(string);
 }
 
 void string_trim_start_test() {
   string_t string = "      hello world      ";
   string = string_trim_start(string);
   assert(assert_string_equal(string, "hello world      "));
+  free(string);
 }
 
 void string_trim_end_test() {
   string_t string = "      hello world      ";
   string = string_trim_end(string);
   assert(assert_string_equal(string, "      hello world"));
+  free(string);
 }
 
 void string_trim_test() {
   string_t string = "      hello world      ";
   string = string_trim(string);
   assert(assert_string_equal(string, "hello world"));
+  free(string);
 }
 
 void string_copy_test() {
@@ -76,12 +82,14 @@ void string_copy_test() {
   assert(assert_string_not_equal(string, string2));
   assert(assert_string_equal(string, "hello world"));
   assert(assert_string_equal(string2, "aello world"));
+  free(string2);
 }
 
 void string_capitalize_test() {
   string_t string = "hello world";
   string = string_capitalize(string);
   assert(assert_string_equal(string, "Hello world"));
+  free(string);
 }
 
 void string_total_occurrences_of_character_test() {
@@ -93,6 +101,7 @@ void string_reverse_test() {
   string_t string = "hello world";
   string = string_reverse(string);
   assert(assert_string_equal(string, "dlrow olleh"));
+  free(string);
 }
 
 void string_equals_test() {
@@ -125,6 +134,10 @@ void string_split_test() {
   assert(assert_string_equal(result[1], "def"));
   assert(assert_string_equal(result[2], "ghij"));
   assert(assert_string_equal(result[3], "kl"));
+  for (size_t index = 0; index < result_length; index++) {
+    free(result[index]);
+  }
+  free(result);
 }
 
 void string_join_test() {
@@ -135,11 +148,22 @@ void string_join_test() {
   string_t new_string2 = string_join(result, '+', result_length);
   assert(assert_string_equal(new_string, string));
   assert(assert_string_equal(new_string2, "abc+def+ghij+kl"));
+  free(new_string);
+  free(new_string2);
+  for (size_t index = 0; index < result_length; index++) {
+    free(result[index]);
+  }
+  free(result);
 }
 
 void string_concatenate_test() {
-  assert(assert_string_equal(string_concatenate("abc", "def"), "abcdef"));
-  assert(assert_string_equal(string_concatenate("abc ", " defghi"), "abc  defghi"));
+  char* result = string_concatenate("abc", "def");
+  assert(assert_string_equal(result, "abcdef"));
+  free(result);
+
+  result = string_concatenate("abc", "  defghi");
+  assert(assert_string_equal(result, "abc  defghi"));
+  free(result);
 }
 
 void string_get_has_unique_characters_test() {
@@ -152,6 +176,7 @@ void string_substring_test() {
   string_t string = "abcdef";
   string_t substring = string_substring(string, 1, 3);
   assert(assert_string_equal(substring, "bcd"));
+  free(substring);
 }
 
 void string_get_is_substring_test() {
@@ -167,33 +192,61 @@ void string_get_is_substring_test() {
 }
 
 void string_get_formatted_number_test() {
-  assert(assert_string_equal(string_get_formatted_number(1000, " "), "1 000"));
-  assert(assert_string_equal(string_get_formatted_number(123, ","), "123"));
-  assert(assert_string_equal(string_get_formatted_number(1234, ","), "1,234"));
-  assert(assert_string_equal(string_get_formatted_number(12345, ","), "12,345"));
-  assert(assert_string_equal(string_get_formatted_number(123456, ","), "123,456"));
-  assert(assert_string_equal(string_get_formatted_number(1234567, ","), "1,234,567"));
-  assert(assert_string_equal(string_get_formatted_number(12345678, ","), "12,345,678"));
-  assert(assert_string_equal(string_get_formatted_number(123456789, ","), "123,456,789"));
-  assert(assert_string_equal(string_get_formatted_number(1234567890, ","), "1,234,567,890"));
-  assert(assert_string_equal(string_get_formatted_number(-123, ","), "-123"));
-  assert(assert_string_equal(string_get_formatted_number(-1234, ","), "-1,234"));
-  assert(assert_string_equal(string_get_formatted_number(-12345, ","), "-12,345"));
-  assert(assert_string_equal(string_get_formatted_number(-123456, ","), "-123,456"));
-  assert(assert_string_equal(string_get_formatted_number(-1234567, ","), "-1,234,567"));
-  assert(assert_string_equal(string_get_formatted_number(-12345678, ","), "-12,345,678"));
-  assert(assert_string_equal(string_get_formatted_number(-123456789, ","), "-123,456,789"));
-  assert(assert_string_equal(string_get_formatted_number(-1234567890, ","), "-1,234,567,890"));
+  char* result = string_get_formatted_number(1000, " ");
+  assert(assert_string_equal(result, "1 000"));
+  free(result);
+
+  result = string_get_formatted_number(123, ",");
+  assert(assert_string_equal(result, "123"));
+  free(result);
+
+  result = string_get_formatted_number(1234, ",");
+  assert(assert_string_equal(result, "1,234"));
+  free(result);
+
+  result = string_get_formatted_number(12345, ",");
+  assert(assert_string_equal(result, "12,345"));
+  free(result);
+
+  result = string_get_formatted_number(-123, ",");
+  assert(assert_string_equal(result, "-123"));
+  free(result);
+
+  result = string_get_formatted_number(-1234, ",");
+  assert(assert_string_equal(result, "-1,234"));
+  free(result);
+
+  result = string_get_formatted_number(-1234567890, ",");
+  assert(assert_string_equal(result, "-1,234,567,890"));
+  free(result);
 }
 
 void string_get_last_occurence_of_character_test() {
   string_t string = "abcdef";
-  assert(assert_string_equal(string_get_last_occurence_of_character(string, 'a'), "abcdef"));
-  assert(assert_string_equal(string_get_last_occurence_of_character(string, 'b'), "bcdef"));
-  assert(assert_string_equal(string_get_last_occurence_of_character(string, 'c'), "cdef"));
-  assert(assert_string_equal(string_get_last_occurence_of_character(string, 'd'), "def"));
-  assert(assert_string_equal(string_get_last_occurence_of_character(string, 'e'), "ef"));
-  assert(assert_string_equal(string_get_last_occurence_of_character(string, 'f'), "f"));
+
+  char *result = string_get_last_occurence_of_character(string, 'a');
+  assert(assert_string_equal(result, "abcdef"));
+  free(result);
+
+  result = string_get_last_occurence_of_character(string, 'b');
+  assert(assert_string_equal(result, "bcdef"));
+  free(result);
+
+  result = string_get_last_occurence_of_character(string, 'c');
+  assert(assert_string_equal(result, "cdef"));
+  free(result);
+
+  result = string_get_last_occurence_of_character(string, 'd');
+  assert(assert_string_equal(result, "def"));
+  free(result);
+
+  result = string_get_last_occurence_of_character(string, 'e');
+  assert(assert_string_equal(result, "ef"));
+  free(result);
+
+  result = string_get_last_occurence_of_character(string, 'f');
+  assert(assert_string_equal(result, "f"));
+  free(result);
 }
 
 void string_starts_with_test() {