created a function for inserting a substring into a string by index

README.md

@@ -60,7 +60,7 @@ Contributions are what make the open-source community such an amazing place to l
 - [ ] size_t stringCountSubstring(TString s, TString pattern); - Count occurrences of a substring.
 - [ ] void stringCapitalize(TString *s); - Capitalize the first letter of each word.
 - [x] void stringFilter(TString *s, bool (*predicate)(char)); - Remove characters not satisfying a predicate.
-- [ ] void stringInsert(TString *s, size_t pos, TString toInsert); - Insert a substring at a specified position.
+- [x] void stringInsert(TString *s, size_t pos, TString toInsert); - Insert a substring at a specified position.
 - [ ] void stringInsertCharArr(TString *s, size_t pos, const char *toInsert); - Insert a substring from a char array.
 - [x] void stringMap(TString *s, char (*func)(char)); - Apply a function to every character of the string.
 - [x] void stringMapIndex(TString *s, char (*func)(size_t, char)); - Apply a function to every character of the string with access to its index.

cstring.h

@@ -941,4 +941,36 @@ double stringToDouble(TString s) {
     return number + decimal;
 }
 
+void stringInsert(TString *s, size_t pos, TString toInsert) {
+    size_t len_sub = stringLen(toInsert);
+    size_t len_str = stringLen(*s);
+
+    if (pos > len_str) {
+        setError(ERR_NAN);
+    }
+
+    char* new_data = malloc(len_str + len_sub + 1);
+    if(!new_data){
+        setError(ERR_ALLOCATE_SPACE);
+    }
+
+    clearError();
+
+    for(size_t i = 0; i < pos; i++){
+        new_data[i] = s->data[i];
+    }
+
+    for(size_t i = 0; i < len_sub; i++){
+        new_data[i + pos] = toInsert.data[i];
+    }
+
+    for (size_t i = pos; i < len_str; i++){
+        new_data[i + len_sub] = s->data[i];
+    }
+
+    free(s->data);
+    s->data = new_data;
+    s->size = len_str + len_sub;
+}
+
 #endif

tests/main.c

@@ -483,6 +483,18 @@ void test_stringToDouble() {
     printGreen("stringToDouble\n");
 }
 
+void test_stringInsert(){
+    TString s1 = stringInitWithCharArr("abcdef");
+    TString s2 = stringInitWithCharArr("123");
+    stringInsert(&s1, 2, s2);
+
+    TString s1expected = stringInitWithCharArr("ab123cdef");
+
+    assertEq(strncmp(s1.data, s1expected.data, stringLen(s1expected)), 0);
+
+    printGreen("stringInsert\n");
+}
+
 int main() {
     test_stringStartWith();
     test_stringEndWith();
@@ -511,5 +523,6 @@ int main() {
     test_stringIsPalindrome();
     test_stringPad();
     test_stringRemove();
+    test_stringInsert();
     return 0;
 }