add string functions, cleanup vector docs

src/string.c

@@ -0,0 +1,51 @@
+#include <string.h>
+#include <stdlib.h>
+#include <stdio.h>
+
+#include "lfstring.h"
+
+size_t *find_substrings(const char* haystack, const char* needle, size_t *num_substrings) {
+    size_t sz_h = strlen(haystack);
+    size_t sz_n = strlen(needle);
+
+    *num_substrings = 0;
+    for (size_t i = 0; i <= sz_h - sz_n; ++i) {
+        if (strncmp(haystack + i, needle, sz_n) == 0) {
+            ++(*num_substrings);
+        }
+    }
+
+    size_t *indicies = malloc(sizeof(size_t) * *num_substrings);
+    if (indicies == NULL) {
+        fprintf(stderr, "Memory allocation failed in find_substrings\n");
+        return NULL;
+    }
+
+    size_t idx = 0;
+    for (size_t i = 0; i <= sz_h - sz_n; ++i) {
+        if (strncmp(haystack + i, needle, sz_n) == 0) {
+            indicies[idx++] = i;
+        }
+    }
+
+    return indicies;
+}
+
+const char* substr(const char* str, size_t idx, size_t len) {
+   size_t sz_str = strlen(str);
+   if (sz_str < len || idx + len > sz_str) {
+       fprintf(stderr, "Improper size arguments in substr\n");
+       return NULL;
+   }
+
+   char *substr = malloc(sizeof(char) * len + 1);
+   if (substr == NULL) {
+       fprintf(stderr, "Memory allocation error in substr\n");
+       return NULL;
+   }
+
+   memcpy(substr, str + idx, len);
+   substr[len] = '\0';
+
+   return substr;
+}