diff --git a/SLS_C/src/string.c b/SLS_C/src/string.c
index cb8d69d..61138ce 100644
--- a/SLS_C/src/string.c
+++ b/SLS_C/src/string.c
@@ -27,8 +27,8 @@ size_t strnlen(const char *s, size_t maxlen) {
 
 SlsStr malloc_str(const char *s, size_t maxlen) {
     size_t length = strnlen(s, maxlen);
-    char *new_str = (char *)malloc(sizeof(char) * length);
-    strncpy(new_str, s, length);
+    char *new_str = (char *)malloc(sizeof(char) * (length + 1));
+    strncpy(new_str, s, length + 1);
     return (SlsStr){length, new_str, TRUE};
 }
 
@@ -92,6 +92,16 @@ typedef struct {
     size_t self_length;
 } FormatStringItem;
 
+static size_t number_length(int64_t i) {
+    if (i == 0) return 1;
+    else return (i < 0 ? 1 : 0) + (int)log10(llabs(i) + 1);
+}
+
+static size_t unsigned_number_length(uint64_t i) {
+    if (i == 0) return 1;
+    else return (int)log10(i + 1);
+}
+
 SlsStr format(const SlsStr s, ...) {
     va_list args;
     va_start(args, s);
@@ -120,7 +130,7 @@ SlsStr format(const SlsStr s, ...) {
 
     FormatStringItem *items = (FormatStringItem *)malloc(sizeof(FormatStringItem) * count);
     size_t i = 0;
-    size_t last_index = 0;
+    size_t last_index = (size_t)s.str;
     current = strchr(s.str, '%');
     do {
         switch (current[1]) {
@@ -177,8 +187,8 @@ SlsStr format(const SlsStr s, ...) {
             items[i].boolean = va_arg(args, Boolean);
             break;
         }
-        items[i].str_index = (size_t)(current - last_index);
-        last_index = (size_t)(current + 2);
+        items[i].str_index = (size_t)current - last_index;
+        last_index = (size_t)current + 2;
         i++;
         current = strchr(current + 2, '%');
     } while (current);
@@ -193,19 +203,19 @@ SlsStr format(const SlsStr s, ...) {
             length += items[i].self_length = 1;
             break;
         case FORMAT_INTEGER_32:
-            length += items[i].self_length = ceil(log10(items[i].integer_32 + 1));
+            length += items[i].self_length = number_length(items[i].integer_32);
             break;
         case FORMAT_INTEGER_64:
-            length += items[i].self_length = ceil(log10(items[i].integer_64 + 1));
+            length += items[i].self_length = number_length(items[i].integer_64);
             break;
         case FORMAT_UNSIGNED_INTEGER_64:
-            length += items[i].self_length = ceil(log10(items[i].unsigned_integer_64 + 1));
+            length += items[i].self_length = unsigned_number_length(items[i].unsigned_integer_64);
             break;
         case FORMAT_SIZE_INTEGER:
-            length += items[i].self_length = ceil(log10(items[i].size_integer + 1));
+            length += items[i].self_length = unsigned_number_length(items[i].size_integer);
             break;
         case FORMAT_FLOAT:
-            length += items[i].self_length = ceil(log10(items[i].ffloat + 1) + 3);
+            length += items[i].self_length = number_length(items[i].ffloat) + 3;
             break;
         case FORMAT_SLS_STR:
             length += items[i].self_length = items[i].sls_str.len - 1;
@@ -240,7 +250,7 @@ SlsStr format(const SlsStr s, ...) {
         target_i += items[item_i].str_index;
         source_i += items[item_i].str_index + 2;
 
-        switch (items[i].type) {
+        switch (items[item_i].type) {
         case FORMAT_C_STRINGS:
             snprintf(temp, items[item_i].self_length + 1, "%s", items[item_i].c_string);
             break;