1 files changed, 13 insertions, 9 deletions

diff --git a/src/std/str.c b/src/std/str.c
index 1e092bc..7dafef1 100644
--- a/src/std/str.c
+++ b/src/std/str.c
@@ -1,6 +1,6 @@
 #include "str.h"
 #include <string.h>
-
+#include <assert.h>
 #include "mem.h"
 
 str8 str8_create(u8* buf, size_t len) { return (str8){ .buf = buf, .len = len }; }
@@ -43,25 +43,29 @@ str8 str8_concat(arena* a, str8 left, str8 right) {
   return str8_create(dest, n_bytes);
 }
 
-str8 str8_substr(str8 s, u64 min, u64 max){
-  s.buf = s.buf + (ptrdiff_t)min;
-  s.len = max - min;
-  return s;
+str8 str8_substr(str8 s, u64 min, u64 max) {
+  assert(min >= 0);
+  assert(min < s.len);
+  assert(max >= 0);
+  assert(max <= s.len);
+  uint8_t * start = s.buf + (ptrdiff_t)min;
+  size_t new_len = max - min;
+  return (str8) {.buf = start, .len = new_len };
 }
 
-str8 str8_take(str8 s, u64 first_n){
+str8 str8_take(str8 s, u64 first_n) {
   return str8_substr(s, 0, first_n);
 }
 
-str8 str8_drop(str8 s, u64 last_n){
+str8 str8_drop(str8 s, u64 last_n) {
   return str8_substr(s, s.len - last_n, s.len);
 }
 
-str8 str8_skip(str8 s, u64 n){
+str8 str8_skip(str8 s, u64 n) {
   return str8_substr(s, n, s.len);
 }
 
-str8 str8_chop(str8 s, u64 n){
+str8 str8_chop(str8 s, u64 n) {
   return str8_substr(s, 0, s.len - n);
 }