handle negative numbers more accurately in str-insert

src/builtin/string.rs

@@ -185,11 +185,17 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
                 );
             } else if index == Number::from(0) {
                 string.insert_str(0, &substr);
+            } else {
+                let idx = index.abs().to_integer().to_usize().unwrap();
+                if idx > len {
+                    string.insert_str(0, &substr)
                 } else {
                     string.insert_str(
-                    len - index.abs().to_integer().to_usize().unwrap().min(len),
+                        len - idx + 1,
                         &substr,
                     );
+
+                }
             }
 
             Ok(Value::Ident(string, quotes))

tests/strings.rs

@@ -150,9 +150,19 @@ test!(
     "a {\n  color: Xabcd;\n}\n"
 );
 test!(
-    str_insert_negative_idx,
+    str_insert_negative_1,
-    "a {\n  color: str-insert(abcd, \"X\", -2);\n}\n",
+    "a {\n  color: str-insert(abc, \"X\", -1);\n}\n",
-    "a {\n  color: abXcd;\n}\n"
+    "a {\n  color: abcX;\n}\n"
+);
+test!(
+    str_insert_negative_2,
+    "a {\n  color: str-insert(abc, \"X\", -2);\n}\n",
+    "a {\n  color: abXc;\n}\n"
+);
+test!(
+    str_insert_negative_3,
+    "a {\n  color: str-insert(abc, \"X\", -3);\n}\n",
+    "a {\n  color: aXbc;\n}\n"
 );
 error!(
     float_idx,