py/sequence: Fix boundary errors when slicing with a negative step.

6 years ago · eb4c37f7a4
parent d007351b33
commit eb4c37f7a4
1 changed files with 9 additions and 2 deletions
--- a/py/sequence.c
+++ b/py/sequence.c
@ -88,15 +88,22 @@ bool mp_seq_get_fast_slice_indexes(mp_uint_t len, mp_obj_t slice, mp_bound_slice
    if (start < 0) {
        start = len + start;
        if (start < 0) {
-            start = 0;
+            if (indexes->step < 0) {
+                start = -1;
+            } else {
+                start = 0;
+            }
        }
    } else if (indexes->step > 0 && (mp_uint_t)start > len) {
        start = len;
-    } else if (indexes->step < 0 && (mp_uint_t)start > len - 1) {
+    } else if (indexes->step < 0 && (mp_uint_t)start >= len) {
        start = len - 1;
    }
    if (stop < 0) {
        stop = len + stop;
+        if (stop < 0) {
+            stop = -1;
+        }
        if (indexes->step < 0) {
            stop += 1;
        }