Factor out m_seq_get_fast_slice_indexes() fucntions as sequence helper.

Takes slice object and sequence length and computes subsequence indexes
for case of slice step=1.

1 files changed, 30 insertions, 0 deletions

diff --git a/py/sequence.c b/py/sequence.c
index 56718c6f8..1e851a9f8 100644
--- a/py/sequence.c
+++ b/py/sequence.c
@@ -23,3 +23,33 @@ void mp_seq_multiply(const void *items, uint item_sz, uint len, uint times, void
         dest = (char*)dest + copy_sz;
     }
 }
+
+bool m_seq_get_fast_slice_indexes(machine_uint_t len, mp_obj_t slice, machine_uint_t *begin, machine_uint_t *end) {
+    machine_int_t start, stop, step;
+    mp_obj_slice_get(slice, &start, &stop, &step);
+    if (step != 1) {
+        return false;
+    }
+
+    // Unlike subscription, out-of-bounds slice indexes are never error
+    if (start < 0) {
+        start = len + start;
+        if (start < 0) {
+            start = 0;
+        }
+    } else if (start > len) {
+        start = len;
+    }
+    if (stop <= 0) {
+        stop = len + stop;
+        // CPython returns empty sequence in such case
+        if (stop < 0) {
+            stop = start;
+        }
+    } else if (stop > len) {
+        stop = len;
+    }
+    *begin = start;
+    *end = stop;
+    return true;
+}