1 files changed, 65 insertions, 0 deletions
diff --git a/py/objslice.c b/py/objslice.c
index 5a15be55a..cbbee326e 100644
--- a/py/objslice.c
+++ b/py/objslice.c
@@ -152,6 +152,71 @@ mp_obj_t mp_obj_new_slice(mp_obj_t ostart, mp_obj_t ostop, mp_obj_t ostep) {
     return MP_OBJ_FROM_PTR(o);
 }
 
+// Return the real index and step values for a slice when applied to a sequence of
+// the given length, resolving missing components, negative values and values off
+// the end of the sequence.
+void mp_obj_slice_indices(mp_obj_t self_in, mp_int_t length, mp_bound_slice_t *result) {
+    mp_obj_slice_t *self = MP_OBJ_TO_PTR(self_in);
+    mp_int_t start, stop, step;
+
+    if (self->step == mp_const_none) {
+        step = 1;
+    } else {
+        step = mp_obj_get_int(self->step);
+        if (step == 0) {
+            mp_raise_ValueError(translate("slice step cannot be zero"));
+        }
+    }
+
+    if (step > 0) {
+        // Positive step
+        if (self->start == mp_const_none) {
+            start = 0;
+        } else {
+            start = mp_obj_get_int(self->start);
+            if (start < 0) {
+                start += length;
+            }
+            start = MIN(length, MAX(start, 0));
+        }
+
+        if (self->stop == mp_const_none) {
+            stop = length;
+        } else {
+            stop = mp_obj_get_int(self->stop);
+            if (stop < 0) {
+                stop += length;
+            }
+            stop = MIN(length, MAX(stop, 0));
+        }
+    } else {
+        // Negative step
+        if (self->start == mp_const_none) {
+            start = length - 1;
+        } else {
+            start = mp_obj_get_int(self->start);
+            if (start < 0) {
+                start += length;
+            }
+            start = MIN(length - 1, MAX(start, -1));
+        }
+
+        if (self->stop == mp_const_none) {
+            stop = -1;
+        } else {
+            stop = mp_obj_get_int(self->stop);
+            if (stop < 0) {
+                stop += length;
+            }
+            stop = MIN(length - 1, MAX(stop, -1));
+        }
+    }
+
+    result->start = start;
+    result->stop = stop;
+    result->step = step;
+}
+
 #if MICROPY_PY_BUILTINS_SLICE_ATTRS
 STATIC mp_obj_t slice_make_new(const mp_obj_type_t *type,
         size_t n_args, const mp_obj_t *args, mp_map_t *kw_args) {