Fix Array#[] with ArithmeticSequence with negative steps (#5739)

* Fix Array#[] with ArithmeticSequence with negative steps

Previously, Array#[] when called with an ArithmeticSequence
with a negative step did not handle all cases correctly,
especially cases involving infinite ranges, inverted ranges,
and/or exclusive ends.

Fixes [Bug #18247]

* Add Array#slice tests for ArithmeticSequence with negative step to test_array

Add tests of rb_arithmetic_sequence_beg_len_step C-API function.

* Fix ext/-test-/arith_seq/beg_len_step/depend

* Rename local variables

* Fix a variable name

Co-authored-by: Kenta Murata <3959+mrkn@users.noreply.github.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/array.c b/array.c
index b2ebf3c0e9..793a53f17b 100644
--- a/array.c
+++ b/array.c
@@ -1373,13 +1373,16 @@ ary_make_partial_step(VALUE ary, VALUE klass, long offset, long len, long step)
     const VALUE *values = RARRAY_CONST_PTR_TRANSIENT(ary);
     const long orig_len = len;
 
-    if ((step > 0 && step >= len) || (step < 0 && (step < -len))) {
+    if (step > 0 && step >= len) {
         VALUE result = ary_new(klass, 1);
         VALUE *ptr = (VALUE *)ARY_EMBED_PTR(result);
         RB_OBJ_WRITE(result, ptr, values[offset]);
         ARY_SET_EMBED_LEN(result, 1);
         return result;
     }
+    else if (step < 0 && step < -len) {
+        step = -len;
+    }
 
     long ustep = (step < 0) ? -step : step;
     len = (len + ustep - 1) / ustep;