From 748afc183c2c44b7b2a582d3078cf3d8b4d5270a Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Sat, 15 Aug 2020 12:32:56 -0700
Subject: [PATCH] Fix recently-introduced Fdelete bug
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Problem reported by Pip Cet in:
https://lists.gnu.org/r/emacs-devel/2020-08/msg00444.html
* src/fns.c (Fdelete): Fix correctness bug via a simpler (though more
memory-intensive) approach.  It’s probably not worth optimizing
the memory usage yere.
* test/src/fns-tests.el (test-vector-delete): Add test for the bug.
---
 src/fns.c             | 29 ++++-------------------------
 test/src/fns-tests.el |  1 +
 2 files changed, 5 insertions(+), 25 deletions(-)

diff --git a/src/fns.c b/src/fns.c
index 069edbe90e..a3b8d6ef57 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -1749,38 +1749,17 @@ DEFUN ("delete", Fdelete, Sdelete, 2, 2, 0,
     {
       ptrdiff_t n = 0;
       ptrdiff_t size = ASIZE (seq);
-      ptrdiff_t neqbits_words = ((size + BITS_PER_BITS_WORD - 1)
-				 / BITS_PER_BITS_WORD);
       USE_SAFE_ALLOCA;
-      bits_word *neqbits = SAFE_ALLOCA (neqbits_words * sizeof *neqbits);
-      bits_word neqword = 0;
+      Lisp_Object *kept = SAFE_ALLOCA (size * sizeof *kept);
 
       for (ptrdiff_t i = 0; i < size; i++)
 	{
-	  bool neq = NILP (Fequal (AREF (seq, i), elt));
-	  n += neq;
-	  neqbits[i / BITS_PER_BITS_WORD] = neqword = (neqword << 1) + neq;
+	  kept[n] = AREF (seq, i);
+	  n += NILP (Fequal (AREF (seq, i), elt));
 	}
 
       if (n != size)
-	{
-	  struct Lisp_Vector *p = allocate_vector (n);
-
-	  if (n != 0)
-	    {
-	      ptrdiff_t j = 0;
-	      for (ptrdiff_t i = 0; ; i++)
-		if (neqbits[i / BITS_PER_BITS_WORD]
-		    & ((bits_word) 1 << (i % BITS_PER_BITS_WORD)))
-		  {
-		    p->contents[j++] = AREF (seq, i);
-		    if (j == n)
-		      break;
-		  }
-	    }
-
-	  XSETVECTOR (seq, p);
-	}
+	seq = Fvector (n, kept);
 
       SAFE_FREE ();
     }
diff --git a/test/src/fns-tests.el b/test/src/fns-tests.el
index 141de1d226..400e912648 100644
--- a/test/src/fns-tests.el
+++ b/test/src/fns-tests.el
@@ -898,5 +898,6 @@ test-secure-hash
 
 (ert-deftest test-vector-delete ()
   (let ((v1 (make-vector 1000 1)))
+    (should (equal (delete t [nil t]) [nil]))
     (should (equal (delete 1 v1) (vector)))
     (should (equal (delete 2 v1) v1))))
-- 
2.17.1