From 57c8e010cea8dfb8d0e6d54992a7543c640c4f9f Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet@gmail.com>
Date: Thu, 27 Jun 2019 21:04:18 +0000
Subject: [PATCH] Make `equal' symmetric.

* src/fns.c (internal_equal): Make symmetric. Copy tortoise-hare
  algorithm from lisp.h
---
 src/fns.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/src/fns.c b/src/fns.c
index 2fc000a7f4..623435445a 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -2401,16 +2401,27 @@ internal_equal (Lisp_Object o1, Lisp_Object o2, enum equal_kind equal_kind,
 	      return true;
 	  }
       else
-	FOR_EACH_TAIL (o1)
+	for (struct for_each_tail_internal li1 = { o1, 2, 0, 2 },
+	       li2 = { o2, 2, 0, 2 };
+	     CONSP (o1) && CONSP (o2);
+	     (o1 = XCDR (o1),
+	      o2 = XCDR (o2),
+	      ((--li1.q != 0
+		|| (maybe_quit (), 0 < --li1.n)
+		|| (li1.q = li1.n = li1.max <<= 1, li1.n >>= USHRT_WIDTH,
+		    li1.tortoise = (o1), false))
+	       && EQ (o1, li1.tortoise)) ? circular_list (o1) : (void) 0,
+	      ((--li2.q != 0
+		|| (maybe_quit (), 0 < --li2.n)
+		|| (li2.q = li2.n = li2.max <<= 1, li2.n >>= USHRT_WIDTH,
+		    li2.tortoise = (o2), false))
+	       && EQ (o2, li2.tortoise)) ? circular_list (o2) : (void) 0))
 	  {
-	    if (! CONSP (o2))
-	      return false;
+	    if (EQ (o1, o2))
+	      return true;
 	    if (! internal_equal (XCAR (o1), XCAR (o2),
 				  equal_kind, depth + 1, ht))
 	      return false;
-	    o2 = XCDR (o2);
-	    if (EQ (XCDR (o1), o2))
-	      return true;
 	  }
       depth++;
       goto tail_recurse;
-- 
2.20.1