From bc00e06c7ec5575f405bee4e12a062d4269f4eab Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ludovic=20Court=C3=A8s?= <ludo@gnu.org>
Date: Sun, 8 May 2011 18:15:10 +0200
Subject: [PATCH] Optimize `vlist-fold-right'.

* module/ice-9/vlist.scm (vlist-fold-right): Avoid `vlist-reverse' and
  instead `vlist-ref' individual elements.  The result is about twice
  faster.  Thanks Andy for suggesting it indirectly.  :-)
---
 module/ice-9/vlist.scm | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/module/ice-9/vlist.scm b/module/ice-9/vlist.scm
index 34c7c00c1..b554e1105 100644
--- a/module/ice-9/vlist.scm
+++ b/module/ice-9/vlist.scm
@@ -245,7 +245,14 @@ tail."
 (define (vlist-fold-right proc init vlist)
   "Fold over @var{vlist}, calling @var{proc} for each element, starting from
 the last element."
-  (vlist-fold proc init (vlist-reverse vlist)))
+  (define len (vlist-length vlist))
+
+  (let loop ((index  (1- len))
+             (result init))
+    (if (< index 0)
+        result
+        (loop (1- index)
+              (proc (vlist-ref vlist index) result)))))
 
 (define (vlist-reverse vlist)
   "Return a new @var{vlist} whose content are those of @var{vlist} in reverse