From 138ac70130b55a0a97dce3ad742896f675bb1ddc Mon Sep 17 00:00:00 2001
From: koba-e964 <koba-e964@debian-koba-e964-0.(none)>
Date: Sun, 16 Mar 2014 01:38:12 +0900
Subject: [PATCH] list? for circular lists

pic_list_p() detects circular lists using Floyd's cycle-finding algorithm.
---
 src/pair.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/src/pair.c b/src/pair.c
index 1b993722..2a24583e 100644
--- a/src/pair.c
+++ b/src/pair.c
@@ -49,12 +49,24 @@ bool
 pic_list_p(pic_state *pic, pic_value obj)
 {
   UNUSED(pic);
+  pic_value twice=obj;
 
-  while (pic_pair_p(obj)) {
+  while (1) {
+    int i;
+    /* variable twice are scanned twice as fast as obj */
+    for(i=0;i<2;i++){
+      if(pic_pair_p(twice)){
+        twice=pic_pair_ptr(twice)->cdr;
+       }
+      else{
+        return pic_nil_p(twice);
+      }
+    }
     obj = pic_pair_ptr(obj)->cdr;
+    if(pic_eq_p(obj,twice)){ /* circular list */
+      return false;
+    }
   }
-
-  return pic_nil_p(obj);
 }
 
 pic_value