From 68ddbfb634809be74722e9252993c6089a0c4d7a Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Tue, 21 Nov 2017 19:25:54 -0800
Subject: bugfix: two issues in mappend* and append*.

Two bugs in these functions, both attributable to
the lazy_appendv implementation:

They destructively catenate the input lists,
much like nconc, even though documented as
non-destructive.

If any input list is infinite, other than the
last input list, that list is forced, resulting
in an infinite loop.

* lib.c (lazy_appendv_func): Rewritten to use a
different algorithm which earnestly allocates a
new lazy cons for each element of the output
sequence, except for the tail part corresponding
to the last list.
(lazy_appendv): Set up the lazy cons according
to the new representation and just return it.
No searching for the tail of the nonempty list,
and no destructive manipulation.
---
 lib.c | 52 +++++++++++++++++++++++-----------------------------
 1 file changed, 23 insertions(+), 29 deletions(-)

diff --git a/lib.c b/lib.c
index 18ba967f..97d0eece 100644
--- a/lib.c
+++ b/lib.c
@@ -1357,34 +1357,31 @@ val replace_list(val list, val items, val from, val to)
 
 static val lazy_appendv_func(val env, val lcons)
 {
-  cons_bind (last, lists, env);
-  val nonempty = nil;
-
-  while (lists) {
-    nonempty = nullify(pop(&lists));
-    if (nonempty)
-      break;
-  }
+  cons_bind (fl, rl, env);
+  cons_bind (fe, re, fl);
 
-  rplaca(lcons, last);
+  rplaca(lcons, fe);
 
-  if (nilp(lists)) {
-    rplacd(lcons, nonempty);
-    return nil;
+  if (re) {
+    rplaca(env, re);
+    rplacd(lcons, make_lazy_cons(lcons_fun(lcons)));
+  } else if (rl) {
+    do {
+      fl = car(rl);
+      rl = cdr(rl);
+    } while (!fl && rl);
+
+    if (fl) {
+      if (rl) {
+        rplaca(env, fl);
+        rplacd(env, rl);
+        rplacd(lcons, make_lazy_cons(lcons_fun(lcons)));
+      } else {
+        rplacd(lcons, fl);
+      }
+    }
   }
 
-  if (atom(nonempty))
-    uw_throwf(error_s, lit("append*: cannot append to atom ~s"),
-              nonempty, nao);
-
-  rplacd(env, lists);
-
-  {
-    loc ptail = ltail(mkcloc(nonempty));
-    rplaca(env, car(deref(ptail)));
-    set(ptail, make_lazy_cons(lcons_fun(lcons)));
-    rplacd(lcons, nonempty);
-  }
   return nil;
 }
 
@@ -1407,11 +1404,8 @@ val lazy_appendv(struct args *args)
               nonempty, nao);
 
   {
-    loc ptail = ltail(mkcloc(nonempty));
-    set(ptail, make_lazy_cons(func_f1(cons(car(deref(ptail)),
-                                           args_get_rest(args, index)),
-                                      lazy_appendv_func)));
-    return nonempty;
+    return make_lazy_cons(func_f1(cons(nonempty, args_get_rest(args, index)),
+                                  lazy_appendv_func));
   }
 }
 
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