[Guile-commits] 03/09: Fix intset-subtract to reliably produce empty-intset

[Andy Wingo]

wingo pushed a commit to branch master
in repository guile.

commit dc27708f0b0b039d9356e8d93d71daf563906fb7
Author: Andy Wingo <address@hidden>
Date:   Mon Jul 13 11:03:00 2015 +0200

    Fix intset-subtract to reliably produce empty-intset
    
    * module/language/cps/intset.scm (intset-subtract): Reliably produce
      empty-intset if the result is empty.
---
 module/language/cps/intset.scm |    8 +++++---
 1 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/module/language/cps/intset.scm b/module/language/cps/intset.scm
index 7f8316e..005bb7e 100644
--- a/module/language/cps/intset.scm
+++ b/module/language/cps/intset.scm
@@ -646,10 +646,10 @@
           (else (make-intset/prune a-min a-shift root)))))))))
 
 (define (intset-subtract a b)
-  (define tmp (new-leaf))
   ;; Intersect leaves.
   (define (subtract-leaves a b)
-    (logand a (lognot b)))
+    (let ((out (logand a (lognot b))))
+      (if (zero? out) #f out)))
   ;; Subtract B from A starting at index I; the result will be fresh.
   (define (subtract-branches/fresh shift a b i fresh)
     (let lp ((i 0))
@@ -721,7 +721,9 @@
                      (new (lp a-min a-shift old)))
                 (if (eq? old new)
                     a-root
-                    (clone-branch-and-set a-root a-idx new)))))))))))
+                    (let ((root (clone-branch-and-set a-root a-idx new)))
+                      (and (or new (not (branch-empty? root)))
+                           root))))))))))))
 
 (define (bitvector->intset bv)
   (define (finish-tail out min tail)