integer-length on negative bignum

integer-length on negative bignum

[Kevin Ryde]

[-- Attachment #1: Type: text/plain, Size: 584 bytes --]

A small bug in integer-length, assuming I understand what that
function is meant to be doing.  This lets it correspond to the inum
case at least.

	* numbers.c (scm_integer_length): On negative bignums, adjust
	mpz_sizeinbase to account for it looking at absolute value where we
	want ones-complement.

	* tests/numbers.test (integer-length): Exercise some negatives, in
	particular "...11100..00".

In the tests here, it's only ...11100..00 which is affected by the
problem, the others are already right, I just added them for coverage
and confirmation of what integer-length does.


[-- Attachment #2: numbers.c.integer-length.diff --]
[-- Type: text/plain, Size: 756 bytes --]

--- numbers.c.~1.181.~	2003-05-04 09:09:49.000000000 +1000
+++ numbers.c	2003-05-04 09:44:31.000000000 +1000
@@ -1438,7 +1438,14 @@
     };
     return SCM_MAKINUM (c - 4 + l);
   } else if (SCM_BIGP (n)) {
+    /* mpz_sizeinbase looks at the absolute value of negatives, whereas we
+       want a ones-complement.  If n is ...111100..00 then mpz_sizeinbase is
+       1 too big, so check for that and adjust.  */
     size_t size = mpz_sizeinbase (SCM_I_BIG_MPZ (n), 2);
+    if (mpz_sgn (SCM_I_BIG_MPZ (n)) < 0
+        && mpz_scan0 (SCM_I_BIG_MPZ (n),  /* no 0 bits above the lowest 1 */
+                      mpz_scan1 (SCM_I_BIG_MPZ (n), 0)) == ULONG_MAX)
+      size--;
     scm_remember_upto_here_1 (n);
     return SCM_MAKINUM (size);
   } else {

[-- Attachment #3: numbers.test.integer-length.diff --]
[-- Type: text/plain, Size: 748 bytes --]

--- numbers.test.~1.17.~	2003-05-04 09:11:34.000000000 +1000
+++ numbers.test	2003-05-04 09:41:02.000000000 +1000
@@ -1739,3 +1739,32 @@
 ;;;
 ;;; inexact->exact
 ;;;
+
+;;;
+;;; integer-length
+;;;
+
+(with-test-prefix "integer-length"
+  
+  (with-test-prefix "negatives ...11100..00"
+    (do ((n -1 (ash n 1))
+	 (i 0  (1+ i)))
+	((> i 256))
+      (pass-if (list n "expect" i)
+	(= i (integer-length n)))))
+  
+  (with-test-prefix "negatives ...11100..01"
+    (do ((n -3 (logxor 3 (ash n 1)))
+	 (i 2  (1+ i)))
+	((> i 256))
+      (pass-if n
+	(= i (integer-length n)))))
+  
+  (with-test-prefix "negatives ...111011..11"
+    (do ((n -2 (1+ (ash n 1)))
+	 (i 1  (1+ i)))
+	((> i 256))
+      (pass-if n
+	(= i (integer-length n))))))
+
+

[-- Attachment #4: Type: text/plain, Size: 142 bytes --]

_______________________________________________
Guile-devel mailing list
Guile-devel@gnu.org
http://mail.gnu.org/mailman/listinfo/guile-devel