frac -0x80..00 / 0x80..00

frac -0x80..00 / 0x80..00

[Kevin Ryde]

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I think scm_make_ratio is missing one case of an integer result,
namely when the numerator is most-negative-fixnum and the denominator
is the negative of that (as a bignum).  Viz,

	(integer? (/ -536870912 536870912))
	=> #f

where I would expect #t.  (Attempting to actually print that division
seems to go into an infinite recursion in number->string.)

        * numbers.c (scm_make_ratio): Check for numerator equal to
        SCM_MOST_NEGATIVE_FIXNUM and bignum denominator the negative of that,
        giving integer -1.

        * tests/fractions.test: Exercise most-negative-fixnum over -ve of
        most-negative-fixnum.


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--- numbers.c.~1.215.~	1970-01-01 10:00:01.000000000 +1000
+++ numbers.c	2003-11-21 17:36:45.000000000 +1000
@@ -357,18 +357,26 @@
   */
   if (SCM_INUMP (numerator))
     {
+      long  x = SCM_INUM (numerator);
       if (SCM_EQ_P (numerator, SCM_INUM0))
 	return SCM_INUM0;
       if (SCM_INUMP (denominator))
 	{
-	  long x, y;
-	  x = SCM_INUM (numerator);
+	  long y;
 	  y = SCM_INUM (denominator);
 	  if (x == y)
 	    return SCM_MAKINUM(1);
 	  if ((x % y) == 0)
 	    return SCM_MAKINUM (x / y);
 	}
+      else
+        {
+          /* When x == SCM_MOST_NEGATIVE_FIXNUM we could have the negative
+             of that value for the denominator (as a bignum of course).  */
+          long  abs_x = (x >= 0 ? x : -x);
+          if (mpz_cmpabs_ui (SCM_I_BIG_MPZ (denominator), abs_x) == 0)
+	    return SCM_MAKINUM(-1);
+        }
     }
   else if (SCM_BIGP (numerator))
     {

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--- fractions.test.~1.2.~	1970-01-01 10:00:01.000000000 +1000
+++ fractions.test	2003-11-21 17:27:24.000000000 +1000
@@ -25,6 +25,10 @@
   (testeqv 3/4 3000000000000/4000000000000)
   (testeqv 3 3/1)
   (test= 1/3 (/ 1.0 3.0))
+
+  (test= -1 (/ most-negative-fixnum (- most-negative-fixnum)))
+  (testeq #t (integer? (/ most-negative-fixnum (- most-negative-fixnum))))
+
   (testeqv (+ 1/4 1/2) 3/4)
   (testeqv (* 1/4 2/3) 1/6)
   (testeqv (/ 1/4 2/3) 3/8)

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