From 424b193be7f3350808cad097047a63685576b74f Mon Sep 17 00:00:00 2001
From: David Scherfgen <d.scherfgen@googlemail.com>
Date: Fri, 21 Jun 2024 10:38:20 +0200
Subject: [PATCH] Fix bug #4317: "Maxima doesn't know that signum(real) can
 only be -1, 0 or +1" and add tests

---
 src/compar.lisp   | 17 +++++++++++++++++
 tests/rtest16.mac | 43 ++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/src/compar.lisp b/src/compar.lisp
index 1c3af3685..9d9ebf22d 100644
--- a/src/compar.lisp
+++ b/src/compar.lisp
@@ -1489,6 +1489,23 @@ TDNEG TDZERO TDPN) to store it, and also sets SIGN."
 	     (setq sgn '$pz))
 		((zerop1 (add xrhs -1))				;; c = -1
 		 (setq sgn '$nz))))
+    
+    ;; sign(signum(x)+c)
+    (when (and (not (atom xlhs))
+	          (eq (caar xlhs) '%signum)
+	          (zerop1 ($imagpart (cadr xlhs))))
+      (cond ((eq (sign* (add xrhs 1)) '$neg) ;; c > 1
+	      (setq sgn '$pos))
+	    ((eq (sign* (add xrhs -1)) '$pos)	;; c < -1
+	      (setq sgn '$neg))
+		  ((zerop1 (add xrhs 1))  ;; c = 1
+	      (setq sgn '$pz))
+		  ((zerop1 (add xrhs -1))  ;; c = -1
+		    (setq sgn '$nz))
+      ((zerop1 xrhs)  ;; c = 0 (necessary?)
+        (setq sgn '$pnz))
+      (t  ;; -1 < c < 1, but c # 0
+        (setq sgn '$pn))))
 	   
     (when (and $useminmax (or (minmaxp xlhs) (minmaxp xrhs)))
       (setq sgn (signdiff-minmax xlhs xrhs)))
diff --git a/tests/rtest16.mac b/tests/rtest16.mac
index 9ddbe972e..b1586a5ca 100644
--- a/tests/rtest16.mac
+++ b/tests/rtest16.mac
@@ -2841,4 +2841,45 @@ closeto(float(li[2](%i))-(0.915965594177219*%i-0.20561675835602827), 1e-15);
 true;
 
 closeto(bfloat(li[2](%i))-(9.15965594177219015054603b-1*%i - 2.05616758356028304559052b-1), 10*10^(-fpprec));
-true;
\ No newline at end of file
+true;
+
+/* Bug #4317: "Maxima doesn't know that signum(real) can only be -1, 0 or +1" */
+
+is(signum(x)<=1);
+true;
+
+is(signum(x)>=-1);
+true;
+
+is(signum(x)<2);
+true;
+
+is(signum(x)>-2);
+true;
+
+is(equal(signum(x),1));
+unknown;
+
+is(equal(signum(x),-1));
+unknown;
+
+is(equal(signum(x),0));
+unknown;
+
+is(equal(signum(x),1/2));
+false;
+
+is(equal(signum(x),-1/2));
+false;
+
+declare(x,complex);
+done;
+
+is(signum(x)<=1);
+unknown;
+
+is(signum(x)>=-1);
+unknown;
+
+kill(x);
+done;
\ No newline at end of file
-- 
2.11.4.GIT