From e368f9e22b29cf3be52b543d39940a03092219cb Mon Sep 17 00:00:00 2001
From: Matthew Flatt <mflatt@racket-lang.org>
Date: Thu, 6 May 2021 08:08:38 -0600
Subject: [PATCH] Chez Scheme: special case for `/` on 1 or -1 and an exact
 rational

Related to #3816, but Sam already has better changes on the way.
---
 racket/src/ChezScheme/mats/5_3.ms |  6 ++++++
 racket/src/ChezScheme/s/5_3.ss    | 12 ++++++++++--
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/racket/src/ChezScheme/mats/5_3.ms b/racket/src/ChezScheme/mats/5_3.ms
index bb77e06e3d..5017759400 100644
--- a/racket/src/ChezScheme/mats/5_3.ms
+++ b/racket/src/ChezScheme/mats/5_3.ms
@@ -1724,6 +1724,12 @@
     (eqv? (/ 1 -2) -1/2)
     (eqv? (/ 1/2 -2) -1/4)
     (eqv? (/ 1 -1/2) -2)
+    (eqv? (/ 1 -1/2) -2)
+    (eqv? (/ -1 -1/2) 2)
+    (eqv? (/ 1 3/2) 2/3)
+    (eqv? (/ -1 3/2) -2/3)
+    (eqv? (/ 1 -3/2) -2/3)
+    (eqv? (/ -1 -3/2) 2/3)
     (fl~= (/ 1.0 2) 0.5)
     (fl~= (/ 1 2.0) 0.5)
     (eqv? (/ 0 2.0) 0)
diff --git a/racket/src/ChezScheme/s/5_3.ss b/racket/src/ChezScheme/s/5_3.ss
index 471585aee1..12a9ab66d8 100644
--- a/racket/src/ChezScheme/s/5_3.ss
+++ b/racket/src/ChezScheme/s/5_3.ss
@@ -86,7 +86,7 @@
 (define schoolbook-intquotient (schemeop2 "(cs)ss_trunc"))
 (define schoolbook-intquotient-remainder (schemeop2 "(cs)ss_trunc_rem"))
 (define schoolbook-intremainder (schemeop2 "(cs)rem"))
-(define make-ratnum (schemeop2 "(cs)s_rational"))
+(define make-ratnum (schemeop2 "(cs)s_rational")) ; does not normalize, except detecting 1 as demoninator
 (define exgcd (schemeop2 "(cs)gcd"))
 
 (define $flsin (cflop1 "(cs)sin"))
@@ -2562,7 +2562,15 @@
          [(ratnum?)
           (type-case x
              [(fixnum? bignum?)
-              (integer/ (* x ($ratio-denominator y)) ($ratio-numerator y))]
+              (cond
+                [(eq? x 1) (if (negative? ($ratio-numerator y))
+                               (make-ratnum ($negate who ($ratio-denominator y)) ($negate who ($ratio-numerator y)))
+                               (make-ratnum ($ratio-denominator y) ($ratio-numerator y)))]
+                [(eq? x -1) (if (negative? ($ratio-numerator y))
+                                (make-ratnum ($ratio-denominator y) ($negate who ($ratio-numerator y)))
+                                (make-ratnum ($negate who ($ratio-denominator y)) ($ratio-numerator y)))]
+                [else
+                 (integer/ (* x ($ratio-denominator y)) ($ratio-numerator y))])]
              [(ratnum?)
               (integer/ (* ($ratio-numerator x) ($ratio-denominator y))
                         (* ($ratio-denominator x) ($ratio-numerator y)))]