Re: Strange problems with data-tests.el

[Paul Eggert <eggert@cs.ucla.edu>]

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Pip Cet wrote:
> Which is what I meant when I said the test
> was misleading: it's true for subtly different reasons on different
> hosts

Who would have thought that the tedium of writing floating-point tests could be 
overwhelmed by the tedium of documenting them and worse yet, writing ChangeLog 
entries for them? Anyway, I just now experienced this, and installed the 
attached into emacs-26. Though I was strongly tempted to remove the tests instead.

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From 217a88557da50be8bcd76f55b90137d121eb2ecf Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Mon, 13 Aug 2018 09:36:11 -0700
Subject: [PATCH] Add comment about floating point test

* test/src/data-tests.el (data-tests--float-greater-than-fixnums):
New constant.
(data-tests-=, data-tests-<, data-tests->, data-tests-<=)
(data-tests->=, data-tests-min): Use it.
---
 test/src/data-tests.el | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/test/src/data-tests.el b/test/src/data-tests.el
index 91463db113..b444dc70f1 100644
--- a/test/src/data-tests.el
+++ b/test/src/data-tests.el
@@ -23,13 +23,21 @@
 
 (require 'cl-lib)
 
+(defconst data-tests--float-greater-than-fixnums (+ 1.0 most-positive-fixnum)
+  "A floating-point value that is greater than all fixnums.
+It is also as small as conveniently possible, to make the tests sharper.
+Adding 1.0 to most-positive-fixnum should suffice on all
+practical Emacs platforms, since the result is a power of 2 and
+this is exactly representable and is greater than
+most-positive-fixnum, which is just less than a power of 2.")
+
 (ert-deftest data-tests-= ()
   (should-error (=))
   (should (= 1))
   (should (= 2 2))
   (should (= 9 9 9 9 9 9 9 9 9))
   (should (= most-negative-fixnum (float most-negative-fixnum)))
-  (should-not (= most-positive-fixnum (+ 1.0 most-positive-fixnum)))
+  (should-not (= most-positive-fixnum data-tests--float-greater-than-fixnums))
   (should-not (apply #'= '(3 8 3)))
   (should-error (= 9 9 'foo))
   ;; Short circuits before getting to bad arg
@@ -40,7 +48,7 @@
   (should (< 1))
   (should (< 2 3))
   (should (< -6 -1 0 2 3 4 8 9 999))
-  (should (< 0.5 most-positive-fixnum (+ 1.0 most-positive-fixnum)))
+  (should (< 0.5 most-positive-fixnum data-tests--float-greater-than-fixnums))
   (should-not (apply #'< '(3 8 3)))
   (should-error (< 9 10 'foo))
   ;; Short circuits before getting to bad arg
@@ -51,7 +59,7 @@
   (should (> 1))
   (should (> 3 2))
   (should (> 6 1 0 -2 -3 -4 -8 -9 -999))
-  (should (> (+ 1.0 most-positive-fixnum) most-positive-fixnum 0.5))
+  (should (> data-tests--float-greater-than-fixnums most-positive-fixnum 0.5))
   (should-not (apply #'> '(3 8 3)))
   (should-error (> 9 8 'foo))
   ;; Short circuits before getting to bad arg
@@ -62,7 +70,7 @@
   (should (<= 1))
   (should (<= 2 3))
   (should (<= -6 -1 -1 0 0 0 2 3 4 8 999))
-  (should (<= 0.5 most-positive-fixnum (+ 1.0 most-positive-fixnum)))
+  (should (<= 0.5 most-positive-fixnum data-tests--float-greater-than-fixnums))
   (should-not (apply #'<= '(3 8 3 3)))
   (should-error (<= 9 10 'foo))
   ;; Short circuits before getting to bad arg
@@ -73,7 +81,7 @@
   (should (>= 1))
   (should (>= 3 2))
   (should (>= 666 1 0 0 -2 -3 -3 -3 -4 -8 -8 -9 -999))
-  (should (>= (+ 1.0 most-positive-fixnum) most-positive-fixnum))
+  (should (>= data-tests--float-greater-than-fixnums most-positive-fixnum))
   (should-not (apply #'>= '(3 8 3)))
   (should-error (>= 9 8 'foo))
   ;; Short circuits before getting to bad arg
@@ -97,7 +105,7 @@
   (should (= 2 (min 3 2)))
   (should (= -999 (min 666 1 0 0 -2 -3 -3 -3 -4 -8 -8 -9 -999)))
   (should (= most-positive-fixnum
-             (min (+ 1.0 most-positive-fixnum) most-positive-fixnum)))
+             (min data-tests--float-greater-than-fixnums most-positive-fixnum)))
   (should (= 3 (apply #'min '(3 8 3))))
   (should-error (min 9 8 'foo))
   (should-error (min (make-marker)))
-- 
2.17.1