[goffice] Math: further fixes to go_add_epsilon fallback code.

[Morten Welinder <mortenw src gnome org>]

commit 099667409dfa2ce506e0aeedfedf65fc739cde3b
Author: Morten Welinder <terra gnome org>
Date:   Thu Feb 17 17:53:39 2022 -0500

    Math: further fixes to go_add_epsilon fallback code.
    
    Largely untested, but should handle switch-over from one binade
    to the next.

 ChangeLog              |  4 ++++
 goffice/math/go-math.c | 12 ++++++++----
 2 files changed, 12 insertions(+), 4 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index a673c5f6..0a46ba3f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2022-02-17  Morten Welinder  <terra gnome org>
+
+       * goffice/math/go-math.c (go_add_epsilon): Fix fallback code.
+
 2022-01-21  Morten Welinder  <terra gnome org>
 
        * goffice/gtk/go-action-combo-stack.c: Reimplement based on
diff --git a/goffice/math/go-math.c b/goffice/math/go-math.c
index b50961f4..ccd8e5af 100644
--- a/goffice/math/go-math.c
+++ b/goffice/math/go-math.c
@@ -213,7 +213,8 @@ go_add_epsilon (double x)
        else {
                int e;
                double mant = frexp (fabs (x), &e);
-               return ldexp (mant + DBL_EPSILON, e);
+               // mant is in range [0.5; 1)
+               return ldexp (mant + DBL_EPSILON / 2, e);
        }
 #endif
 }
@@ -238,7 +239,8 @@ go_sub_epsilon (double x)
        else {
                int e;
                double mant = frexp (fabs (x), &e);
-               return ldexp (mant - DBL_EPSILON, e);
+               // mant is in range [0.5; 1)
+               return ldexp (mant - (mant == 0.5 ? DBL_EPSILON / 4 : DBL_EPSILON / 2), e);
        }
 #endif
 }
@@ -764,7 +766,8 @@ go_add_epsilonl (long double x)
        else {
                int e;
                long double mant = frexpl (fabsl (x), &e);
-               return ldexpl (mant + LDBL_EPSILON, e);
+               // mant is in range [0.5; 1)
+               return ldexpl (mant + LDBL_EPSILON / 2, e);
        }
 #endif
 }
@@ -789,7 +792,8 @@ go_sub_epsilonl (long double x)
        else {
                int e;
                long double mant = frexpl (fabsl (x), &e);
-               return ldexpl (mant - LDBL_EPSILON, e);
+               // mant is in range [0.5; 1)
+               return ldexpl (mant - (mant == 0.5L ? LDBL_EPSILON / 4 : LDBL_EPSILON / 2), e);
        }
 #endif
 }