Do not overflow when converting large rationals to float

gap-system · Oct 18, 2020 · 7d8f948 · 7d8f948
1 parent e62d395
commit 7d8f948
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Showing 3 changed files with 40 additions and 4 deletions.
diff --git a/doc/ref/floats.xml b/doc/ref/floats.xml
@@ -106,7 +106,7 @@ or floating-point numbers. Floating-point numbers may
 also be created, in any floating-point representation, using
 <Ref Constr="NewFloat"/> as in <C>NewFloat(IsIEEE754FloatRep,355/113)</C>,
 by supplying the category filter of the desired new floating-point number;
-or using <Ref Oper="MakeFloat"/> as in <C>NewFloat(1.0,355/113)</C>,
+or using <Ref Oper="MakeFloat"/> as in <C>MakeFloat(1.0,355/113)</C>,
 by supplying a sample floating-point number.
 <P/>
 

diff --git a/lib/float.gi b/lib/float.gi
@@ -110,7 +110,10 @@ BindGlobal("INSTALLFLOATCONSTRUCTORS", function(arg)
     fi;
 
     InstallMethod(NewFloat, [filter,IsRat], -1, function(filter,obj)
-        return NewFloat(filter,NumeratorRat(obj))/NewFloat(filter,DenominatorRat(obj));
+        local num, den;
+        num := NumeratorRat(obj);
+        den := DenominatorRat(obj);
+        return NewFloat(filter, QuoInt(num, den)) + NewFloat(filter, RemInt(num, den)) / NewFloat(filter, den);
     end);
 
     InstallMethod(NewFloat, [filter,IsInfinity], -1, function(filter,obj)
@@ -138,7 +141,10 @@ BindGlobal("INSTALLFLOATCONSTRUCTORS", function(arg)
     end);
 
     InstallMethod(MakeFloat, [filter,IsRat], -1, function(filter,obj)
-        return MakeFloat(filter,NumeratorRat(obj))/MakeFloat(filter,DenominatorRat(obj));
+        local num, den;
+        num := NumeratorRat(obj);
+        den := DenominatorRat(obj);
+        return MakeFloat(filter, QuoInt(num, den)) + MakeFloat(filter, RemInt(num, den)) / MakeFloat(filter, den);
     end);
 
     InstallMethod(MakeFloat, [filter,IsInfinity], -1, function(filter,obj)

diff --git a/tst/testinstall/float.tst b/tst/testinstall/float.tst
@@ -1,4 +1,4 @@
-#@local neginf,posinf,r,nan,l,f,g
+#@local neginf,posinf,r,nan,l,f,g,a,b
 gap> START_TEST("float.tst");
 
 # make sure we are testing the built-in machine floats
@@ -32,6 +32,36 @@ inf
 gap> Float(-infinity);
 -inf
 
+#
+gap> r := (10^309 + 1) / 10^308;;
+gap> Float(r);
+10.
+gap> Float(-r);
+-10.
+gap> NewFloat(IsIEEE754FloatRep, r);
+10.
+gap> NewFloat(IsIEEE754FloatRep, -r);
+-10.
+gap> MakeFloat(0.0, r);
+10.
+gap> MakeFloat(0.0, -r);
+-10.
+
+#
+gap> r := (10^309-1) / 10^308;;
+gap> Float(r);
+10.
+gap> Float(-r);
+-10.
+gap> NewFloat(IsIEEE754FloatRep, r);
+10.
+gap> NewFloat(IsIEEE754FloatRep, -r);
+-10.
+gap> MakeFloat(0.0, r);
+10.
+gap> MakeFloat(0.0, -r);
+-10.
+
 #
 # input floats directly
 #