Add an optional random source to RandomList

hpcgap/lib/coll.gd

@@ -1661,15 +1661,17 @@ DeclareOperation( "Random", [ IS_INT, IS_INT ] );
 ##
 ## <#GAPDoc Label="RandomList">
 ## <ManSection>
-## <Func Name="RandomList" Arg='list'/>
+## <Func Name="RandomList" Arg='[rs,] list'/>
 ##
 ## <Description>
 ## <Index>random seed</Index>
 ## For a dense list <A>list</A>,
 ## <Ref Func="RandomList"/> returns a (pseudo-)random element with equal
 ## distribution.
 ## <P/>
-## This function uses the <Ref Var="GlobalMersenneTwister"/> to produce the
+## The random source <A>rs</A> is used to choose a random number.
+## If <A>rs</A> is absent,
+## this function uses the <Ref Var="GlobalMersenneTwister"/> to produce the
 ## random elements (a source of high quality random numbers).
 ## <P/>
 ## </Description>

lib/coll.gd

@@ -1631,15 +1631,17 @@ DeclareOperation( "Random", [ IS_INT, IS_INT ] );
 ##
 ## <#GAPDoc Label="RandomList">
 ## <ManSection>
-## <Func Name="RandomList" Arg='list'/>
+## <Func Name="RandomList" Arg='[rs,] list'/>
 ##
 ## <Description>
 ## <Index>random seed</Index>
 ## For a dense list <A>list</A>,
 ## <Ref Func="RandomList"/> returns a (pseudo-)random element with equal
 ## distribution.
 ## <P/>
-## This function uses the <Ref Var="GlobalMersenneTwister"/> to produce the
+## The random source <A>rs</A> is used to choose a random number.
+## If <A>rs</A> is absent,
+## this function uses the <Ref Var="GlobalMersenneTwister"/> to produce the
 ## random elements (a source of high quality random numbers).
 ## <P/>
 ## </Description>

lib/coll.gi

@@ -246,8 +246,23 @@ InstallMethod( RepresentativeSmallest,
 ## an enumerator of <C> and selects a random element of this list using the
 ## function `RandomList', which is a pseudo random number generator.
 ##
-InstallGlobalFunction( RandomList, function(list)
- return list[Random(GlobalMersenneTwister, 1, Length(list))];
+
+# RandomList is not an operation to avoid the (often expensive) cost of
+# dispatch for lists
+InstallGlobalFunction( RandomList, function(args...)
+ local len, source, list;
+ len := Length(args);
+ if len = 1 then
+ source := GlobalMersenneTwister;
+ list := args[1];
+ elif len = 2 then
+ source := args[1];
+ list := args[2];
+ else
+ Error(" Usage: RandomList([rs], list))");
+ fi;
+
+ return list[Random(source, 1, Length(list))];
 end );
 InstallMethod( Random, "for a (finite) collection",
  [ IsCollection and IsFinite ],

tst/testinstall/random.tst

@@ -1,6 +1,12 @@
 gap> START_TEST("random.tst");
 gap> ReadGapRoot( "tst/testrandom.g" );
 
+# Test RandomList
+gap> randomTest([1,2,3], RandomList);
+gap> randomTest([1..100], RandomList);
+gap> randomTest("abcdef", RandomList);
+gap> randomTestForSizeOneCollection([1], RandomList);
+
 #
 # fields and rings
 #