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#############################################################################
##
#W random.g GAP library Martin Schönert
##
##
#Y Copyright (C) 1996, Lehrstuhl D für Mathematik, RWTH Aachen, Germany
#Y (C) 1998 School Math and Comp. Sci., University of St Andrews, Scotland
#Y Copyright (C) 2002 The GAP Group
##
## This file contains the random number generator
##
#############################################################################
##
#F RANDOM_LIST( <list> ) . . . . . . . . return a random element from a list
##
if IsHPCGAP then
MakeThreadLocal("R_N");
MakeThreadLocal("R_X");
else
R_N := 1;
R_X := [];
fi;
# We need to compute modulo 2^28 repeatedly. If we just write 2^28 into the
# code, though, GAP needs to evaluate it to 268435456 each time it executes
# the function. To avoid this, we put it into a constant. As an additional
# trick, we actually store -2^28, which gives identical results, but has the
# benefit of being an immediate integer even on 32 bit systems.
BIND_CONSTANT("R_228", -2^28);
RANDOM_LIST := function ( list )
R_N := R_N mod 55 + 1;
R_X[R_N] := (R_X[R_N] + R_X[(R_N+30) mod 55+1]) mod R_228;
return list[ QUO_INT( R_X[R_N] * LEN_LIST(list), -R_228 ) + 1 ];
end;
RANDOM_SEED := function ( n )
local i;
R_N := 1;
R_X := [ n mod R_228 ];
for i in [2..55] do
R_X[i] := (1664525 * R_X[i-1] + 1) mod R_228;
od;
for i in [1..99] do
R_N := R_N mod 55 + 1;
R_X[R_N] := (R_X[R_N] + R_X[(R_N+30) mod 55+1]) mod R_228;
od;
end;
if IsHPCGAP then
BIND_GLOBAL("RANDOM_SEED_COUNTER", FixedAtomicList(1, 0));
BIND_GLOBAL("GET_RANDOM_SEED_COUNTER", function()
local r;
r := ATOMIC_ADDITION(RANDOM_SEED_COUNTER, 1, 1);
return r;
end);
BIND_GLOBAL("RANDOM_SEED_CONSTRUCTOR", function()
RANDOM_SEED( GET_RANDOM_SEED_COUNTER() );
end);
BindThreadLocalConstructor("R_N", RANDOM_SEED_CONSTRUCTOR);
BindThreadLocalConstructor("R_X", RANDOM_SEED_CONSTRUCTOR);
else
if R_X = [] then RANDOM_SEED( 1 ); fi;
fi;
#############################################################################
##
#E random.g . . . . . . . . . . . . . . . . . . . . . . . . . . . ends here
##
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