sub combinations {
my @list= @_; # List of items to choose from
my @pick= (0) x @list; # Whether we want each item
# $pick[$i] means include $list[$i] in results.
# So @pick currently describes the empty subset.
# Return a closure that, each time it is called, returns
# the next subset:
return sub {
# Treat @pick as a base-2 number and increment it.
# Note that @pick started as all 0s and we stop
# after it is all 1s so all cases get covered.
# (See original node for handling the empty subset)
# Start at least-significant bit, $pick[0]:
my $i= 0;
# Increment a bit. If the bit was already 1, then
# set it to 0 and continue to next bit:
while( 1 < ++$pick[$i] ) {
$pick[$i]= 0;
# If we've run out of bits, then we were at
# all 1s and so are done. Return empty list:
return if $#pick < ++$i;
}
# The grep() below returns the indices for which
# $pick[$_] is not 0. The @list[...] is an array
# slice that returns the list of elements of @list
# at the indices returned by grep. That is, we
# return all items $list[$i] where $pick[$i] is
# not 0. Same as:
# map { $pick[$_] ? $list[$_] : () } 0..$#list;
return @list[ grep $pick[$_], 0..$#pick ];
};
}
my $next= combinations( 50..59 );
my @comb;
while( @comb= $next->() ) {
# do work with @comb here
}