function [ a, done ] = equiv_next2 ( n, a, done )
%*****************************************************************************80
%
%% equiv_next2() computes, one at a time, the partitions of a set.
%
% Discussion:
%
% A partition of a set assigns each element to exactly one subset.
%
% The number of partitions of a set of size N is the Bell number B(N).
%
% The entries of IARRAY are the partition subset to which each
% element of the original set belongs. If there are NPART distinct
% parts of the partition, then each entry of IARRAY will be a
% number between 1 and NPART. Every number from 1 to NPART will
% occur somewhere in the list. If the entries of IARRAY are
% examined in order, then each time a new partition subset occurs,
% it will be the next unused integer.
%
% For instance, for N = 4, the program will describe the set
% where each element is in a separate subset as 1, 2, 3, 4,
% even though such a partition might also be described as
% 4, 3, 2, 1 or even 1, 5, 8, 19.
%
% Licensing:
%
% This code is distributed under the MIT license.
%
% Modified:
%
% 29 June 2004
%
% Author:
%
% This version by John Burkardt
%
% Input:
%
% integer N, the number of elements in the set.
%
% integer A(N), the previous partition, that is, the output value
% of A on the previous call. On the first call, with DONE = TRUE,
% the value of A is not needed.
%
% logical DONE, should be set to TRUE for the first call, to set
% up initialization, and should be FALSE thereafter.
%
% Output:
%
% integer A(N), the next partition.
%
% logical DONE, is TRUE if there are more partitions to generate.
%
if ( done )
done = 0;
a(1:n) = 1;
else
%
% Find the last element J that can be increased by 1.
% This is the element that is not equal to its maximum possible value,
% which is the maximum value of all preceding elements +1.
%
jmax = a(1);
imax = 1;
for j = 2 : n
if ( jmax < a(j) )
jmax = a(j);
else
imax = j;
end
end
%
% If no element can be increased by 1, we are done.
%
if ( imax == 1 )
done = 1;
return
end
%
% Increase the value of the IMAX-th element by 1, set its successors to 1.
%
done = 0;
a(imax) = a(imax) + 1;
a(imax+1:n) = 1;
end
return
end