function a = randsubset(v, k)
% A = RANDSUBSET(N, K)
% 
% returns a random array A composed of K distinct and sorted elements of
% the set (1:N). The probablity of the subsets is uniform.
%
% A = RANDSUBSET(V, K), where V is a (cell) vector of length N, will pick K
% distinct elements of the set V. The elements of A respect the same order
% with respect to V.
%
% To get random sampling without replacement of the set, call RANDPERM
% after RANDSUBSET.
%
% The function uses Robert Floyd's sampling algorithm.
%
% See also: randperm, nchoosek
% 
% Author: Bruno Luong
% Original: 24-Jun-2010
%           24-Jun-2010: mex engine

if isscalar(v)
    n = v;
else
    n = length(v);
end

if k>n
    error('Calling: randsubset(n,k) with n>=k')
end

a64 = int64(ceil(rand(1,k).*(n-k+1:n)));
n64 = int64(n);
% Mex Robert Floyd engine, inplace change of a64
rssfloyd(a64, n64);
a = double(a64);

if ~isscalar(v)
    a = v(a);
end

end % randsubset