random.cpp

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/*
    Genesis - A toolkit for working with phylogenetic data.
    Copyright (C) 2014-2020 Lucas Czech and HITS gGmbH
 
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation,  either version 3 of the License,  or
    (at your option) any later version.
 
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
 
    You should have received a copy of the GNU General Public License
    along with this program.  If not,  see <http://www.gnu.org/licenses/>.
 
    Contact:
    Lucas Czech <lucas.czech@h-its.org>
    Exelixis Lab,  Heidelberg Institute for Theoretical Studies
    Schloss-Wolfsbrunnenweg 35,  D-69118 Heidelberg,  Germany
*/
 
#include "genesis/utils/math/random.hpp"
 
#include "genesis/utils/core/options.hpp"
 
#include <cassert>
#include <random>
#include <stdexcept>
#include <string>
 
namespace genesis {
namespace utils {
 
// ================================================================================================
//     Sampling
// ================================================================================================
 
std::vector<size_t> select_without_replacement( size_t const k, size_t const n )
{
    // Following http://stackoverflow.com/a/311716/4184258
    // Knuth's variable names: k=n, n=N
 
    std::vector<size_t> result;
    result.reserve( k );
 
    if( k > n ) {
        throw std::invalid_argument(
            "Cannot select more unique elements than there are elements: k == "  +
            std::to_string(k) + " > n == " + std::to_string(n) + "."
        );
    }
 
    auto& engine = Options::get().random_engine();
    std::uniform_real_distribution<double> distribution( 0.0, 1.0 );
 
    size_t t = 0; // total input records dealt with
    size_t m = 0; // number of items selected so far
 
    while( m < k ) {
        // call a uniform( 0, 1 ) random number generator
        double const u = distribution( engine );
 
        if( (n - t) * u >= k - m ) {
            t++;
        } else {
            assert( t < n );
            result.push_back( t );
            t++;
            m++;
        }
    }
    assert( m == k );
    assert( result.size() == k );
 
    return result;
}
 
} // namespace utils
} // namespace genesis