randutils.hpp

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#ifndef MEDUSA_BITS_UTILS_RANDUTILS_HPP_
#define MEDUSA_BITS_UTILS_RANDUTILS_HPP_
 
#include <medusa/Config.hpp>
#include <medusa/bits/utils/assert.hpp>
#include <medusa/bits/types/Range.hpp>
#include <random>
#include <chrono>
 
namespace mm {
 
inline unsigned int get_seed() {
    try {
        std::random_device rd;
        return rd();
    } catch (std::exception& e) {
        return std::chrono::system_clock::now().time_since_epoch().count();
    }
}
 
template <typename T, typename URNG>
T random_choice(const Range<T>& elements, const Range<double>& weights, bool normed,
                URNG& generator);
 
template <typename T>
T random_choice(const Range<T>& elements, const Range<double>& weights = {},
                bool normed = false) {
    std::mt19937 generator(get_seed());
    return random_choice(elements, weights, normed, generator);
}
 
template <typename T, typename URNG>
T random_choice(const Range<T>& elements, const Range<double>& weights, bool normed,
                URNG& generator) {
    Range<double> actual_weights = weights;
    if (actual_weights.empty()) {
        normed = true;
        actual_weights.assign(elements.size(), 1.0 / elements.size());
    }
    assert_msg(actual_weights.size() == elements.size(),
               "Weights not specified for all elements. Got %d weights but %d elements.",
               weights.size(), elements.size());
    if (!normed) {  // must be 0.0 so that the inferred type is double
        double sum = std::accumulate(actual_weights.begin(), actual_weights.end(), 0.0);
        for (auto& w : actual_weights) w /= sum;
    }
    std::uniform_real_distribution<double> uniform(0, 1);
    double x = uniform(generator);
    for (int i = 0; i < elements.size(); ++i) {
        if (x < actual_weights[i]) return elements[i];
        x -= actual_weights[i];
    }
    assert_msg(false, "Should never get here, you might have claimed the weights "
                      "are normalized, but weren't.");
    return elements[0];
}
 
}  // namespace mm
 
#endif  // MEDUSA_BITS_UTILS_RANDUTILS_HPP_