ランダムに指定個数を非復元抽出するRange Adapterを作ってみた。

RandomSamplingAdapter.cpp

#include <vector>
#include <boost/range.hpp>
using namespace std;

namespace RangeAdapter {
    namespace Detail {
        template<class Range>
        struct RandomSampledRange;
        // Range
        template<class Range>
        struct RandomSampledRange {
            // イテレータ
            struct iterator {
                typedef forward_iterator_tag iterator_category;
                typedef typename boost::range_value<Range>::type value_type;
                typedef typename boost::range_difference<Range>::type difference_type;
                typedef typename boost::range_pointer<Range>::type pointer;
                typedef typename boost::range_reference<Range>::type reference;
                const RandomSampledRange<Range>& r;
                size_t index;
                iterator& operator++() { ++index; return *this; }
                iterator operator++(int) { auto t = *this; ++(*this); return t; }
                reference operator*() const { return *(std::begin(r.range) + r.indices[index]); }
                bool operator==(const iterator& other) const { return index == other.index; }
                bool operator!=(const iterator& other) const { return index != other.index; }
            };
            typedef iterator const_iterator;
            Range& range;
            vector<size_t> indices;
            const size_t count;
            template<class URNG>
            RandomSampledRange(Range& range, URNG&& g, size_t count)
                : range(range), indices(boost::size(range)), count((min)(count, boost::size(range))) {
                for (size_t i = 0, n = indices.size(); i < n; i++)
                    indices[i] = i;
                shuffle(indices.begin(), indices.end(), g);
            }
            iterator begin() const { return { *this, size_t() }; }
            iterator end() const { return { *this, count }; }
        };
        // Rangeへ渡す値を一時的に持つためのもの
        template<class URNG>
        struct RandomSampleHolder { URNG&& g; size_t count; };
        // operator|
        template<class Range, class URNG>
        inline RandomSampledRange<Range> operator|(Range& range, const Detail::RandomSampleHolder<URNG>& holder) {
            return RandomSampledRange<Range>(range, holder.g, holder.count);
        }
        // operator|
        template<class Range, class URNG>
        inline RandomSampledRange<const Range> operator|(const Range& range, const Detail::RandomSampleHolder<URNG>& holder) {
            return RandomSampledRange<const Range>(range, holder.g, holder.count);
        }
    }
    using Detail::RandomSampledRange;
    // アダプタ
    template<class URNG>
    inline Detail::RandomSampleHolder<URNG> RandomSampled(URNG&& g, size_t count = (numeric_limits<size_t>::max)()) {
        return Detail::RandomSampleHolder<URNG> { g, count };
    }
}

#include <iostream>
#include <random>
#include <boost/range/irange.hpp>
#include <boost/range/adaptors.hpp>
#include <boost/range/algorithm_ext.hpp>
#include <boost/algorithm/string.hpp>

int main(int argc, char* argv[])
{
    mt19937 mt((random_device())()); // とても雑に初期化

    vector<int> vec;
    boost::range::push_back(vec, boost::irange(0, 20));
    cout << boost::algorithm::join(
        vec
            | RangeAdapter::RandomSampled(mt, 10) // 非constバージョン
            | boost::adaptors::transformed([] (int& n) { return to_string(n); }),
            ", ") << endl;

    cout << boost::algorithm::join(
        boost::irange(0, 20)
            | RangeAdapter::RandomSampled(mt, 10) // constバージョン
            | boost::adaptors::transformed([] (const int& n) { return to_string(n); }),
            ", ") << endl;

    cout << boost::algorithm::join(
        boost::irange(0, 20)
            | RangeAdapter::RandomSampled(mt) // 全量
            | boost::adaptors::transformed([] (const int& n) { return to_string(n); }),
            ", ") << endl;

    return 0;
}

/*
＜実行結果例＞

4, 2, 16, 12, 5, 19, 8, 17, 1, 7
10, 5, 11, 3, 2, 17, 8, 0, 13, 18
0, 12, 14, 17, 5, 2, 13, 4, 8, 1, 7, 3, 18, 10, 16, 6, 15, 9, 11, 19

*/