cp-library-cpp

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:warning: Inversion Number
(library/algorithm/inversion_number.hpp)

Inversion Number

Depends on

Code

#ifndef SUISEN_INVERSION_NUMBER
#define SUISEN_INVERSION_NUMBER

#include <atcoder/fenwicktree>
#include "library/util/coordinate_compressor.hpp"

namespace suisen {
    template <typename T>
    long long inversion_number(const std::vector<T> &a) {
        const int n = a.size();
        const auto cmp = CoordinateCompressorBuilder<T>::build(a);

        long long ans = 0;
        atcoder::fenwick_tree<int> ft(cmp.size());
        for (int i = n - 1; i >= 0; --i) {
            const int j = cmp[a[i]];
            ans += ft.sum(0, j), ft.add(j, 1);
        }
        return ans;
    }
} // namespace suisen

#endif // SUISEN_INVERSION_NUMBER
#line 1 "library/algorithm/inversion_number.hpp"



#include <atcoder/fenwicktree>
#line 1 "library/util/coordinate_compressor.hpp"



#include <algorithm>
#include <cassert>
#include <vector>

#line 1 "library/type_traits/type_traits.hpp"



#include <limits>
#include <iostream>
#include <type_traits>

namespace suisen {
    template <typename ...Constraints> using constraints_t = std::enable_if_t<std::conjunction_v<Constraints...>, std::nullptr_t>;

    template <typename T, typename = std::nullptr_t> struct bitnum { static constexpr int value = 0; };
    template <typename T> struct bitnum<T, constraints_t<std::is_integral<T>>> { static constexpr int value = std::numeric_limits<std::make_unsigned_t<T>>::digits; };
    template <typename T> static constexpr int bitnum_v = bitnum<T>::value;
    template <typename T, size_t n> struct is_nbit { static constexpr bool value = bitnum_v<T> == n; };
    template <typename T, size_t n> static constexpr bool is_nbit_v = is_nbit<T, n>::value;

    template <typename T, typename = std::nullptr_t> struct safely_multipliable { using type = T; };
    template <typename T> struct safely_multipliable<T, constraints_t<std::is_signed<T>, is_nbit<T, 32>>> { using type = long long; };
    template <typename T> struct safely_multipliable<T, constraints_t<std::is_signed<T>, is_nbit<T, 64>>> { using type = __int128_t; };
    template <typename T> struct safely_multipliable<T, constraints_t<std::is_unsigned<T>, is_nbit<T, 32>>> { using type = unsigned long long; };
    template <typename T> struct safely_multipliable<T, constraints_t<std::is_unsigned<T>, is_nbit<T, 64>>> { using type = __uint128_t; };
    template <typename T> using safely_multipliable_t = typename safely_multipliable<T>::type;

    template <typename T, typename = void> struct rec_value_type { using type = T; };
    template <typename T> struct rec_value_type<T, std::void_t<typename T::value_type>> {
        using type = typename rec_value_type<typename T::value_type>::type;
    };
    template <typename T> using rec_value_type_t = typename rec_value_type<T>::type;

    template <typename T> class is_iterable {
        template <typename T_> static auto test(T_ e) -> decltype(e.begin(), e.end(), std::true_type{});
        static std::false_type test(...);
    public:
        static constexpr bool value = decltype(test(std::declval<T>()))::value;
    };
    template <typename T> static constexpr bool is_iterable_v = is_iterable<T>::value;
    template <typename T> class is_writable {
        template <typename T_> static auto test(T_ e) -> decltype(std::declval<std::ostream&>() << e, std::true_type{});
        static std::false_type test(...);
    public:
        static constexpr bool value = decltype(test(std::declval<T>()))::value;
    };
    template <typename T> static constexpr bool is_writable_v = is_writable<T>::value;
    template <typename T> class is_readable {
        template <typename T_> static auto test(T_ e) -> decltype(std::declval<std::istream&>() >> e, std::true_type{});
        static std::false_type test(...);
    public:
        static constexpr bool value = decltype(test(std::declval<T>()))::value;
    };
    template <typename T> static constexpr bool is_readable_v = is_readable<T>::value;
} // namespace suisen

#line 9 "library/util/coordinate_compressor.hpp"

namespace suisen {
template <typename T>
class CoordinateCompressorBuilder {
    public:
        struct Compressor {
            public:
                static constexpr int absent = -1;

                // default constructor
                Compressor() : _xs(std::vector<T>{}) {}
                // Construct from strictly sorted vector
                Compressor(const std::vector<T> &xs) : _xs(xs) {
                    assert(is_strictly_sorted(xs));
                }

                // Return the number of distinct keys.
                int size() const {
                    return _xs.size();
                }
                // Check if the element is registered.
                bool has_key(const T &e) const {
                    return std::binary_search(_xs.begin(), _xs.end(), e);
                }
                // Compress the element. if not registered, returns `default_value`. (default: Compressor::absent)
                int comp(const T &e, int default_value = absent) const {
                    const int res = min_geq_index(e);
                    return res != size() and _xs[res] == e ? res : default_value;
                }
                // Restore the element from the index.
                T decomp(const int compressed_index) const {
                    return _xs[compressed_index];
                }
                // Compress the element. Equivalent to call `comp(e)`
                int operator[](const T &e) const {
                    return comp(e);
                }
                // Return the minimum registered value greater than `e`. if not exists, return `default_value`.
                T min_gt(const T &e, const T &default_value) const {
                    auto it = std::upper_bound(_xs.begin(), _xs.end(), e);
                    return it == _xs.end() ? default_value : *it;
                }
                // Return the minimum registered value greater than or equal to `e`. if not exists, return `default_value`.
                T min_geq(const T &e, const T &default_value) const {
                    auto it = std::lower_bound(_xs.begin(), _xs.end(), e);
                    return it == _xs.end() ? default_value : *it;
                }
                // Return the maximum registered value less than `e`. if not exists, return `default_value`
                T max_lt(const T &e, const T &default_value) const {
                    auto it = std::upper_bound(_xs.rbegin(), _xs.rend(), e, std::greater<T>());
                    return it == _xs.rend() ? default_value : *it;
                }
                // Return the maximum registered value less than or equal to `e`. if not exists, return `default_value`
                T max_leq(const T &e, const T &default_value) const {
                    auto it = std::lower_bound(_xs.rbegin(), _xs.rend(), e, std::greater<T>());
                    return it == _xs.rend() ? default_value : *it;
                }
                // Return the compressed index of the minimum registered value greater than `e`. if not exists, return `compressor.size()`.
                int min_gt_index(const T &e) const {
                    return std::upper_bound(_xs.begin(), _xs.end(), e) - _xs.begin();
                }
                // Return the compressed index of the minimum registered value greater than or equal to `e`. if not exists, return `compressor.size()`.
                int min_geq_index(const T &e) const {
                    return std::lower_bound(_xs.begin(), _xs.end(), e) - _xs.begin();
                }
                // Return the compressed index of the maximum registered value less than `e`. if not exists, return -1.
                int max_lt_index(const T &e) const {
                    return int(_xs.rend() - std::upper_bound(_xs.rbegin(), _xs.rend(), e, std::greater<T>())) - 1;
                }
                // Return the compressed index of the maximum registered value less than or equal to `e`. if not exists, return -1.
                int max_leq_index(const T &e) const {
                    return int(_xs.rend() - std::lower_bound(_xs.rbegin(), _xs.rend(), e, std::greater<T>())) - 1;
                }
            private:
                std::vector<T> _xs;
                static bool is_strictly_sorted(const std::vector<T> &v) {
                    return std::adjacent_find(v.begin(), v.end(), std::greater_equal<T>()) == v.end();
                }
        };
        CoordinateCompressorBuilder() : _xs(std::vector<T>{}) {}
        explicit CoordinateCompressorBuilder(const std::vector<T> &xs) : _xs(xs) {}
        explicit CoordinateCompressorBuilder(std::vector<T> &&xs) : _xs(std::move(xs)) {}
        template <typename Gen, constraints_t<std::is_invocable_r<T, Gen, int>> = nullptr>
        CoordinateCompressorBuilder(const int n, Gen generator) {
            reserve(n);
            for (int i = 0; i < n; ++i) push(generator(i));
        }
        // Attempt to preallocate enough memory for specified number of elements.
        void reserve(int n) {
            _xs.reserve(n);
        }
        // Add data.
        void push(const T &first) {
            _xs.push_back(first);
        }
        // Add data.
        void push(T &&first) {
            _xs.push_back(std::move(first));
        }
        // Add data in the range of [first, last). 
        template <typename Iterator>
        auto push(const Iterator &first, const Iterator &last) -> decltype(std::vector<T>{}.push_back(*first), void()) {
            for (auto it = first; it != last; ++it) _xs.push_back(*it);
        }
        // Add all data in the container. Equivalent to `push(iterable.begin(), iterable.end())`.
        template <typename Iterable>
        auto push(const Iterable &iterable) -> decltype(std::vector<T>{}.push_back(*iterable.begin()), void()) {
            push(iterable.begin(), iterable.end());
        }
        // Add data.
        template <typename ...Args>
        void emplace(Args &&...args) {
            _xs.emplace_back(std::forward<Args>(args)...);
        }
        // Build compressor.
        auto build() {
            std::sort(_xs.begin(), _xs.end()), _xs.erase(std::unique(_xs.begin(), _xs.end()), _xs.end());
            return Compressor {_xs};
        }
        // Build compressor from vector.
        static auto build(const std::vector<T> &xs) {
            return CoordinateCompressorBuilder(xs).build();
        }
        // Build compressor from vector.
        static auto build(std::vector<T> &&xs) {
            return CoordinateCompressorBuilder(std::move(xs)).build();
        }
        // Build compressor from generator.
        template <typename Gen, constraints_t<std::is_invocable_r<T, Gen, int>> = nullptr>
        static auto build(const int n, Gen generator) {
            return CoordinateCompressorBuilder<T>(n, generator).build();
        }
    private:
        std::vector<T> _xs;
};

} // namespace suisen


#line 6 "library/algorithm/inversion_number.hpp"

namespace suisen {
    template <typename T>
    long long inversion_number(const std::vector<T> &a) {
        const int n = a.size();
        const auto cmp = CoordinateCompressorBuilder<T>::build(a);

        long long ans = 0;
        atcoder::fenwick_tree<int> ft(cmp.size());
        for (int i = n - 1; i >= 0; --i) {
            const int j = cmp[a[i]];
            ans += ft.sum(0, j), ft.add(j, 1);
        }
        return ans;
    }
} // namespace suisen
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