cp-library

C++ Library for Competitive Programming

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:heavy_check_mark: 遅延伝播セグメント木
(include/emthrm/data_structure/lazy_segment_tree.hpp)

セグメント木 (segment tree)

モノイドであるデータに対して高速に区間クエリを処理する（完全）二分木である。

時間計算量

$\langle O(N), O(\log{N}) \rangle$

仕様

セグメント木

template <typename T>
requires requires {
  typename T::Monoid;
  {T::id()} -> std::same_as<typename T::Monoid>;
  {T::merge(std::declval<typename T::Monoid>(),
            std::declval<typename T::Monoid>())}
      -> std::same_as<typename T::Monoid>;
}
struct SegmentTree;

メンバ関数

名前 効果・戻り値
explicit SegmentTree(const int n); 要素数 $N$ のオブジェクトを構築する。
explicit SegmentTree(const std::vector<Monoid>& a); $A$ に対してオブジェクトを構築する。
void set(int idx, const Monoid val); $A_{\mathrm{idx}} \gets \mathrm{val}$
Monoid get(int left, int right) const; $A_{\mathrm{left}} \circ \cdots \circ A_{\mathrm{right}}$
Monoid operator[](const int idx) const; $A_{\mathrm{idx}}$
template <typename G>
int find_right(int left, const G g); 		g(get(left, right + 1)) = false を満たす最小の $\mathrm{right}$。ただし存在しないときは $N$ を返す。
template <typename G>
int find_left(int right, const G g); 		g(get(left, right)) = false を満たす最大の $\mathrm{left}$。ただし存在しないときは $-1$ を返す。

メンバ型

名前 説明
Monoid T::Monoid

遅延伝播セグメント木

template <typename T>
requires requires {
  typename T::Monoid;
  typename T::OperatorMonoid;
  {T::m_id()} -> std::same_as<typename T::Monoid>;
  {T::o_id()} -> std::same_as<typename T::OperatorMonoid>;
  {T::m_merge(std::declval<typename T::Monoid>(),
              std::declval<typename T::Monoid>())}
      -> std::same_as<typename T::Monoid>;
  {T::o_merge(std::declval<typename T::OperatorMonoid>(),
              std::declval<typename T::OperatorMonoid>())}
      -> std::same_as<typename T::OperatorMonoid>;
  {T::apply(std::declval<typename T::Monoid>(),
            std::declval<typename T::OperatorMonoid>())}
      -> std::same_as<typename T::Monoid>;
}
struct LazySegmentTree;

メンバ関数

名前 効果・戻り値
explicit LazySegmentTree(const int n); 要素数 $N$ のオブジェクトを構築する。
explicit LazySegmentTree(const std::vector<Monoid>& a); $A$ に対してオブジェクトを構築する。
void set(int idx, const Monoid val); $A_{\mathrm{idx}} \gets \mathrm{val}$
void apply(int idx, const OperatorMonoid val); $\mathrm{idx}$ における変更クエリ
void apply(int left, int right, const OperatorMonoid val); $[\mathrm{left}, \mathrm{right})$ における変更クエリ
Monoid get(int left, int right); $[\mathrm{left}, \mathrm{right})$ における解答クエリ
Monoid operator[](const int idx); $A_{\mathrm{idx}}$
template <typename G>
int find_right(int left, const G g); 		g(get(left, right + 1)) = false を満たす最小の $\mathrm{right}$。ただし存在しない場合は $N$ を返す。
template <typename G>
int find_left(int right, const G g); 		g(get(left, right)) = false を満たす最大の $\mathrm{left}$。ただし存在しない場合は $-1$ を返す。

メンバ型

名前 説明
Monoid T::Monoid
OperatorMonoid T::OperatorMonoid

参考文献

セグメント木

遅延伝播セグメント木

TODO

Submissons

Verified with

Code

#ifndef EMTHRM_DATA_STRUCTURE_LAZY_SEGMENT_TREE_HPP_
#define EMTHRM_DATA_STRUCTURE_LAZY_SEGMENT_TREE_HPP_

#include <algorithm>
#include <bit>
// #include <cassert>
#include <limits>
#include <type_traits>
#include <vector>

namespace emthrm {

template <typename T>
requires requires {
  typename T::Monoid;
  typename T::OperatorMonoid;
  {T::m_id()} -> std::same_as<typename T::Monoid>;
  {T::o_id()} -> std::same_as<typename T::OperatorMonoid>;
  {T::m_merge(std::declval<typename T::Monoid>(),
              std::declval<typename T::Monoid>())}
      -> std::same_as<typename T::Monoid>;
  {T::o_merge(std::declval<typename T::OperatorMonoid>(),
              std::declval<typename T::OperatorMonoid>())}
      -> std::same_as<typename T::OperatorMonoid>;
  {T::apply(std::declval<typename T::Monoid>(),
            std::declval<typename T::OperatorMonoid>())}
      -> std::same_as<typename T::Monoid>;
}
struct LazySegmentTree {
  using Monoid = typename T::Monoid;
  using OperatorMonoid = typename T::OperatorMonoid;

  explicit LazySegmentTree(const int n)
      : LazySegmentTree(std::vector<Monoid>(n, T::m_id())) {}

  explicit LazySegmentTree(const std::vector<Monoid>& a)
      : n(a.size()), height(std::countr_zero(std::bit_ceil(a.size()))),
        p2(1 << height) {
    lazy.assign(p2, T::o_id());
    data.assign(p2 << 1, T::m_id());
    std::copy(a.begin(), a.end(), data.begin() + p2);
    for (int i = p2 - 1; i > 0; --i) {
      data[i] = T::m_merge(data[i << 1], data[(i << 1) + 1]);
    }
  }

  void set(int idx, const Monoid val) {
    idx += p2;
    for (int i = height; i > 0; --i) {
      propagate(idx >> i);
    }
    data[idx] = val;
    for (int i = 1; i <= height; ++i) {
      const int current_idx = idx >> i;
      data[current_idx] =
          T::m_merge(data[current_idx << 1], data[(current_idx << 1) + 1]);
    }
  }

  void apply(int idx, const OperatorMonoid val) {
    idx += p2;
    for (int i = height; i > 0; --i) {
      propagate(idx >> i);
    }
    data[idx] = T::apply(data[idx], val);
    for (int i = 1; i <= height; ++i) {
      const int current_idx = idx >> i;
      data[current_idx] =
          T::m_merge(data[current_idx << 1], data[(current_idx << 1) + 1]);
    }
  }

  void apply(int left, int right, const OperatorMonoid val) {
    if (right <= left) [[unlikely]] return;
    left += p2;
    right += p2;
    const int ctz_left = std::countr_zero(static_cast<unsigned int>(left));
    for (int i = height; i > ctz_left; --i) {
      propagate(left >> i);
    }
    const int ctz_right = std::countr_zero(static_cast<unsigned int>(right));
    for (int i = height; i > ctz_right; --i) {
      propagate(right >> i);
    }
    for (int l = left, r = right; l < r; l >>= 1, r >>= 1) {
      if (l & 1) apply_sub(l++, val);
      if (r & 1) apply_sub(--r, val);
    }
    for (int i = left >> (ctz_left + 1); i > 0; i >>= 1) {
      data[i] = T::m_merge(data[i << 1], data[(i << 1) + 1]);
    }
    for (int i = right >> (ctz_right + 1); i > 0; i >>= 1) {
      data[i] = T::m_merge(data[i << 1], data[(i << 1) + 1]);
    }
  }

  Monoid get(int left, int right) {
    if (right <= left) [[unlikely]] return T::m_id();
    left += p2;
    right += p2;
    const int ctz_left = std::countr_zero(static_cast<unsigned int>(left));
    for (int i = height; i > ctz_left; --i) {
      propagate(left >> i);
    }
    const int ctz_right = std::countr_zero(static_cast<unsigned int>(right));
    for (int i = height; i > ctz_right; --i) {
      propagate(right >> i);
    }
    Monoid res_l = T::m_id(), res_r = T::m_id();
    for (; left < right; left >>= 1, right >>= 1) {
      if (left & 1) res_l = T::m_merge(res_l, data[left++]);
      if (right & 1) res_r = T::m_merge(data[--right], res_r);
    }
    return T::m_merge(res_l, res_r);
  }

  Monoid operator[](const int idx) {
    const int node = idx + p2;
    for (int i = height; i > 0; --i) {
      propagate(node >> i);
    }
    return data[node];
  }

  template <typename G>
  int find_right(int left, const G g) {
    if (left >= n) [[unlikely]] return n;
    left += p2;
    for (int i = height; i > 0; --i) {
      propagate(left >> i);
    }
    Monoid val = T::m_id();
    do {
      while (!(left & 1)) left >>= 1;
      Monoid nxt = T::m_merge(val, data[left]);
      if (!g(nxt)) {
        while (left < p2) {
          propagate(left);
          left <<= 1;
          nxt = T::m_merge(val, data[left]);
          if (g(nxt)) {
            val = nxt;
            ++left;
          }
        }
        return left - p2;
      }
      val = nxt;
      ++left;
    } while (!std::has_single_bit(static_cast<unsigned int>(left)));
    return n;
  }

  template <typename G>
  int find_left(int right, const G g) {
    if (right <= 0) [[unlikely]] return -1;
    right += p2;
    for (int i = height; i > 0; --i) {
      propagate((right - 1) >> i);
    }
    Monoid val = T::m_id();
    do {
      --right;
      while (right > 1 && (right & 1)) right >>= 1;
      Monoid nxt = T::m_merge(data[right], val);
      if (!g(nxt)) {
        while (right < p2) {
          propagate(right);
          right = (right << 1) + 1;
          nxt = T::m_merge(data[right], val);
          if (g(nxt)) {
            val = nxt;
            --right;
          }
        }
        return right - p2;
      }
      val = nxt;
    } while (!std::has_single_bit(static_cast<unsigned int>(right)));
    return -1;
  }

 private:
  const int n, height, p2;
  std::vector<Monoid> data;
  std::vector<OperatorMonoid> lazy;

  void apply_sub(const int idx, const OperatorMonoid& val) {
    data[idx] = T::apply(data[idx], val);
    if (idx < p2) lazy[idx] = T::o_merge(lazy[idx], val);
  }

  void propagate(const int idx) {
    // assert(1 <= idx && idx < p2);
    apply_sub(idx << 1, lazy[idx]);
    apply_sub((idx << 1) + 1, lazy[idx]);
    lazy[idx] = T::o_id();
  }
};

namespace monoid {

template <typename T>
struct RangeMinimumAndUpdateQuery {
  using Monoid = T;
  using OperatorMonoid = T;
  static constexpr Monoid m_id() { return std::numeric_limits<Monoid>::max(); }
  static constexpr OperatorMonoid o_id() {
    return std::numeric_limits<OperatorMonoid>::max();
  }
  static Monoid m_merge(const Monoid& a, const Monoid& b) {
    return std::min(a, b);
  }
  static OperatorMonoid o_merge(const OperatorMonoid& a,
                                const OperatorMonoid& b) {
    return b == o_id() ? a : b;
  }
  static Monoid apply(const Monoid& a, const OperatorMonoid& b) {
    return b == o_id() ? a : b;
  }
};

template <typename T>
struct RangeMaximumAndUpdateQuery {
  using Monoid = T;
  using OperatorMonoid = T;
  static constexpr Monoid m_id() {
    return std::numeric_limits<Monoid>::lowest();
  }
  static constexpr OperatorMonoid o_id() {
    return std::numeric_limits<OperatorMonoid>::lowest();
  }
  static Monoid m_merge(const Monoid& a, const Monoid& b) {
    return std::max(a, b);
  }
  static OperatorMonoid o_merge(const OperatorMonoid& a,
                                const OperatorMonoid& b) {
    return b == o_id() ? a : b;
  }
  static Monoid apply(const Monoid& a, const OperatorMonoid& b) {
    return b == o_id()? a : b;
  }
};

template <typename T, T Inf>
struct RangeMinimumAndAddQuery {
  using Monoid = T;
  using OperatorMonoid = T;
  static constexpr Monoid m_id() { return Inf; }
  static constexpr OperatorMonoid o_id() { return 0; }
  static Monoid m_merge(const Monoid& a, const Monoid& b) {
    return std::min(a, b);
  }
  static OperatorMonoid o_merge(const OperatorMonoid& a,
                                const OperatorMonoid& b) {
    return a + b;
  }
  static Monoid apply(const Monoid& a, const OperatorMonoid& b) {
    return a + b;
  }
};

template <typename T, T Inf>
struct RangeMaximumAndAddQuery {
  using Monoid = T;
  using OperatorMonoid = T;
  static constexpr Monoid m_id() { return -Inf; }
  static constexpr OperatorMonoid o_id() { return 0; }
  static Monoid m_merge(const Monoid& a, const Monoid& b) {
    return std::max(a, b);
  }
  static OperatorMonoid o_merge(const OperatorMonoid& a,
                                const OperatorMonoid& b) {
    return a + b;
  }
  static Monoid apply(const Monoid& a, const OperatorMonoid& b) {
    return a + b;
  }
};

template <typename T>
struct RangeSumAndUpdateQuery {
  using Monoid = struct { T sum; int len; };
  using OperatorMonoid = T;
  static std::vector<Monoid> init(const int n) {
    return std::vector<Monoid>(n, Monoid{0, 1});
  }
  static constexpr Monoid m_id() { return {0, 0}; }
  static constexpr OperatorMonoid o_id() {
    return std::numeric_limits<OperatorMonoid>::max();
  }
  static Monoid m_merge(const Monoid& a, const Monoid& b) {
    return Monoid{a.sum + b.sum, a.len + b.len};
  }
  static OperatorMonoid o_merge(const OperatorMonoid& a,
                                const OperatorMonoid& b) {
    return b == o_id() ? a : b;
  }
  static Monoid apply(const Monoid& a, const OperatorMonoid& b) {
    return Monoid{b == o_id() ? a.sum : b * a.len, a.len};
  }
};

template <typename T>
struct RangeSumAndAddQuery {
  using Monoid = struct { T sum; int len; };
  using OperatorMonoid = T;
  static std::vector<Monoid> init(const int n) {
    return std::vector<Monoid>(n, Monoid{0, 1});
  }
  static constexpr Monoid m_id() { return {0, 0}; }
  static constexpr OperatorMonoid o_id() { return 0; }
  static Monoid m_merge(const Monoid& a, const Monoid& b) {
    return Monoid{a.sum + b.sum, a.len + b.len};
  }
  static OperatorMonoid o_merge(const OperatorMonoid& a,
                                const OperatorMonoid& b) {
    return a + b;
  }
  static Monoid apply(const Monoid& a, const OperatorMonoid& b) {
    return Monoid{a.sum + b * a.len, a.len};
  }
};

}  // namespace monoid

}  // namespace emthrm

#endif  // EMTHRM_DATA_STRUCTURE_LAZY_SEGMENT_TREE_HPP_
#line 1 "include/emthrm/data_structure/lazy_segment_tree.hpp"



#include <algorithm>
#include <bit>
// #include <cassert>
#include <limits>
#include <type_traits>
#include <vector>

namespace emthrm {

template <typename T>
requires requires {
  typename T::Monoid;
  typename T::OperatorMonoid;
  {T::m_id()} -> std::same_as<typename T::Monoid>;
  {T::o_id()} -> std::same_as<typename T::OperatorMonoid>;
  {T::m_merge(std::declval<typename T::Monoid>(),
              std::declval<typename T::Monoid>())}
      -> std::same_as<typename T::Monoid>;
  {T::o_merge(std::declval<typename T::OperatorMonoid>(),
              std::declval<typename T::OperatorMonoid>())}
      -> std::same_as<typename T::OperatorMonoid>;
  {T::apply(std::declval<typename T::Monoid>(),
            std::declval<typename T::OperatorMonoid>())}
      -> std::same_as<typename T::Monoid>;
}
struct LazySegmentTree {
  using Monoid = typename T::Monoid;
  using OperatorMonoid = typename T::OperatorMonoid;

  explicit LazySegmentTree(const int n)
      : LazySegmentTree(std::vector<Monoid>(n, T::m_id())) {}

  explicit LazySegmentTree(const std::vector<Monoid>& a)
      : n(a.size()), height(std::countr_zero(std::bit_ceil(a.size()))),
        p2(1 << height) {
    lazy.assign(p2, T::o_id());
    data.assign(p2 << 1, T::m_id());
    std::copy(a.begin(), a.end(), data.begin() + p2);
    for (int i = p2 - 1; i > 0; --i) {
      data[i] = T::m_merge(data[i << 1], data[(i << 1) + 1]);
    }
  }

  void set(int idx, const Monoid val) {
    idx += p2;
    for (int i = height; i > 0; --i) {
      propagate(idx >> i);
    }
    data[idx] = val;
    for (int i = 1; i <= height; ++i) {
      const int current_idx = idx >> i;
      data[current_idx] =
          T::m_merge(data[current_idx << 1], data[(current_idx << 1) + 1]);
    }
  }

  void apply(int idx, const OperatorMonoid val) {
    idx += p2;
    for (int i = height; i > 0; --i) {
      propagate(idx >> i);
    }
    data[idx] = T::apply(data[idx], val);
    for (int i = 1; i <= height; ++i) {
      const int current_idx = idx >> i;
      data[current_idx] =
          T::m_merge(data[current_idx << 1], data[(current_idx << 1) + 1]);
    }
  }

  void apply(int left, int right, const OperatorMonoid val) {
    if (right <= left) [[unlikely]] return;
    left += p2;
    right += p2;
    const int ctz_left = std::countr_zero(static_cast<unsigned int>(left));
    for (int i = height; i > ctz_left; --i) {
      propagate(left >> i);
    }
    const int ctz_right = std::countr_zero(static_cast<unsigned int>(right));
    for (int i = height; i > ctz_right; --i) {
      propagate(right >> i);
    }
    for (int l = left, r = right; l < r; l >>= 1, r >>= 1) {
      if (l & 1) apply_sub(l++, val);
      if (r & 1) apply_sub(--r, val);
    }
    for (int i = left >> (ctz_left + 1); i > 0; i >>= 1) {
      data[i] = T::m_merge(data[i << 1], data[(i << 1) + 1]);
    }
    for (int i = right >> (ctz_right + 1); i > 0; i >>= 1) {
      data[i] = T::m_merge(data[i << 1], data[(i << 1) + 1]);
    }
  }

  Monoid get(int left, int right) {
    if (right <= left) [[unlikely]] return T::m_id();
    left += p2;
    right += p2;
    const int ctz_left = std::countr_zero(static_cast<unsigned int>(left));
    for (int i = height; i > ctz_left; --i) {
      propagate(left >> i);
    }
    const int ctz_right = std::countr_zero(static_cast<unsigned int>(right));
    for (int i = height; i > ctz_right; --i) {
      propagate(right >> i);
    }
    Monoid res_l = T::m_id(), res_r = T::m_id();
    for (; left < right; left >>= 1, right >>= 1) {
      if (left & 1) res_l = T::m_merge(res_l, data[left++]);
      if (right & 1) res_r = T::m_merge(data[--right], res_r);
    }
    return T::m_merge(res_l, res_r);
  }

  Monoid operator[](const int idx) {
    const int node = idx + p2;
    for (int i = height; i > 0; --i) {
      propagate(node >> i);
    }
    return data[node];
  }

  template <typename G>
  int find_right(int left, const G g) {
    if (left >= n) [[unlikely]] return n;
    left += p2;
    for (int i = height; i > 0; --i) {
      propagate(left >> i);
    }
    Monoid val = T::m_id();
    do {
      while (!(left & 1)) left >>= 1;
      Monoid nxt = T::m_merge(val, data[left]);
      if (!g(nxt)) {
        while (left < p2) {
          propagate(left);
          left <<= 1;
          nxt = T::m_merge(val, data[left]);
          if (g(nxt)) {
            val = nxt;
            ++left;
          }
        }
        return left - p2;
      }
      val = nxt;
      ++left;
    } while (!std::has_single_bit(static_cast<unsigned int>(left)));
    return n;
  }

  template <typename G>
  int find_left(int right, const G g) {
    if (right <= 0) [[unlikely]] return -1;
    right += p2;
    for (int i = height; i > 0; --i) {
      propagate((right - 1) >> i);
    }
    Monoid val = T::m_id();
    do {
      --right;
      while (right > 1 && (right & 1)) right >>= 1;
      Monoid nxt = T::m_merge(data[right], val);
      if (!g(nxt)) {
        while (right < p2) {
          propagate(right);
          right = (right << 1) + 1;
          nxt = T::m_merge(data[right], val);
          if (g(nxt)) {
            val = nxt;
            --right;
          }
        }
        return right - p2;
      }
      val = nxt;
    } while (!std::has_single_bit(static_cast<unsigned int>(right)));
    return -1;
  }

 private:
  const int n, height, p2;
  std::vector<Monoid> data;
  std::vector<OperatorMonoid> lazy;

  void apply_sub(const int idx, const OperatorMonoid& val) {
    data[idx] = T::apply(data[idx], val);
    if (idx < p2) lazy[idx] = T::o_merge(lazy[idx], val);
  }

  void propagate(const int idx) {
    // assert(1 <= idx && idx < p2);
    apply_sub(idx << 1, lazy[idx]);
    apply_sub((idx << 1) + 1, lazy[idx]);
    lazy[idx] = T::o_id();
  }
};

namespace monoid {

template <typename T>
struct RangeMinimumAndUpdateQuery {
  using Monoid = T;
  using OperatorMonoid = T;
  static constexpr Monoid m_id() { return std::numeric_limits<Monoid>::max(); }
  static constexpr OperatorMonoid o_id() {
    return std::numeric_limits<OperatorMonoid>::max();
  }
  static Monoid m_merge(const Monoid& a, const Monoid& b) {
    return std::min(a, b);
  }
  static OperatorMonoid o_merge(const OperatorMonoid& a,
                                const OperatorMonoid& b) {
    return b == o_id() ? a : b;
  }
  static Monoid apply(const Monoid& a, const OperatorMonoid& b) {
    return b == o_id() ? a : b;
  }
};

template <typename T>
struct RangeMaximumAndUpdateQuery {
  using Monoid = T;
  using OperatorMonoid = T;
  static constexpr Monoid m_id() {
    return std::numeric_limits<Monoid>::lowest();
  }
  static constexpr OperatorMonoid o_id() {
    return std::numeric_limits<OperatorMonoid>::lowest();
  }
  static Monoid m_merge(const Monoid& a, const Monoid& b) {
    return std::max(a, b);
  }
  static OperatorMonoid o_merge(const OperatorMonoid& a,
                                const OperatorMonoid& b) {
    return b == o_id() ? a : b;
  }
  static Monoid apply(const Monoid& a, const OperatorMonoid& b) {
    return b == o_id()? a : b;
  }
};

template <typename T, T Inf>
struct RangeMinimumAndAddQuery {
  using Monoid = T;
  using OperatorMonoid = T;
  static constexpr Monoid m_id() { return Inf; }
  static constexpr OperatorMonoid o_id() { return 0; }
  static Monoid m_merge(const Monoid& a, const Monoid& b) {
    return std::min(a, b);
  }
  static OperatorMonoid o_merge(const OperatorMonoid& a,
                                const OperatorMonoid& b) {
    return a + b;
  }
  static Monoid apply(const Monoid& a, const OperatorMonoid& b) {
    return a + b;
  }
};

template <typename T, T Inf>
struct RangeMaximumAndAddQuery {
  using Monoid = T;
  using OperatorMonoid = T;
  static constexpr Monoid m_id() { return -Inf; }
  static constexpr OperatorMonoid o_id() { return 0; }
  static Monoid m_merge(const Monoid& a, const Monoid& b) {
    return std::max(a, b);
  }
  static OperatorMonoid o_merge(const OperatorMonoid& a,
                                const OperatorMonoid& b) {
    return a + b;
  }
  static Monoid apply(const Monoid& a, const OperatorMonoid& b) {
    return a + b;
  }
};

template <typename T>
struct RangeSumAndUpdateQuery {
  using Monoid = struct { T sum; int len; };
  using OperatorMonoid = T;
  static std::vector<Monoid> init(const int n) {
    return std::vector<Monoid>(n, Monoid{0, 1});
  }
  static constexpr Monoid m_id() { return {0, 0}; }
  static constexpr OperatorMonoid o_id() {
    return std::numeric_limits<OperatorMonoid>::max();
  }
  static Monoid m_merge(const Monoid& a, const Monoid& b) {
    return Monoid{a.sum + b.sum, a.len + b.len};
  }
  static OperatorMonoid o_merge(const OperatorMonoid& a,
                                const OperatorMonoid& b) {
    return b == o_id() ? a : b;
  }
  static Monoid apply(const Monoid& a, const OperatorMonoid& b) {
    return Monoid{b == o_id() ? a.sum : b * a.len, a.len};
  }
};

template <typename T>
struct RangeSumAndAddQuery {
  using Monoid = struct { T sum; int len; };
  using OperatorMonoid = T;
  static std::vector<Monoid> init(const int n) {
    return std::vector<Monoid>(n, Monoid{0, 1});
  }
  static constexpr Monoid m_id() { return {0, 0}; }
  static constexpr OperatorMonoid o_id() { return 0; }
  static Monoid m_merge(const Monoid& a, const Monoid& b) {
    return Monoid{a.sum + b.sum, a.len + b.len};
  }
  static OperatorMonoid o_merge(const OperatorMonoid& a,
                                const OperatorMonoid& b) {
    return a + b;
  }
  static Monoid apply(const Monoid& a, const OperatorMonoid& b) {
    return Monoid{a.sum + b * a.len, a.len};
  }
};

}  // namespace monoid

}  // namespace emthrm
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