multidim-index

utilities/multidim_index.hpp

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+#include <cassert>
+#include <vector>
+
+// n-dimentional index <-> 1-dimentional index converter
+// [a_0, ..., a_{dim - 1}] <-> a_0 + a_1 * size_0 + ... + a_{dim - 1} * (size_0 * ... * size_{dim - 2})
+template <class T = int> struct multidim_index {
+    int dim = 0;
+    T _size = 1;
+    std::vector<T> sizes;
+    std::vector<T> weights;
+
+    multidim_index() = default;
+
+    explicit multidim_index(const std::vector<T> &sizes)
+        : dim(sizes.size()), sizes(sizes), weights(dim, T(1)) {
+        for (int d = 0; d < (int)sizes.size(); ++d) {
+            assert(sizes.at(d) > 0);
+            _size *= sizes.at(d);
+            if (d >= 1) weights.at(d) = weights.at(d - 1) * sizes.at(d - 1);
+        }
+    }
+
+    T size() const { return _size; }
+
+    T flat_index(const std::vector<T> &encoded_vec) const {
+        assert((int)encoded_vec.size() == (int)sizes.size());
+        T ret = 0;
+        for (int d = 0; d < (int)sizes.size(); ++d) {
+            assert(0 <= encoded_vec.at(d) and encoded_vec.at(d) < sizes.at(d));
+            ret += encoded_vec.at(d) * weights.at(d);
+        }
+        return ret;
+    }
+
+    std::vector<T> encode(T flat_index) const {
+        assert(0 <= flat_index and flat_index < size());
+        std::vector<T> ret(sizes.size());
+        for (int d = (int)sizes.size() - 1; d >= 0; --d) {
+            ret.at(d) = flat_index / weights.at(d);
+            flat_index %= weights.at(d);
+        }
+        return ret;
+    }
+
+    template <class F> void lo_to_hi(F f) {
+        for (int d = 0; d < (int)sizes.size(); ++d) {
+            if (sizes.at(d) == 1) continue;
+
+            T i = 0;
+            std::vector<T> ivec(sizes.size());
+
+            int cur = sizes.size();
+
+            while (true) {
+                f(i, i + weights.at(d));
+                --cur;
+
+                while (cur >= 0 and ivec.at(cur) + 1 == sizes.at(cur) - (cur == d)) {
+                    i -= ivec.at(cur) * weights.at(cur);
+                    ivec.at(cur--) = 0;
+                }
+
+                if (cur < 0) break;
+
+                ++ivec.at(cur);
+                i += weights.at(cur);
+                cur = sizes.size();
+            }
+        }
+    }
+
+    // Subset sum (fast zeta transform)
+    template <class U> void subset_sum(std::vector<U> &vec) {
+        assert((T)vec.size() == size());
+        lo_to_hi([&](T lo, T hi) { vec.at(hi) += vec.at(lo); });
+    }
+
+    // Inverse of subset sum (fast moebius transform)
+    template <class U> void subset_sum_inv(std::vector<U> &vec) {
+        assert((T)vec.size() == size());
+        const T s = size() - 1;
+        lo_to_hi([&](T dummylo, T dummyhi) { vec.at(s - dummylo) -= vec.at(s - dummyhi); });
+    }
+
+    // Superset sum (fast zeta transform)
+    template <class U> void superset_sum(std::vector<U> &vec) {
+        assert((T)vec.size() == size());
+        const T s = size() - 1;
+        lo_to_hi([&](T dummylo, T dummyhi) { vec.at(s - dummyhi) += vec.at(s - dummylo); });
+    }
+
+    // Inverse of superset sum (fast moebius transform)
+    template <class U> void superset_sum_inv(std::vector<U> &vec) {
+        assert((T)vec.size() == size());
+        lo_to_hi([&](T lo, T hi) { vec.at(lo) -= vec.at(hi); });
+    }
+};

utilities/multidim_index.md

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+---
+title: Multidimensional index
+documentation_of: ./multidim_index.hpp
+---
+
+多次元の添え字を 1 次元に潰す処理とその逆変換の実装．
+
+## 使用方法
+
+### 添字変換
+
+以下のような C++ コードを実行すると，以下のような結果を得る．
+
+```cpp
+multidim_index mi({2, 3, 4});
+
+for (int i = 0; i < mi.size(); ++i) {
+    const auto vec = mi.encode(i);
+    assert(mi.flat_index(vec) == i);
+
+    cout << i << ": (";
+    for (int x : vec) cout << x << ",";
+    cout << ")\n";
+}
+```
+
+```txt
+0: (0,0,0,)
+1: (1,0,0,)
+2: (0,1,0,)
+3: (1,1,0,)
+4: (0,2,0,)
+5: (1,2,0,)
+6: (0,0,1,)
+...
+23: (1,2,3,)
+```
+
+### 累積和やその逆変換
+
+通常の `+=` 演算子による累積和処理に関しては， `subset_sum()` / `subset_sum_inv()` （下側累積和およびその逆変換）・ `superset_sum()` / `superset_sum_inv()` （上側累積和およびその逆変換）が提供されている．
+
+より一般の演算を行いたい場合は，ラムダ式　`f` を用意した上で `lo_to_hi(F f)` 関数を使えばよい．
+
+## 問題例
+
+- [AtCoder Beginner Contest 335（Sponsored by Mynavi） G - Discrete Logarithm Problems](https://atcoder.jp/contests/abc335/tasks/abc335_g)
+  - $P - 1$ の正の約数全てをキーとする DP テーブルで，整序関係をもとに累積和を取る． [参考提出](https://atcoder.jp/contests/abc335/submissions/49118789)

utilities/test/multidim_index.zeta.test.cpp

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+#define PROBLEM "https://judge.yosupo.jp/problem/bitwise_and_convolution"
+#include "../../modint.hpp"
+#include "../multidim_index.hpp"
+
+#include <iostream>
+#include <vector>
+using namespace std;
+
+int main() {
+    cin.tie(nullptr), ios::sync_with_stdio(false);
+    int N;
+    cin >> N;
+
+    multidim_index mi(std::vector<int>(N, 2));
+    vector<ModInt<998244353>> A(1 << N), B(1 << N);
+
+    for (auto &x : A) cin >> x;
+    for (auto &x : B) cin >> x;
+    mi.superset_sum(A);
+    mi.superset_sum(B);
+    for (int i = 0; i < 1 << N; ++i) A.at(i) *= B.at(i);
+    mi.superset_sum_inv(A);
+
+    for (auto x : A) cout << x << ' ';
+}