first draft of hilbert

Author: lrvideckis

library/data_structures/uncommon/hilbert_mos.hpp

@@ -0,0 +1,23 @@
+#pragma once
+//! https://codeforces.com/blog/entry/61203?#comment-1064868
+//! @code
+//!   vector<pii> queries;
+//!   ranges::sort(queries, {}, [&](pii& x) {
+//!     return hilbert(x.first, x.second);
+//!   });
+//! @endcode
+//! @time O(log(n))
+//! @space O(1)
+ll hilbert(int x, int y) {
+  ll d = 0, mx = 1;
+  while(mx <= max(x, y)) mx *= 4;
+  for (int s = mx / 2; s; s /= 2) {
+    bool rx = x & s, ry = y & s;
+    d = d * 4 | (ry * 3 ^ rx);
+    if (!rx) {
+      if (ry) x ^= mx - 1, y ^= mx - 1;
+      swap(x, y);
+    }
+  }
+  return d;
+}

tests/library_checker_aizu_tests/handmade_tests/hilbert_mos.test.cpp

@@ -0,0 +1,38 @@
+#define PROBLEM \
+  "https://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ITP1_1_A"
+#include "../template.hpp"
+#include "../../../library/contest/random.hpp"
+#include "../../../library/data_structures/uncommon/hilbert_mos.hpp"
+const vector<int> di = {1, -1, 0, 0};
+const vector<int> dj = {0, 0, 1, -1};
+void test(int i, int j) {
+  ll res = hilbert(i, j);
+  int cnt_prev = 0;
+  int cnt_next = 0;
+  for (int k = 0; k < 4; k++) {
+    int to_i = i + di[k];
+    int to_j = j + dj[k];
+    if(to_i >= 0 && to_j >= 0) {
+      cnt_prev += (res - 1 == hilbert(to_i, to_j));
+      cnt_next += (res + 1 == hilbert(to_i, to_j));
+    }
+  }
+  if(i==0 && j==0) assert(res == 0);
+  else assert(cnt_prev == 1);
+  assert(cnt_next == 1);
+}
+
+int main() {
+  cin.tie(0)->sync_with_stdio(0);
+  for (int i = 0; i < 3000; i++) {
+    for (int j = 0; j < 3000; j++) {
+      test(i,j);
+    }
+  }
+  for (int iter = 50000; iter--;) {
+    int i = rnd(0, 1'000'000'000);
+    int j = rnd(0, 1'000'000'000);
+    test(i,j);
+  }
+  cout << "Hello World\n";
+}