Bitonic sort problems

Author: dolphingarlic

Baltic/Baltic 21-swaps.cpp

@@ -0,0 +1,37 @@
+/*
+Baltic 2021 The Collection Game
+- This is a buffed version of COI '21 MalnaRISC, so see that solution too
+- We basically want to sort the artworks by swapping them in parallel
+- https://en.wikipedia.org/wiki/Bitonic_sorter
+- https://en.wikipedia.org/wiki/Batcher_odd%E2%80%93even_mergesort
+- When a comparison returns 1, don't do anything because they've been sorted/swapped
+- When a comparison returns 0, manually swap the two artworks in the answer array
+- O(log^2 N) visits
+*/
+
+#include "swaps.h"
+
+#include <numeric>
+#include <algorithm>
+
+void solve(int N, int V) {
+    std::vector<int> ans(N);
+    std::iota(ans.begin(), ans.end(), 1);
+    int P2 = 1;
+    while (P2 < N) P2 <<= 1;
+    for (int P = 1; P < P2; P <<= 1)
+        for (int k = P; k; k >>= 1) {
+            std::vector<std::pair<int, int>> visited;
+            for (int j = k % P; j < P2 - k; j += 2 * k)
+                for (int i = 0; i < k && i + j + k < N; i++)
+                    if ((i + j) / (2 * P) == (i + j + k) / (2 * P)) {
+                        schedule(ans[i + j], ans[i + j + k]);
+                        visited.push_back({i + j, i + j + k});
+                    }
+            std::vector<int> visit_res = visit();
+            for (int i = 0; i < visit_res.size(); i++)
+                if (!visit_res[i])
+                    std::swap(ans[visited[i].first], ans[visited[i].second]);
+        }
+    answer(ans);
+}

COI/COI 21-malnarisc.cpp

@@ -1,32 +1,26 @@
-#include <bits/stdc++.h>
-typedef long long ll;
-using namespace std;
+/*
+COI 2021 MalnaRISC
+- This is a nerfed version of Baltic '21 Swaps, so see that solution too
+- https://en.wikipedia.org/wiki/Bitonic_sorter
+- https://en.wikipedia.org/wiki/Batcher_odd%E2%80%93even_mergesort
+- O(log^2 N) steps to sort in parallel
+*/
 
-vector<pair<int, int>> swaps[5000];
-
-int swapsort(int l, int r) {
-    if (l == r) return 0;
-
-    int mid = (l + r) / 2;
-    int depth = max(swapsort(l, mid), swapsort(mid + 1, r));
-
-    for (int i = 0; i <= mid - l; i++) {
-        for (int x = mid, y = r + (mid - l != r - mid - 1) - i; x >= l + i; x--, y--)
-            if (y <= r) swaps[depth + 1 + i].push_back({x, y});
-    }
-    return depth + mid - l + 1;
-}
+#include <stdio.h>
 
 int main() {
-    cin.tie(0)->sync_with_stdio(0);
     int n;
-    cin >> n;
-    int depth = swapsort(1, n);
-    cout << depth << '\n';
-    for (int i = 1; i <= depth; i++) {
-        for (pair<int, int> j : swaps[i]) if (j.second <= n)
-            cout << "CMPSWP R" << j.first << " R" << j.second << ' ';
-        cout << '\n';
-    }
+    scanf("%d", &n);
+    int p2 = 1, blocks = 0;
+    while (p2 < n) p2 <<= 1, blocks++;
+    printf("%d\n", blocks * (blocks + 1) / 2);
+    for (int p = 1; p < p2; p <<= 1)
+        for (int k = p; k; k >>= 1) {
+            for (int j = k % p; j < p2 - k; j += 2 * k)
+                for (int i = 0; i < k && i + j + k < n; i++)
+                    if ((i + j) / (2 * p) == (i + j + k) / (2 * p))
+                        printf("CMPSWP R%d R%d ", i + j + 1, i + j + k + 1);
+            printf("\n");
+        }
     return 0;
 }