39 lines
1.1 KiB
C++
39 lines
1.1 KiB
C++
struct convex_hull {
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int n, m;
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vector<int> on_hull;
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// WARN: counter-clockwise
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vector<int> hull;
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// WARN: sort the points before initializing
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template <typename T>
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convex_hull(const vector<point<T>>& a) : n(a.size()), on_hull(n) {
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for (int i = 0; i < n; ++i) {
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int m = hull.size();
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// WARN: keeping the points that are on edges
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while (m > 1 and (a[hull[m - 1]] - a[hull[m - 2]]) * (a[i] - a[hull[m - 2]]) < 0) {
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on_hull[hull[m - 1]] -= 1;
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hull.pop_back();
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m -= 1;
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}
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on_hull[i] += 1;
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hull.emplace_back(i);
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}
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int low = hull.size();
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for (int i = n - 2; ~i; --i) {
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int m = hull.size();
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while (m > low and (a[hull[m - 1]] - a[hull[m - 2]]) * (a[i] - a[hull[m - 2]]) < 0) {
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on_hull[hull[m - 1]] -= 1;
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hull.pop_back();
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m -= 1;
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}
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on_hull[i] += 1;
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hull.emplace_back(i);
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}
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if (n > 1) {
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hull.pop_back();
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}
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m = hull.size();
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}
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};
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