Add Convex Hull in C++ (#5537)

Author: pascalecu

archive/c/c-plus-plus/convex-hull.cpp

@@ -0,0 +1,153 @@
+#include <algorithm>
+#include <iostream>
+#include <sstream>
+#include <string>
+#include <vector>
+
+struct Point
+{
+    int x{};
+    int y{};
+
+    Point() = default;
+    Point(int x_, int y_) : x(x_), y(y_)
+    {
+    }
+
+    bool operator<(const Point &other) const
+    {
+        if (x != other.x)
+        {
+            return x < other.x;
+        }
+        return y < other.y;
+    }
+};
+
+[[noreturn]] void usage()
+{
+    std::cerr
+        << R"(Usage: please provide at least 3 x and y coordinates as separate lists (e.g. "100, 440, 210"))"
+        << '\n';
+    std::exit(1);
+}
+
+std::vector<int> parseList(const std::string &input)
+{
+    std::vector<int> out;
+    std::stringstream ss(input);
+
+    for (std::string token; std::getline(ss, token, ',');)
+    {
+        auto start = token.find_first_not_of(" \t");
+        if (start == std::string::npos)
+        {
+            usage();
+        }
+
+        const auto end = token.find_last_not_of(" \t");
+        token = token.substr(start, end - start + 1);
+
+        size_t pos = 0;
+        int value = 0;
+
+        try
+        {
+            size_t pos = 0;
+            const int value = std::stoi(token, &pos);
+
+            if (pos != token.size())
+            {
+                usage();
+            }
+
+            out.push_back(value);
+        }
+        catch (...)
+        {
+            usage();
+        }
+    }
+
+    return out;
+}
+
+std::vector<Point> parseCoordinates(const std::string &xs,
+                                    const std::string &ys)
+{
+    auto x = parseList(xs);
+    auto y = parseList(ys);
+
+    if (x.size() != y.size() || x.size() < 3)
+    {
+        usage();
+    }
+
+    std::vector<Point> pts;
+    pts.reserve(x.size());
+
+    std::transform(x.begin(), x.end(), y.begin(), std::back_inserter(pts),
+                   [](int a, int b) { return Point{a, b}; });
+
+    return pts;
+}
+
+long long cross(const Point &a, const Point &b, const Point &c)
+{
+    return 1LL * (b.x - a.x) * (c.y - a.y) - 1LL * (b.y - a.y) * (c.x - a.x);
+}
+
+std::vector<Point> convexHull(std::vector<Point> pts)
+{
+    if (pts.size() < 3)
+    {
+        usage();
+    }
+
+    std::sort(pts.begin(), pts.end());
+
+    std::vector<Point> lower, upper;
+    lower.reserve(pts.size());
+    upper.reserve(pts.size());
+
+    auto build_half = [&](auto begin, auto end, auto &hull) {
+        for (auto it = begin; it != end; ++it)
+        {
+            const auto &p = *it;
+
+            while (hull.size() >= 2 &&
+                   cross(hull[hull.size() - 2], hull.back(), p) <= 0)
+            {
+                hull.pop_back();
+            }
+
+            hull.push_back(p);
+        }
+    };
+
+    build_half(pts.begin(), pts.end(), lower);
+    build_half(pts.rbegin(), pts.rend(), upper);
+
+    lower.pop_back();
+    upper.pop_back();
+
+    lower.insert(lower.end(), std::make_move_iterator(upper.begin()),
+                 std::make_move_iterator(upper.end()));
+
+    return lower;
+}
+
+int main(int argc, char *argv[])
+{
+    if (argc != 3)
+    {
+        usage();
+    }
+
+    const auto hull = convexHull(parseCoordinates(argv[1], argv[2]));
+
+    for (const auto &[x, y] : hull)
+    {
+        std::cout << '(' << x << ", " << y << ")\n";
+    }
+}