Add Convex Hull in C# (TheRenegadeCoder#4873)

Author: pascalecu

archive/c/c-sharp/ConvexHull.cs

@@ -0,0 +1,135 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+public record Point(int X, int Y) : IComparable<Point>
+{
+    public int CompareTo(Point? other)
+         => other is null ? 1 : X != other.X ? X.CompareTo(other.X) : Y.CompareTo(other.Y);
+
+    public override string ToString() => $"({X}, {Y})";
+
+    public static bool operator <(Point left, Point right) => left.CompareTo(right) < 0;
+    public static bool operator >(Point left, Point right) => left.CompareTo(right) > 0;
+}
+
+public static class ConvexHull
+{
+    private static void ShowUsage()
+    {
+        Console.Error.WriteLine("Usage: please provide at least 3 x and y coordinates as separate lists (e.g. \"100, 440, 210\")");
+    }
+
+    private static List<int> ParseIntegerList(string input)
+    {
+        if (string.IsNullOrWhiteSpace(input))
+        {
+            ShowUsage();
+            Environment.Exit(1);
+        }
+
+        var list = input
+            .Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+            .Select(part => int.TryParse(part, out var val)
+                ? val
+                : throw new ArgumentException($"Invalid integer value: '{part}'"))
+            .ToList();
+
+        if (list.Count < 3)
+        {
+            ShowUsage();
+            Environment.Exit(1);
+        }
+
+        return list;
+    }
+
+
+    /// <summary>
+    /// Calculates the cross product of vectors OA and OB.
+    /// Positive result means counter-clockwise turn.
+    /// Negative result means clockwise turn.
+    /// Zero means points are colinear.
+    /// </summary>
+    private static long Cross(Point o, Point a, Point b)
+    {
+        var (ox, oy) = (o.X, o.Y);
+        var (ax, ay) = (a.X, a.Y);
+        var (bx, by) = (b.X, b.Y);
+        return (long)(ax - ox) * (by - oy) - (long)(ay - oy) * (bx - ox);
+    }
+
+
+    /// <summary>
+    /// Constructs the convex hull using the Jarvis March algorithm.
+    /// </summary>
+    private static List<Point> BuildHull(List<Point> points)
+    {
+        int n = points.Count;
+        if (n < 3) return [.. points];
+
+        int startIndex = 0;
+        for (int i = 1; i < n; i++)
+        {
+            if (points[i] < points[startIndex])
+                startIndex = i;
+        }
+
+        var hull = new List<Point>();
+        int currentIndex = startIndex;
+
+        do
+        {
+            hull.Add(points[currentIndex]);
+            int candidateIndex = (currentIndex + 1) % n;
+
+            for (int i = 0; i < n; i++)
+            {
+                if (Cross(points[currentIndex], points[i], points[candidateIndex]) > 0)
+                    candidateIndex = i;
+            }
+
+            currentIndex = candidateIndex;
+
+        } while (currentIndex != startIndex);
+
+        return hull;
+    }
+
+    public static int Main(string[] args)
+    {
+        if (args.Length != 2)
+        {
+            ShowUsage();
+            return 1;
+        }
+
+        try
+        {
+            var xCoords = ParseIntegerList(args[0]);
+            var yCoords = ParseIntegerList(args[1]);
+            if (xCoords.Count != yCoords.Count)
+            {
+                ShowUsage();
+                return 1;
+            }
+
+            if (xCoords.Count < 3)
+            {
+                ShowUsage();
+                return 1;
+            }
+
+            var points = xCoords.Zip(yCoords, (x, y) => new Point(x, y)).ToList();
+
+            BuildHull(points).ForEach(Console.WriteLine);
+
+            return 0;
+        }
+        catch
+        {
+            ShowUsage();
+            return 1;
+        }
+    }
+}