riversong code showcase

Source/Engine/Helpers/TopologicalSort.cs

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+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace DanieleMarotta.RiversongCodeShowcase
+{
+    public class TopologicalSort<T>
+    {
+        private HashSet<T> _currentDependencies;
+
+        private HashSet<T> _closed;
+
+        public TopologicalSort(IEqualityComparer<T> comparer)
+        {
+            _currentDependencies = new HashSet<T>(comparer);
+            _closed = new HashSet<T>(comparer);
+        }
+
+        public TopologicalSort() : this(EqualityComparer<T>.Default)
+        {
+        }
+
+        public static TopologicalSort<T> Default { get; } = new();
+
+        public IEnumerable<T> Sort(IEnumerable<T> source, Func<T, List<T>> dependenciesGetter, ICollection<T> sorted = null)
+        {
+            sorted ??= new List<T>();
+
+            try
+            {
+                foreach (var item in source) Visit(item, dependenciesGetter, sorted);
+            }
+            finally
+            {
+                _currentDependencies.Clear();
+                _closed.Clear();
+            }
+
+            return sorted;
+        }
+
+        private void Visit(T item, Func<T, List<T>> dependenciesGetter, ICollection<T> sorted)
+        {
+            if (!_currentDependencies.Add(item))
+            {
+                var ex = new StringBuilder();
+                ex.AppendLine(item.ToString());
+                foreach (var dependency in _currentDependencies) ex.AppendLine(dependency.ToString());
+
+                throw new InvalidOperationException(ex.ToString());
+            }
+
+            var dependencies = dependenciesGetter.Invoke(item);
+            if (dependencies != null)
+                foreach (var dependency in dependencies)
+                    Visit(dependency, dependenciesGetter, sorted);
+
+            _currentDependencies.Remove(item);
+
+            if (_closed.Add(item)) sorted.Add(item);
+        }
+    }
+}