Экскурс в историю

public interface IPath
{
    IReadOnlyCollection<Vector2Int> Calculate(Vector2Int start, Vector2Int target, 
        IReadOnlyCollection<Vector2Int> obstacles);
}

Поиск в глубину

1. Добавить в стопку все непосещенные вершины рядом с текущей.
2. Перейти в первую вершину из стопки.
3. Если вершина искомая — выходим из цикла. Если пришли в тупик — вернуться назад.
4. GOTO 1.

private Node DFS(Node first, Node target)
{
    List<Node> visited = new List<Node>();
    Stack<Node> frontier = new Stack<Node>();
    frontier.Push(first);
            
    while (frontier.Count > 0)
    {
        var current = frontier.Pop();
        visited.Add(current);
               
        if (current == target)
            return Node;

        var neighbours = current.GenerateNeighbours();
        foreach(var neighbour in neighbours)
            if (!visited.Contains(neighbour))
                frontier.Push(neighbour);
    }

    return default;
}

1. Задать минимальную область для поиска в глубину.
2. Запустить поиск.
3. Если путь не найден, увеличиваем область поиска на 1.
4. GOTO 2.

Поиск в ширину

1. Добавить всех непосещённых соседей в очередь.
2. Перейти в первую вершину из очереди.
3. Если вершина искомая — выходим из цикла, иначе GOTO 1.

private Node BFS(Node first, Node target)
{
    List<Node> visited = new List<Node>();
    Queue<Node> frontier = new Queue<Node>();
    frontier.Enqueue(first);
            
    while (frontier.Count > 0)
    {
        var current = frontier.Dequeue();
        visited.Add(current);
                
        if (current == target)
            return Node;

        var neighbours = current.GenerateNeighbours();
        foreach(var neighbour in neighbours)
            if (!visited.Contains(neighbour))
                frontier.Enqueue(neighbour);
    }

    return default;
}

A*

public readonly struct Vector2Int : IEquatable<Vector2Int>
{
    private static readonly double Sqr = Math.Sqrt(2);

    public Vector2Int(int x, int y)
    {
        X = x;
        Y = y;
    }

    public int X { get; }
    public int Y { get; }
        
    /// <summary>
    /// Estimated path distance without obstacles.
    /// </summary>
    public double DistanceEstimate()
    {
        int linearSteps = Math.Abs(Math.Abs(Y) - Math.Abs(X));
        int diagonalSteps = Math.Max(Math.Abs(Y), Math.Abs(X)) - linearSteps;
        return linearSteps + Sqr * diagonalSteps;
    }
        
    public static Vector2Int operator +(Vector2Int a, Vector2Int b) 
        => new Vector2Int(a.X + b.X, a.Y + b.Y);
    public static Vector2Int operator -(Vector2Int a, Vector2Int b) 
        => new Vector2Int(a.X - b.X, a.Y - b.Y);
    public static bool operator ==(Vector2Int a, Vector2Int b) 
        => a.X == b.X && a.Y == b.Y;
    public static bool operator !=(Vector2Int a, Vector2Int b) 
        => !(a == b);

    public bool Equals(Vector2Int other)
        => X == other.X && Y == other.Y;

    public override bool Equals(object obj)
    {
        if (!(obj is Vector2Int))
            return false;

        var other = (Vector2Int) obj;
        return X == other.X && Y == other.Y;
    }

    public override int GetHashCode()
        => HashCode.Combine(X, Y);

    public override string ToString()
        => $"({X}, {Y})";
}

return Math.Max(Math.Abs(X), Math.Abs(Y))

internal class PathNode
{
    public PathNode(Vector2Int position, double traverseDist, double heuristicDist, PathNode parent)
    {
        Position = position;
        TraverseDistance = traverseDist;
        Parent = parent;
        EstimatedTotalCost = TraverseDistance + heuristicDist;
    }

    public Vector2Int Position { get; }
    public double TraverseDistance { get; }
    public double EstimatedTotalCost { get; }
    public PathNode Parent { get; }
}

internal static class NodeExtensions
{
    private static readonly (Vector2Int position, double cost)[] NeighboursTemplate = {
        (new Vector2Int(1, 0), 1),
        (new Vector2Int(0, 1), 1),
        (new Vector2Int(-1, 0), 1),
        (new Vector2Int(0, -1), 1),
        (new Vector2Int(1, 1), Math.Sqrt(2)),
        (new Vector2Int(1, -1), Math.Sqrt(2)),
        (new Vector2Int(-1, 1), Math.Sqrt(2)),
        (new Vector2Int(-1, -1), Math.Sqrt(2))
    };
        
    public static IEnumerable<PathNode> GenerateNeighbours(this PathNode parent, Vector2Int target)
    {
        foreach ((Vector2Int position, double cost) in NeighboursTemplate)
        {
            Vector2Int nodePosition = position + parent.Position;
            double traverseDistance = parent.TraverseDistance + cost;
            double heuristicDistance = (position - target).DistanceEstimate();
            yield return new PathNode(nodePosition, traverseDistance, heuristicDistance, parent);
        }
    }
}

public class Path
{
    private readonly List<PathNode> frontier = new List<PathNode>();
    private readonly List<Vector2Int> ignoredPositions = new List<Vector2Int>();

    public IReadOnlyCollection<Vector2Int> Calculate(Vector2Int start, Vector2Int target, 
        IReadOnlyCollection<Vector2Int> obstacles) {...}
		
    private bool GenerateNodes(Vector2Int start, Vector2Int target, 
        IEnumerable<Vector2Int> obstacles, out PathNode node)
    {
        frontier.Clear();
        ignoredPositions.Clear();
        ignoredPositions.AddRange(obstacles);

        // Add starting point.
        frontier.Add(new PathNode(start, 0, 0, null));

        while (frontier.Any())
        {
            // Find node with lowest estimated cost.
            var lowest = frontier.Min(a => a.EstimatedTotalCost);
            PathNode current = frontier.First(a => a.EstimatedTotalCost == lowest);
            ignoredPositions.Add(current.Position);

            if (current.Position.Equals(target))
            {
                node = current;
                return true;
            }

            GenerateFrontierNodes(current, target);
        }

        // All nodes analyzed - no path detected.
        node = null;
        return false;
    }

    private void GenerateFrontierNodes(PathNode parent, Vector2Int target)
    {
        foreach (PathNode newNode in parent.GenerateNeighbours(target))
        {
            // Position is already checked or occupied by an obstacle.
            if (ignoredPositions.Contains(newNode.Position)) continue;

            // Node is not present in queue.
            var node = frontier.FirstOrDefault(a => a.Position == newNode.Position);
            if (node == null)
                frontier.Add(newNode);

            // Node is present in queue and new optimal path is detected.
            else if (newNode.TraverseDistance < node.TraverseDistance)
            {
                frontier.Remove(node);
                frontier.Add(newNode);
            }
        }
    }
}

public IReadOnlyCollection<Vector2Int> Calculate(Vector2Int start, Vector2Int target,
     IReadOnlyCollection<Vector2Int> obstacles)
{
    if (!GenerateNodes(start, target, obstacles, out PathNode node)) 
        return Array.Empty<Vector2Int>();

    var output = new List<Vector2Int>();

    while (node.Parent != null)
    {
        output.Add(node.Position);
        node = node.Parent;
    }

    output.Add(start);
    return output.AsReadOnly();
}

Оптимизация алгоритма

1. PathNode

internal readonly struct PathNode
{
    public PathNode(Vector2Int position, Vector2Int target, double traverseDistance)
    {
        Position = position;
        TraverseDistance = traverseDistance;
        double heuristicDistance = (position - target).DistanceEstimate();
        EstimatedTotalCost = traverseDistance + heuristicDistance;
    }

    public Vector2Int Position { get; }
    public double TraverseDistance { get; }
    public double EstimatedTotalCost { get; }
}

private readonly Dictionary<Vector2Int, Vector2Int> links;

private void GenerateFrontierNodes(PathNode parent, Vector2Int target)
{
    foreach (PathNode newNode in parent.GenerateNeighbours(target))
    {
        // Position is already checked or occupied by an obstacle.
        if (ignoredPositions.Contains(newNode.Position)) continue;

        // Node is not present in queue.
        if (!frontier.TryGet(newNode.Position, out PathNode existingNode))
        {
            frontier.Enqueue(newNode);
            links[newNode.Position] = parent.Position; // Add link to dictionary.
        }

        // Node is present in queue and new optimal path is detected.
        else if (newNode.TraverseDistance < existingNode.TraverseDistance)
        {
            frontier.Modify(newNode);
            links[newNode.Position] = parent.Position; // Add link to dictionary.
        }
    }
}

public IReadOnlyCollection<Vector2Int> Calculate(Vector2Int start, Vector2Int target, 
    IReadOnlyCollection<Vector2Int> obstacles)
{
    if (!GenerateNodes(start, target, obstacles)) 
        return Array.Empty<Vector2Int>();

    var output = new List<Vector2Int> {target};
    while (links.TryGetValue(target, out target)) 
        output.Add(target);
    return output.AsReadOnly();
}

2. NodeExtensions

internal static class NodeExtensions
{
    private static readonly (Vector2Int position, double cost)[] NeighboursTemplate = {
        (new Vector2Int(1, 0), 1),
        (new Vector2Int(0, 1), 1),
        (new Vector2Int(-1, 0), 1),
        (new Vector2Int(0, -1), 1),
        (new Vector2Int(1, 1), Math.Sqrt(2)),
        (new Vector2Int(1, -1), Math.Sqrt(2)),
        (new Vector2Int(-1, 1), Math.Sqrt(2)),
        (new Vector2Int(-1, -1), Math.Sqrt(2))
    };
        
    public static void Fill(this PathNode[] buffer, PathNode parent, Vector2Int target)
    {
        int i = 0;
        foreach ((Vector2Int position, double cost) in NeighboursTemplate)
        {
            Vector2Int nodePosition = position + parent.Position;
            double traverseDistance = parent.TraverseDistance + cost;
            buffer[i++] = new PathNode(nodePosition, target, traverseDistance);
        }
    }
}

private const int MaxNeighbours = 8;
private readonly PathNode[] neighbours = new PathNode[MaxNeighbours];

neighbours.Fill(parent, target);
foreach(var neighbour in neighbours)
{
   // ...

3. HashSet

private readonly HashSet<Vector2Int> ignoredPositions;

4. BinaryHeap

// Find node with lowest estimated cost.
var lowest = frontier.Min(a => a.EstimatedTotalCost);
PathNode current = frontier.First(a => a.EstimatedTotalCost == lowest);

• Автоматической сортировкой.
• Низким ростом асимптотической сложности.
• Быстрой операцией вставки.
• Быстрой операцией удаления.
• Легкой навигацией по элементам.

internal interface IBinaryHeap<in TKey, T>
{
    void Enqueue(T item);
    T Dequeue();
    void Clear();
    bool TryGet(TKey key, out T value);
    void Modify(T value);
    int Count { get; }
}

internal class BinaryHeap : IBinaryHeap<Vector2Int, PathNode>
{
    private readonly IDictionary<Vector2Int, int> map;
    private readonly IList<PathNode> collection;
    private readonly IComparer<PathNode> comparer;
        
    public BinaryHeap(IComparer<PathNode> comparer)
    {
        this.comparer = comparer;
        collection = new List<PathNode>();
        map = new Dictionary<Vector2Int, int>();
    }

    public int Count => collection.Count;

    public void Clear()
    {
    collection.Clear();
    map.Clear();
    }		

    // ...
}

public void Enqueue(PathNode item)
{
    collection.Add(item);
    int i = collection.Count - 1;
    map[item.Position] = i;

    // Sort nodes from bottom to top.
    while(i > 0)
    {
        int j = (i - 1) / 2;
                
        if (comparer.Compare(collection[i], collection[j]) <= 0)
            break;

        Swap(i, j);
        i = j;
    }
}
	
private void Swap(int i, int j)
{
    PathNode temp = collection[i];
    collection[i] = collection[j];
    collection[j] = temp;
    map[collection[i].Position] = i;
    map[collection[j].Position] = j;
}

public PathNode Dequeue()
{
    if (collection.Count == 0) return default;
          
    PathNode result = collection.First();
    RemoveRoot();
    map.Remove(result.Position);
    return result;
}
	
private void RemoveRoot()
{
    collection[0] = collection.Last();
    map[collection[0].Position] = 0;
    collection.RemoveAt(collection.Count - 1);

    // Sort nodes from top to bottom.
    var i = 0;
    while(true)
    {
        int largest = LargestIndex(i);
        if (largest == i)
            return;

        Swap(i, largest);
        i = largest;
    }
}
	
private int LargestIndex(int i)
{
    int leftInd = 2 * i + 1;
    int rightInd = 2 * i + 2;
    int largest = i;

    if (leftInd < collection.Count 
        && comparer.Compare(collection[leftInd], collection[largest]) > 0) 
        largest = leftInd;

    if (rightInd < collection.Count 
        && comparer.Compare(collection[rightInd], collection[largest]) > 0) 
        largest = rightInd;
            
    return largest;
}

public bool TryGet(Vector2Int key, out PathNode value)
{
    if (!map.TryGetValue(key, out int index))
    {
        value = default;
        return false;
    }
            
    value = collection[index];
    return true;
}

public void Modify(PathNode value)
{
    if (!map.TryGetValue(value.Position, out int index))
        throw new KeyNotFoundException(nameof(value));
            
    collection[index] = value;
}

internal class BinaryHeap<TKey, T> : IBinaryHeap<TKey, T> where TKey : IEquatable<TKey>
{
    private readonly IDictionary<TKey, int> map;
    private readonly IList<T> collection;
    private readonly IComparer<T> comparer;
    private readonly Func<T, TKey> lookupFunc;
        
    public BinaryHeap(IComparer<T> comparer, Func<T, TKey> lookupFunc, int capacity)
    {
        this.comparer = comparer;
        this.lookupFunc = lookupFunc;
        collection = new List<T>(capacity);
        map = new Dictionary<TKey, int>(capacity);
    }

    public int Count => collection.Count;

    public void Enqueue(T item)
    {
        collection.Add(item);
        int i = collection.Count - 1;
        map[lookupFunc(item)] = i;
        while(i > 0)
        {
            int j = (i - 1) / 2;
                
            if (comparer.Compare(collection[i], collection[j]) <= 0)
                break;

            Swap(i, j);
            i = j;
        }
    }

    public T Dequeue()
    {
        if (collection.Count == 0) return default;
            
        T result = collection.First();
        RemoveRoot();
        map.Remove(lookupFunc(result));
        return result;
    }

    public void Clear()
    {
        collection.Clear();
        map.Clear();
    }

    public bool TryGet(TKey key, out T value)
    {
        if (!map.TryGetValue(key, out int index))
        {
            value = default;
            return false;
        }
            
        value = collection[index];
        return true;
    }

    public void Modify(T value)
    {
        if (!map.TryGetValue(lookupFunc(value), out int index))
            throw new KeyNotFoundException(nameof(value));
            
        collection[index] = value;
    }
        
    private void RemoveRoot()
    {
        collection[0] = collection.Last();
        map[lookupFunc(collection[0])] = 0;
        collection.RemoveAt(collection.Count - 1);

        var i = 0;
        while(true)
        {
            int largest = LargestIndex(i);
            if (largest == i)
                return;

            Swap(i, largest);
            i = largest;
        }
    }

    private void Swap(int i, int j)
    {
        T temp = collection[i];
        collection[i] = collection[j];
        collection[j] = temp;
        map[lookupFunc(collection[i])] = i;
        map[lookupFunc(collection[j])] = j;
    }

    private int LargestIndex(int i)
    {
        int leftInd = 2 * i + 1;
        int rightInd = 2 * i + 2;
        int largest = i;

        if (leftInd < collection.Count && comparer.Compare(collection[leftInd], collection[largest]) > 0) largest = leftInd;

        if (rightInd < collection.Count && comparer.Compare(collection[rightInd], collection[largest]) > 0) largest = rightInd;
            
        return largest;
    }
}

5. Переиспользуемые коллекции

private const int MaxNeighbours = 8;
private readonly PathNode[] neighbours = new PathNode[MaxNeighbours];

private readonly IBinaryHeap<Vector2Int, PathNode> frontier;
private readonly HashSet<Vector2Int> ignoredPositions;
private readonly IList<Vector2Int> output;
private readonly IDictionary<Vector2Int, Vector2Int> links;

public Path()
{
    var comparer = Comparer<PathNode>.Create((a, b) => b.EstimatedTotalCost.CompareTo(a.EstimatedTotalCost));
    frontier = new BinaryHeap(comparer);
    ignoredPositions = new HashSet<Vector2Int>();
    output = new List<Vector2Int>();
    links = new Dictionary<Vector2Int, Vector2Int>();
}

public IReadOnlyCollection<Vector2Int> Calculate(Vector2Int start, Vector2Int target, 
    IReadOnlyCollection<Vector2Int> obstacles)
{
        if (!GenerateNodes(start, target, obstacles)) 
                return Array.Empty<Vector2Int>();

        output.Clear();
        output.Add(target);

        while (links.TryGetValue(target, out target)) 
            output.Add(target);
        return output.AsReadOnly();
}

6. (Бонус) Объявление размера коллекций

• Лучше перенести нагрузку в конструктор класса Path или оставить на первом вызове поиска пути?
• Какую вместимость скармливать коллекциям?
• Дешевле объявить сразу большую вместимость или позволить коллекции расширяться самой?

public Path(int maxSteps)
{
    this.maxSteps = maxSteps;
    var comparer = Comparer<PathNode>.Create((a, b) => b.EstimatedTotalCost.CompareTo(a.EstimatedTotalCost));
    frontier = new BinaryHeap(comparer, maxSteps);
    ignoredPositions = new HashSet<Vector2Int>(maxSteps);
    output = new List<Vector2Int>(maxSteps);
    links = new Dictionary<Vector2Int, Vector2Int>(maxSteps);
}

var step = 0;
while (frontier.Count > 0 && step++ <= maxSteps) 
{
   // ...
}

Что ещё можно сделать?

• Добавить валидацию аргументов.
• Написать XML-документацию.
• Сохранить хеш для метода GetHashCode в локальную переменную в классе Vector2Int. Вектор иммутабелен, соответственно, нет опасности, что хеш будет изменен в течение жизни объекта.
• Перенести запись массива препятствий в конструктор, если все препятствия в нашей модели статичны. Нет смысла с каждой итерацией скармливать алгоритму один и тот же массив препятствий.
• Реализовать интерфейс IComparable для PathNode вместо кастомного EqualityComparer. Это даст очень небольшой рост производительности и сделает конструкторы чуть почище.
• Переиспользование данных: перед очисткой коллекций проверять, изменилось ли положение исходной и конечной точки. Если положение сохранено или изменилось не сильно, можно вычислить путь частичным алгоритмом, а не выполнять всю работу с нуля.
• Изменить сигнатуру метода в нашем интерфейсе, чтобы лучше отражать его суть:
  
  

  bool Calculate(Vector2Int start, Vector2Int target, 
      IReadOnlyCollection<Vector2Int> obstacles, 
      out IReadOnlyCollection<Vector2Int> path);

  
  Таким образом по методу кристально видна логика его работы, в то время как оригинальный метод следовало бы назвать так: CalculateOrReturnEmpty.

/// <summary>
/// Reusable A* path finder.
/// </summary>
public class Path : IPath
{
    private const int MaxNeighbours = 8;
    private readonly PathNode[] neighbours = new PathNode[MaxNeighbours];

    private readonly int maxSteps;
    private readonly IBinaryHeap<Vector2Int, PathNode> frontier;
    private readonly HashSet<Vector2Int> ignoredPositions;
    private readonly List<Vector2Int> output;
    private readonly IDictionary<Vector2Int, Vector2Int> links;

    /// <summary>
    /// Creation of new path finder.
    /// </summary>
    /// <exception cref="ArgumentOutOfRangeException"></exception>
    public Path(int maxSteps = int.MaxValue, int initialCapacity = 0)
    {
        if (maxSteps <= 0) 
            throw new ArgumentOutOfRangeException(nameof(maxSteps));
        if (initialCapacity < 0) 
            throw new ArgumentOutOfRangeException(nameof(initialCapacity));

        this.maxSteps = maxSteps;
        var comparer = Comparer<PathNode>.Create((a, b) => b.EstimatedTotalCost.CompareTo(a.EstimatedTotalCost));
        frontier = new BinaryHeap<Vector2Int, PathNode>(comparer, a => a.Position, initialCapacity);
        ignoredPositions = new HashSet<Vector2Int>(initialCapacity);
        output = new List<Vector2Int>(initialCapacity);
        links = new Dictionary<Vector2Int, Vector2Int>(initialCapacity);
    }

    /// <summary>
    /// Calculate a new path between two points.
    /// </summary>
    /// <exception cref="ArgumentNullException"></exception>
    public bool Calculate(Vector2Int start, Vector2Int target, 
        IReadOnlyCollection<Vector2Int> obstacles, 
        out IReadOnlyCollection<Vector2Int> path)
    {
        if (obstacles == null) 
            throw new ArgumentNullException(nameof(obstacles));

        if (!GenerateNodes(start, target, obstacles))
        {
            path = Array.Empty<Vector2Int>();
            return false;
        }

        output.Clear();
        output.Add(target);

        while (links.TryGetValue(target, out target)) output.Add(target);
        path = output;
        return true;
    }

    private bool GenerateNodes(Vector2Int start, Vector2Int target, IReadOnlyCollection<Vector2Int> obstacles)
    {
        frontier.Clear();
        ignoredPositions.Clear();
        links.Clear();

        frontier.Enqueue(new PathNode(start, target, 0));
        ignoredPositions.UnionWith(obstacles);
        var step = 0;
        while (frontier.Count > 0 && step++ <= maxSteps)
        {
            PathNode current = frontier.Dequeue();
            ignoredPositions.Add(current.Position);

            if (current.Position.Equals(target)) return true;

            GenerateFrontierNodes(current, target);
        }

        // All nodes analyzed - no path detected.
        return false;
    }

    private void GenerateFrontierNodes(PathNode parent, Vector2Int target)
    {
        neighbours.Fill(parent, target);
        foreach (PathNode newNode in neighbours)
        {
            // Position is already checked or occupied by an obstacle.
            if (ignoredPositions.Contains(newNode.Position)) continue;

            // Node is not present in queue.
            if (!frontier.TryGet(newNode.Position, out PathNode existingNode))
            {
                frontier.Enqueue(newNode);
                links[newNode.Position] = parent.Position;
            }

            // Node is present in queue and new optimal path is detected.
            else if (newNode.TraverseDistance < existingNode.TraverseDistance)
            {
                frontier.Modify(newNode);
                links[newNode.Position] = parent.Position;
            }
        }
    }
}