feat: initial project structure with Snake.Core, CLI and Avalonia skeleton

Snake.Core/Snake.cs

@@ -0,0 +1,80 @@
+namespace Snake.Core;
+
+public sealed class Snake
+{
+    private readonly LinkedList<Position> _segments = new();
+
+    public Snake(Position start, int length, Direction direction)
+    {
+        if (length < 1)
+            throw new ArgumentOutOfRangeException(nameof(length), "Length must be at least 1.");
+
+        Direction = direction;
+
+        var offset = DirectionToOffset(direction);
+        for (var i = 0; i < length; i++)
+        {
+            _segments.AddLast(new Position(
+                start.X - offset.X * i,
+                start.Y - offset.Y * i));
+        }
+    }
+
+    public Direction Direction { get; private set; }
+
+    public Position Head => _segments.First!.Value;
+
+    public IReadOnlyCollection<Position> Segments => _segments;
+
+    public void SetDirection(Direction direction)
+    {
+        if (direction == Direction)
+            return;
+
+        if (AreOpposite(Direction, direction))
+            return;
+
+        Direction = direction;
+    }
+
+    public Position PeekNextHead()
+    {
+        var offset = DirectionToOffset(Direction);
+        return new Position(Head.X + offset.X, Head.Y + offset.Y);
+    }
+
+    public Position Move(bool grow = false)
+    {
+        var newHead = PeekNextHead();
+
+        _segments.AddFirst(newHead);
+
+        if (!grow)
+            _segments.RemoveLast();
+
+        return newHead;
+    }
+
+    public bool Occupies(Position position) =>
+        _segments.Contains(position);
+
+    private static Position DirectionToOffset(Direction direction) =>
+        direction switch
+        {
+            Direction.Up => new Position(0, -1),
+            Direction.Down => new Position(0, 1),
+            Direction.Left => new Position(-1, 0),
+            Direction.Right => new Position(1, 0),
+            _ => throw new ArgumentOutOfRangeException(nameof(direction))
+        };
+
+    private static bool AreOpposite(Direction current, Direction next) =>
+        (current, next) switch
+        {
+            (Direction.Up, Direction.Down) => true,
+            (Direction.Down, Direction.Up) => true,
+            (Direction.Left, Direction.Right) => true,
+            (Direction.Right, Direction.Left) => true,
+            _ => false
+        };
+}