Add the Coord extension type

lib/src/models.dart

@@ -223,6 +223,9 @@ extension type const Square._(int value) implements int {
   /// The rank of the square on the board.
   Rank get rank => Rank(value >> 3);
 
+  /// The Zero-based, numeric coordinates of the square on the board.
+  Coord get coord => Coord(value & 0x7, value >> 3);
+
   /// The Algebraic Notation of the square, such as 'a1', 'b2', etc.
   String get algebraicNotation =>
       file.algebraicNotation + rank.algebraicNotation;
@@ -360,6 +363,151 @@ extension type const Square._(int value) implements int {
   ];
 }
 
+/// Zero-based numeric chessboard coordinate.
+///
+/// For instance a1 is (0, 0), a2 is (0, 1), etc.
+extension type const Coord._((int x, int y) value) {
+  /// Constructs a [Coord] from a pair of integers ranging from 0 to 7.
+  const Coord(int x, int y) : this._new((x, y));
+
+  const Coord._new(this.value)
+      : assert(value == (0, 0) ||
+            value == (0, 1) ||
+            value == (0, 2) ||
+            value == (0, 3) ||
+            value == (0, 4) ||
+            value == (0, 5) ||
+            value == (0, 6) ||
+            value == (0, 7) ||
+            value == (1, 0) ||
+            value == (1, 1) ||
+            value == (1, 2) ||
+            value == (1, 3) ||
+            value == (1, 4) ||
+            value == (1, 5) ||
+            value == (1, 6) ||
+            value == (1, 7) ||
+            value == (2, 0) ||
+            value == (2, 1) ||
+            value == (2, 2) ||
+            value == (2, 3) ||
+            value == (2, 4) ||
+            value == (2, 5) ||
+            value == (2, 6) ||
+            value == (2, 7) ||
+            value == (3, 0) ||
+            value == (3, 1) ||
+            value == (3, 2) ||
+            value == (3, 3) ||
+            value == (3, 4) ||
+            value == (3, 5) ||
+            value == (3, 6) ||
+            value == (3, 7) ||
+            value == (4, 0) ||
+            value == (4, 1) ||
+            value == (4, 2) ||
+            value == (4, 3) ||
+            value == (4, 4) ||
+            value == (4, 5) ||
+            value == (4, 6) ||
+            value == (4, 7) ||
+            value == (5, 0) ||
+            value == (5, 1) ||
+            value == (5, 2) ||
+            value == (5, 3) ||
+            value == (5, 4) ||
+            value == (5, 5) ||
+            value == (5, 6) ||
+            value == (5, 7) ||
+            value == (6, 0) ||
+            value == (6, 1) ||
+            value == (6, 2) ||
+            value == (6, 3) ||
+            value == (6, 4) ||
+            value == (6, 5) ||
+            value == (6, 6) ||
+            value == (6, 7) ||
+            value == (7, 0) ||
+            value == (7, 1) ||
+            value == (7, 2) ||
+            value == (7, 3) ||
+            value == (7, 4) ||
+            value == (7, 5) ||
+            value == (7, 6) ||
+            value == (7, 7));
+
+  /// Gets the square from the coordinates.
+  Square get square => Square(value.$2 * 8 + value.$1);
+
+  /// All possible coordinates on the chessboard.
+  static const values = [
+    Coord(0, 0),
+    Coord(0, 1),
+    Coord(0, 2),
+    Coord(0, 3),
+    Coord(0, 4),
+    Coord(0, 5),
+    Coord(0, 6),
+    Coord(0, 7),
+    Coord(1, 0),
+    Coord(1, 1),
+    Coord(1, 2),
+    Coord(1, 3),
+    Coord(1, 4),
+    Coord(1, 5),
+    Coord(1, 6),
+    Coord(1, 7),
+    Coord(2, 0),
+    Coord(2, 1),
+    Coord(2, 2),
+    Coord(2, 3),
+    Coord(2, 4),
+    Coord(2, 5),
+    Coord(2, 6),
+    Coord(2, 7),
+    Coord(3, 0),
+    Coord(3, 1),
+    Coord(3, 2),
+    Coord(3, 3),
+    Coord(3, 4),
+    Coord(3, 5),
+    Coord(3, 6),
+    Coord(3, 7),
+    Coord(4, 0),
+    Coord(4, 1),
+    Coord(4, 2),
+    Coord(4, 3),
+    Coord(4, 4),
+    Coord(4, 5),
+    Coord(4, 6),
+    Coord(4, 7),
+    Coord(5, 0),
+    Coord(5, 1),
+    Coord(5, 2),
+    Coord(5, 3),
+    Coord(5, 4),
+    Coord(5, 5),
+    Coord(5, 6),
+    Coord(5, 7),
+    Coord(6, 0),
+    Coord(6, 1),
+    Coord(6, 2),
+    Coord(6, 3),
+    Coord(6, 4),
+    Coord(6, 5),
+    Coord(6, 6),
+    Coord(6, 7),
+    Coord(7, 0),
+    Coord(7, 1),
+    Coord(7, 2),
+    Coord(7, 3),
+    Coord(7, 4),
+    Coord(7, 5),
+    Coord(7, 6),
+    Coord(7, 7)
+  ];
+}
+
 typedef BySide<T> = IMap<Side, T>;
 typedef ByRole<T> = IMap<Role, T>;
 typedef ByCastlingSide<T> = IMap<CastlingSide, T>;

test/models_test.dart

@@ -55,6 +55,29 @@ void main() {
       expect(Square.h8.offset(-8), Square.h7);
       expect(Square.h8.offset(1), null);
     });
+
+    test('algebraicNotation', () {
+      expect(Square.a1.algebraicNotation, 'a1');
+      expect(Square.h8.algebraicNotation, 'h8');
+    });
+
+    test('coord', () {
+      expect(Square.a1.coord, const Coord(0, 0));
+      expect(Square.c6.coord, const Coord(2, 5));
+      expect(Square.h8.coord, const Coord(7, 7));
+    });
+  });
+
+  group('Coord', () {
+    test('Coord.values', () {
+      expect(Coord.values.length, 64);
+    });
+
+    test('square', () {
+      expect(const Coord(0, 0).square, Square.a1);
+      expect(const Coord(2, 5).square, Square.c6);
+      expect(const Coord(7, 7).square, Square.h8);
+    });
   });
 
   group('Move', () {