Domineering as a combinatorial game.

We define the game of Domineering, played on a chessboard of arbitrary shape (possibly even disconnected). Left moves by placing a domino vertically, while Right moves by placing a domino horizontally.

This is only a fragment of a full development; in order to successfully analyse positions we would need some more theorems. Most importantly, we need a general statement that allows us to discard irrelevant moves. Specifically to domineering, we need the fact that disjoint parts of the chessboard give sums of games.

def SetTheory.PGame.Domineering.shiftUp : ℤ × ℤ ≃ ℤ × ℤ

The equivalence (x, y) ↦ (x, y+1).

@[simp]

theorem SetTheory.PGame.Domineering.shiftUp_apply (a✝ : ℤ × ℤ) : shiftUp a✝ = Prod.map id (fun (x : ℤ) => x + 1) a✝

@[simp]

theorem SetTheory.PGame.Domineering.shiftUp_symm_apply (a✝ : ℤ × ℤ) : shiftUp.symm a✝ = Prod.map id (fun (x : ℤ) => x + -1) a✝

def SetTheory.PGame.Domineering.shiftRight : ℤ × ℤ ≃ ℤ × ℤ

The equivalence (x, y) ↦ (x+1, y).

@[simp]

theorem SetTheory.PGame.Domineering.shiftRight_symm_apply (a✝ : ℤ × ℤ) : shiftRight.symm a✝ = Prod.map (fun (x : ℤ) => x + -1) id a✝

@[simp]

theorem SetTheory.PGame.Domineering.shiftRight_apply (a✝ : ℤ × ℤ) : shiftRight a✝ = Prod.map (fun (x : ℤ) => x + 1) id a✝

@[reducible, inline]

abbrev SetTheory.PGame.Domineering.Board : Type

A Domineering board is an arbitrary finite subset of ℤ × ℤ.

def SetTheory.PGame.Domineering.left (b : Board) : Finset (ℤ × ℤ)

Left can play anywhere that a square and the square below it are open.

def SetTheory.PGame.Domineering.right (b : Board) : Finset (ℤ × ℤ)

Right can play anywhere that a square and the square to the left are open.

theorem SetTheory.PGame.Domineering.mem_left {b : Board} (x : ℤ × ℤ) : x ∈ left b ↔ x ∈ b ∧ (x.1, x.2 - 1) ∈ b

theorem SetTheory.PGame.Domineering.mem_right {b : Board} (x : ℤ × ℤ) : x ∈ right b ↔ x ∈ b ∧ (x.1 - 1, x.2) ∈ b

def SetTheory.PGame.Domineering.moveLeft (b : Board) (m : ℤ × ℤ) : Board

After Left moves, two vertically adjacent squares are removed from the board.

def SetTheory.PGame.Domineering.moveRight (b : Board) (m : ℤ × ℤ) : Board

After Left moves, two horizontally adjacent squares are removed from the board.

theorem SetTheory.PGame.Domineering.fst_pred_mem_erase_of_mem_right {b : Board} {m : ℤ × ℤ} (h : m ∈ right b) : (m.1 - 1, m.2) ∈ Finset.erase b m

theorem SetTheory.PGame.Domineering.snd_pred_mem_erase_of_mem_left {b : Board} {m : ℤ × ℤ} (h : m ∈ left b) : (m.1, m.2 - 1) ∈ Finset.erase b m

theorem SetTheory.PGame.Domineering.card_of_mem_left {b : Board} {m : ℤ × ℤ} (h : m ∈ left b) : 2 ≤ Finset.card b

theorem SetTheory.PGame.Domineering.card_of_mem_right {b : Board} {m : ℤ × ℤ} (h : m ∈ right b) : 2 ≤ Finset.card b

theorem SetTheory.PGame.Domineering.moveLeft_card {b : Board} {m : ℤ × ℤ} (h : m ∈ left b) : Finset.card (moveLeft b m) + 2 = Finset.card b

theorem SetTheory.PGame.Domineering.moveRight_card {b : Board} {m : ℤ × ℤ} (h : m ∈ right b) : Finset.card (moveRight b m) + 2 = Finset.card b

theorem SetTheory.PGame.Domineering.moveLeft_smaller {b : Board} {m : ℤ × ℤ} (h : m ∈ left b) : Finset.card (moveLeft b m) / 2 < Finset.card b / 2

theorem SetTheory.PGame.Domineering.moveRight_smaller {b : Board} {m : ℤ × ℤ} (h : m ∈ right b) : Finset.card (moveRight b m) / 2 < Finset.card b / 2

instance SetTheory.PGame.Domineering.state : State Board

The instance describing allowed moves on a Domineering board.

def SetTheory.PGame.domineering (b : Domineering.Board) : PGame

Construct a pre-game from a Domineering board.

instance SetTheory.PGame.shortDomineering (b : Domineering.Board) : (domineering b).Short

All games of Domineering are short, because each move removes two squares.

def SetTheory.PGame.domineering.one : PGame

The Domineering board with two squares arranged vertically, in which Left has the only move.

def SetTheory.PGame.domineering.L : PGame

The L shaped Domineering board, in which Left is exactly half a move ahead.

instance SetTheory.PGame.shortOne : domineering.one.Short

instance SetTheory.PGame.shortL : domineering.L.Short