added checkers;

x/checkers/rules/checkers.go

@@ -0,0 +1,350 @@
+package rules
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"strings"
+)
+
+const (
+	BOARD_DIM = 8
+	RED       = "red"
+	BLACK     = "black"
+	ROW_SEP   = "|"
+)
+
+type Player struct {
+	Color string
+}
+
+type Piece struct {
+	Player Player
+	King   bool
+}
+
+var PieceStrings = map[Player]string{
+	RED_PLAYER:   "r",
+	BLACK_PLAYER: "b",
+	NO_PLAYER:    "*",
+}
+
+var NO_PIECE = Piece{NO_PLAYER, false}
+
+var StringPieces = map[string]Piece{
+	"r": Piece{RED_PLAYER, false},
+	"b": Piece{BLACK_PLAYER, false},
+	"R": Piece{RED_PLAYER, true},
+	"B": Piece{BLACK_PLAYER, true},
+	"*": NO_PIECE,
+}
+
+type Pos struct {
+	X int
+	Y int
+}
+
+var NO_POS = Pos{-1, -1}
+
+var BLACK_PLAYER = Player{BLACK}
+var RED_PLAYER = Player{RED}
+var NO_PLAYER = Player{
+	Color: "NO_PLAYER",
+}
+
+var Players = map[string]Player{
+	RED:   RED_PLAYER,
+	BLACK: BLACK_PLAYER,
+}
+
+var Opponents = map[Player]Player{
+	BLACK_PLAYER: RED_PLAYER,
+	RED_PLAYER:   BLACK_PLAYER,
+}
+
+var Usable = map[Pos]bool{}
+var Moves = map[Player]map[Pos]map[Pos]bool{}
+var Jumps = map[Player]map[Pos]map[Pos]Pos{}
+var KingMoves = map[Pos]map[Pos]bool{}
+var KingJumps = map[Pos]map[Pos]Pos{}
+
+func Capture(src, dst Pos) Pos {
+	return Pos{(src.X + dst.X) / 2, (src.Y + dst.Y) / 2}
+}
+
+func init() {
+
+	// Initialize usable spaces
+	for y := 0; y < BOARD_DIM; y++ {
+		for x := (y + 1) % 2; x < BOARD_DIM; x += 2 {
+			Usable[Pos{X: x, Y: y}] = true
+		}
+	}
+
+	// Initialize deep maps
+	for _, p := range Players {
+		Moves[p] = map[Pos]map[Pos]bool{}
+		Jumps[p] = map[Pos]map[Pos]Pos{}
+	}
+
+	// Compute possible moves, jumps and captures
+	for pos := range Usable {
+		KingMoves[pos] = map[Pos]bool{}
+		KingJumps[pos] = map[Pos]Pos{}
+		var directions = []int{1, -1}
+		for i, player := range []Player{BLACK_PLAYER, RED_PLAYER} {
+			Moves[player][pos] = map[Pos]bool{}
+			Jumps[player][pos] = map[Pos]Pos{}
+			movOff := 1
+			jmpOff := 2
+			for _, direction := range directions {
+				mov := Pos{pos.X + (movOff * direction), pos.Y + (movOff * directions[i])}
+				if Usable[mov] {
+					Moves[player][pos][mov] = true
+					KingMoves[pos][mov] = true
+				}
+				jmp := Pos{pos.X + (jmpOff * direction), pos.Y + (jmpOff * directions[i])}
+				if Usable[jmp] {
+					capturePos := Capture(pos, jmp)
+					Jumps[player][pos][jmp] = capturePos
+					KingJumps[pos][jmp] = capturePos
+				}
+			}
+		}
+	}
+}
+
+type Game struct {
+	Pieces map[Pos]Piece
+	Turn   Player
+}
+
+func New() *Game {
+	pieces := make(map[Pos]Piece)
+	game := &Game{pieces, BLACK_PLAYER}
+	game.addInitialPieces()
+	return game
+}
+
+func (game *Game) addInitialPieces() {
+	for pos := range Usable {
+		if pos.Y >= 0 && pos.Y < 3 {
+			game.Pieces[pos] = Piece{BLACK_PLAYER, false}
+		}
+		if pos.Y >= BOARD_DIM-3 && pos.Y < BOARD_DIM {
+			game.Pieces[pos] = Piece{RED_PLAYER, false}
+		}
+	}
+}
+
+func (game *Game) PieceAt(pos Pos) bool {
+	_, ok := game.Pieces[pos]
+	return ok
+}
+
+func (game *Game) TurnIs(player Player) bool {
+	return game.Turn == player
+}
+
+func (game *Game) Winner() Player {
+	red_count := 0
+	black_count := 0
+	for _, piece := range game.Pieces {
+		switch {
+		case piece.Player == BLACK_PLAYER:
+			black_count += 1
+		case piece.Player == RED_PLAYER:
+			red_count += 1
+		}
+	}
+	if black_count > 0 && red_count <= 0 {
+		return BLACK_PLAYER
+	} else if red_count > 0 && black_count <= 0 {
+		return RED_PLAYER
+	}
+	return NO_PLAYER
+}
+
+func (game *Game) ValidMove(src, dst Pos) bool {
+	if !game.PieceAt(src) || game.PieceAt(dst) {
+		return false
+	}
+	piece := game.Pieces[src]
+	if (!piece.King && Moves[piece.Player][src][dst]) || (piece.King && KingMoves[src][dst]) {
+		return !game.playerHasJump(piece.Player)
+	}
+	return game.ValidJump(src, dst)
+}
+
+func (game *Game) ValidJump(src, dst Pos) bool {
+	if !game.PieceAt(src) || game.PieceAt(dst) {
+		return false
+	}
+	piece := game.Pieces[src]
+	if !piece.King {
+		capLoc, jumpOk := Jumps[piece.Player][src][dst]
+		return jumpOk && game.PieceAt(capLoc) && game.Pieces[capLoc].Player == Opponents[piece.Player]
+	} else {
+		capLoc, kingJumpOk := KingJumps[src][dst]
+		return kingJumpOk && game.PieceAt(capLoc) && game.Pieces[capLoc].Player == Opponents[piece.Player]
+	}
+}
+
+func (game *Game) kingPiece(dst Pos) {
+	if !game.PieceAt(dst) {
+		return
+	}
+	piece := game.Pieces[dst]
+	if (dst.Y == 0 && piece.Player == RED_PLAYER) ||
+		(dst.Y == BOARD_DIM-1 && piece.Player == BLACK_PLAYER) {
+		piece.King = true
+		game.Pieces[dst] = piece
+	}
+}
+
+func (game *Game) updateTurn(dst Pos, jumped bool) {
+	opponent := Opponents[game.Turn]
+	if (!jumped || !game.jumpPossibleFrom(dst)) && game.playerHasMove(opponent) {
+		game.Turn = opponent
+	}
+}
+
+func (game *Game) jumpPossibleFrom(src Pos) bool {
+	if !game.PieceAt(src) {
+		return false
+	}
+	piece := game.Pieces[src]
+	if !piece.King {
+		// enumerate all player jumps and return true if one is valid
+		for dst := range Jumps[piece.Player][src] {
+			if game.ValidJump(src, dst) {
+				return true
+			}
+		}
+	} else {
+		// enumerate all king jumps and return true if one is valid
+		for dst := range KingJumps[src] {
+			if game.ValidJump(src, dst) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+func (game *Game) movePossibleFrom(src Pos) bool {
+	if !game.PieceAt(src) {
+		return false
+	}
+	piece := game.Pieces[src]
+	if !piece.King {
+		for dst := range Moves[piece.Player][src] {
+			if game.ValidMove(src, dst) {
+				return true
+			}
+		}
+	} else {
+		for dst := range KingMoves[src] {
+			if game.ValidMove(src, dst) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+func (game *Game) playerHasMove(player Player) bool {
+	for loc, piece := range game.Pieces {
+		if piece.Player == player && (game.movePossibleFrom(loc) || game.jumpPossibleFrom(loc)) {
+			return true
+		}
+	}
+	return false
+}
+
+func (game *Game) playerHasJump(player Player) bool {
+	for loc, piece := range game.Pieces {
+		if piece.Player == player && game.jumpPossibleFrom(loc) {
+			return true
+		}
+	}
+	return false
+}
+
+func (game *Game) Move(src, dst Pos) (captured Pos, err error) {
+	captured = NO_POS
+	err = nil
+	if !game.PieceAt(src) {
+		return NO_POS, errors.New(fmt.Sprintf("No piece at source position: %v", src))
+	}
+	if game.PieceAt(dst) {
+		return NO_POS, errors.New(fmt.Sprintf("Already piece at destination position: %v", dst))
+	}
+	if !game.TurnIs(game.Pieces[src].Player) {
+		return NO_POS, errors.New(fmt.Sprintf("Not %v's turn", game.Pieces[src].Player))
+	}
+	if !game.ValidMove(src, dst) {
+		return NO_POS, errors.New(fmt.Sprintf("Invalid move: %v to %v", src, dst))
+	}
+	if game.ValidJump(src, dst) {
+		game.Pieces[dst] = game.Pieces[src]
+		delete(game.Pieces, src)
+		captured = Capture(src, dst)
+		delete(game.Pieces, captured)
+	} else {
+		game.Pieces[dst] = game.Pieces[src]
+		delete(game.Pieces, src)
+	}
+	game.updateTurn(dst, captured != NO_POS)
+	game.kingPiece(dst)
+	return
+}
+
+func (game *Game) String() string {
+	var buf bytes.Buffer
+	for y := 0; y < BOARD_DIM; y++ {
+		for x := 0; x < BOARD_DIM; x++ {
+			pos := Pos{x, y}
+			if game.PieceAt(pos) {
+				piece := game.Pieces[pos]
+				val := PieceStrings[piece.Player]
+				if piece.King {
+					val = strings.ToUpper(val)
+				}
+				buf.WriteString(val)
+			} else {
+				buf.WriteString(PieceStrings[NO_PLAYER])
+			}
+		}
+		if y < (BOARD_DIM - 1) {
+			buf.WriteString(ROW_SEP)
+		}
+	}
+	return buf.String()
+}
+
+func ParsePiece(s string) (Piece, bool) {
+	piece, ok := StringPieces[s]
+	return piece, ok
+}
+
+func Parse(s string) (*Game, error) {
+	if len(s) != BOARD_DIM*BOARD_DIM+(BOARD_DIM-1) {
+		return nil, errors.New(fmt.Sprintf("invalid board string: %v", s))
+	}
+	pieces := make(map[Pos]Piece)
+	result := &Game{pieces, BLACK_PLAYER}
+	for y, row := range strings.Split(s, ROW_SEP) {
+		for x, c := range strings.Split(row, "") {
+			if x >= BOARD_DIM || y >= BOARD_DIM {
+				return nil, errors.New(fmt.Sprintf("invalid board, piece out of bounds: %v, %v", x, y))
+			}
+			if piece, ok := ParsePiece(c); !ok {
+				return nil, errors.New(fmt.Sprintf("invalid board, invalid piece at %v, %v", x, y))
+			} else if piece != NO_PIECE {
+				result.Pieces[Pos{x, y}] = piece
+			}
+		}
+	}
+	return result, nil
+}