Tic Tac Toe Golang

main.go

package main

import (
	"fmt"
	"strings"
)

// set a board type for get and set functions
type board uint16

// position struct
type position struct {
	turn  uint8
	board [2]board
	draw  bool
}

// Masks used for checking if there is a winner
const (
	vertMask  = 0b111
	horzMask  = 0b100100100
	diagMask1 = 0b100010001
	diagMask2 = 0b001010100
)

func (b board) get(index uint16) bool {
	return (1<<index)&b != 0
}

func (b *board) set(index uint16) {
	// Dereferance the board and set the bit at the current index
	*b |= 1 << index
}

func (b board) countBits() int {
	var r int
	for r = 0; b > 0; r++ {
		b &= b - 1
	}
	return r
}

func main() {
	playAgain := true
	// Reset the board after every game until the user quits
	for playAgain {
		pos := &position{}
		pos.gameLoop()
		var answer string
		fmt.Print("Play again? (y/N): ")
		fmt.Scanln(&answer)
		answer = strings.ToLower(answer)
		if playAgain = answer == "y"; playAgain {
			fmt.Print("\n")
		}
	}
}

func (pos *position) gameLoop() {
	ended := false
	for !ended {
		pos.print()
		var toMove string
		if pos.turn == 0 {
			toMove = "X"
		} else {
			toMove = "O"
		}

		fmt.Printf("It is %s's turn\n", toMove)

		validMove := false
		// keep trying to get a move while the move provided is invalid
		for !validMove {
			fmt.Print("Your Move: ")

			var move string
			fmt.Scanln(&move)
			validMove = pos.makeMove(move)
		}

		ended = pos.gameEnded()
		if !ended {
			pos.turn ^= 1 // 0 == 1 && 1 == 0
		}
	}

	if !pos.draw {
		pos.print()

		var winner string
		if pos.turn == 0 {
			winner = "X"
		} else {
			winner = "O"
		}

		fmt.Printf("%s wins! Conratz!\n", winner)
	} else {
		fmt.Println("The game is a draw. No one wins!")
	}
}

func (pos *position) makeMove(move string) bool {
	// Return false if the move is more than 2 chars
	if len(move) != 2 {
		return false
	}
	// Convert all letters to lowercase so we can calculate from a known state
	move = strings.ToLower(move)
	// get the value of the first char and subtract it by the ascii value of 'a' so a wil == 0 b will == 1 and so on
	row := uint16(move[0] - 'a')
	// do the same thing except for the number, 1 == 0, 2 == 1 and so on
	col := uint16(move[1] - '1')

	// Convert our 2D coords into a 1D index for the bitboard
	index := (row * 3) + col
	// Check that no square is already taken
	if pos.board[0].get(index) || pos.board[1].get(index) {
		return false
	}

	// Set the bit for the active player
	pos.board[pos.turn].set(index)
	return true
}

func (pos *position) gameEnded() bool {
	// Check board for player who jsut moved since they are the only possible winner.
	// Check diagnal masks first since they require no loop
	if pos.board[pos.turn]&diagMask1 == diagMask1 || pos.board[pos.turn]&diagMask2 == diagMask2 {
		return true
	}

	// Loop 3 times for each horizantal and vertical mask
	for i := uint16(0); i < 3; i++ {
		// Bit shift the board by i*3 to the right to allign the board with our vertical mask
		if vertMask&(pos.board[pos.turn]>>(i*3)) == vertMask {
			return true
		}

		// Bit shift the board by i to the left to allign the board with our horizantal mask
		if horzMask&(pos.board[pos.turn]<<i) == horzMask {
			return true
		}
	}

	// check for a draw
	if pos.board[0].countBits()+pos.board[1].countBits() >= 9 {
		pos.draw = true
		return true
	}

	return false
}

func (pos *position) print() {
	fmt.Println("    1  2  3\n -------------")
	letter := 'A'
	for i := uint16(0); i < 9; i++ {
		if i%3 == 0 {
			if i > 0 {
				fmt.Println(" |")
			}
			fmt.Printf("%s| ", string(letter))
			letter++
		}

		if pos.board[0].get(i) {
			fmt.Print(" X ")
		} else if pos.board[1].get(i) {
			fmt.Print(" O ")
		} else {
			fmt.Print(" - ")
		}

	}
	fmt.Println(" |\n -------------")
}