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// $G $F.go && $L $F.$A  # don't run it - produces too much output

// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This program solves the (English) peg solitaire board game.
// See also: http://en.wikipedia.org/wiki/Peg_solitaire

package main

const N = 11 + 1 // length of a board row (+1 for newline)

// The board must be surrounded by 2 illegal fields in each direction
// so that move() doesn't need to check the board boundaries. Periods
// represent illegal fields, ● are pegs, and ○ are holes.
var board = []rune(
        `...........
...........
....●●●....
....●●●....
..●●●●●●●..
..●●●○●●●..
..●●●●●●●..
....●●●....
....●●●....
...........
...........
`)

// center is the position of the center hole if there is a single one;
// otherwise it is -1.
var center int

func init() {
        n := 0
        for pos, field := range board {
                if field == '○' {
                        center = pos
                        n++
                }
        }
        if n != 1 {
                center = -1 // no single hole
        }
}

var moves int // number of times move is called

// move tests if there is a peg at position pos that can jump over another peg
// in direction dir. If the move is valid, it is executed and move returns true.
// Otherwise, move returns false.
func move(pos, dir int) bool {
        moves++
        if board[pos] == '●' && board[pos+dir] == '●' && board[pos+2*dir] == '○' {
                board[pos] = '○'
                board[pos+dir] = '○'
                board[pos+2*dir] = '●'
                return true
        }
        return false
}

// unmove reverts a previously executed valid move.
func unmove(pos, dir int) {
        board[pos] = '●'
        board[pos+dir] = '●'
        board[pos+2*dir] = '○'
}

// solve tries to find a sequence of moves such that there is only one peg left
// at the end; if center is >= 0, that last peg must be in the center position.
// If a solution is found, solve prints the board after each move in a backward
// fashion (i.e., the last board position is printed first, all the way back to
// the starting board position).
func solve() bool {
        var last, n int
        for pos, field := range board {
                // try each board position
                if field == '●' {
                        // found a peg
                        for _, dir := range [...]int{-1, -N, +1, +N} {
                                // try each direction
                                if move(pos, dir) {
                                        // a valid move was found and executed,
                                        // see if this new board has a solution
                                        if solve() {
                                                unmove(pos, dir)
                                                println(string(board))
                                                return true
                                        }
                                        unmove(pos, dir)
                                }
                        }
                        last = pos
                        n++
                }
        }
        // tried each possible move
        if n == 1 && (center < 0 || last == center) {
                // there's only one peg left
                println(string(board))
                return true
        }
        // no solution found for this board
        return false
}

func main() {
        if !solve() {
                println("no solution found")
        }
        println(moves, "moves tried")
}