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+// build
+
+// 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.
+
+// Test general operation by solving a peg solitaire game.
+// A version of this is in the Go playground.
+// Don't run it - produces too much output.
+
+// 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")
+}