# Solitare

This program solves the (English) peg solitaire board game.

``````var N = 11 + 1 # length of a row (+1 for \n)

# 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 = '...........
...........
....●●●....
....●●●....
..●●●●●●●..
..●●●○●●●..
..●●●●●●●..
....●●●....
....●●●....
...........
...........'.to_list()

var dirs = [-1, -N, 1, N]

/* center is the position of the center hole if
there is a single one; otherwise it is -1.
moves is the number of times move is called
*/
var center = -1, moves = 0

def init() {
var n = 0

for pos, field in board {
if field == '○' {
center = pos
n++
}
}

if n != 1 center = -1 # no single hole
}

/*
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. */
def move(pos, dir) {
moves++

if board[pos] == '●' and board[pos+dir] == '●' and
board[pos+2*dir] == '○' {
board[pos] = '○'
board[pos+dir] = '○'
board[pos+2*dir] = '●'
return true
}
return false
}

# unmove reverts a previously executed valid move.
def unmove(pos, dir) {
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). */
def solve() {
var last = 0, n = 0

for pos, field in board {

# try each board position
if field == '●' {

# found a peg
for dir in dirs {

# 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)
echo ''.join(board)
echo ''
return true
}

unmove(pos, dir)
}
}

last = pos
n = n + 1
}
}

# tried each possible move
if n == 1 and (center < 0 or last == center) {
# there's only one peg left
echo ''.join(board)
echo ''
return true
}

# no solution found for this board
return false
}

var start = time()

init()

if !solve() echo 'No solution found!'

echo '\${moves} moves tried in \${time() - start}s'
``````