A few months ago, my wife gave me a book with sudoku puzzles. After solving a couple by hand, I decided that it was much more fun to try and write a solver...
My first implementation was a quick hack, the code was brief and ugly with lots of index operations in a bad functional programming style, but it worked well. It used only solution-reduction algorithms, no brute force. As such not all sudoku puzzles could be solved, but the ones that could, were solved pretty fast. Satisfied with my little achievement I soon forgot about it...
Then, recently, I read this article on LinuxJournal. The program in article was unfinished (the third part has still to be published), but the code was much tidier than mine. I took some inspiration to rewrite my solver in a more pythonic way. After that I stumbled on the ASPN recipe, which gave me the idea of adding a brute force solver.
The result is a combination of analytic and brute force solvers that performs 2 or 3 orders of magnitude better than the ASPN code.
Enough talk, here it is:
#!/usr/bin/env python
#
# sudoku-solver version 3
#
# Some ideas ripped-off from:
# http://www.linuxjournal.com/article/8729
# http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/440542
# Pavol solver in
# http://groups.google.com/group/comp.lang.python/browse_thread/thread/5087890f4c5e770d
#
# Copyright 2005 Ago,
# But you are free to copy, reuse, modify and distribute the code as you see fit
from copy import deepcopy
class DeadEnd(Exception): pass
class Matrix:
def __init__(self, input):
self.rows = [[] for i in range(9)]
self.cols = [[] for i in range(9)]
self.submatrices = [[] for i in range(9)]
self.cells = [Cell(i,self) for i in range(81)]
self.subdiagonals = [self.rows[i][j+i%3] for i in range(9) for j in [0,3,6]]
input = [s not in '-*' and int(s) or 0 for s in input if s in '0123456789-*']
for cell,val in zip(self.cells, input):
if val: cell.setSolution(val)
def solve(self): #Brute-force solver
self.solveByReduction()
lensols=[(len(c.solutions),c.index) for c in self.cells if not c.solved]
if not lensols: return True
unsolved = min(lensols)[1]
solutions = list(self.cells[unsolved].solutions)
for s in solutions:
tmpmatrix = deepcopy(self)
try:
tmpmatrix.cells[unsolved].setSolution(s)
if tmpmatrix.solve():
self.update(tmpmatrix)
return True
except DeadEnd: pass
def solveByReduction(self):
while True:
self.changed = False
for c in self.cells: c.solve()
for c in self.subdiagonals: c.skim()
if not self.changed: break
def update(self, m):
self.rows, self.cols, self.submatrices, self.cells, self.subdiagonals=\
m.rows, m.cols, m.submatrices, m.cells, m.subdiagonals
def __str__(self):
return "\n".join(str([c for c in row ])[1:-1] for row in self.rows)
class Cell:
def __init__(self, index, matrix):
self.solved = False
self.matrix = matrix
self.index = index
self.row = matrix.rows[index/9]
self.col = matrix.cols[index%9]
self.submatrix = matrix.submatrices[((index/9)/3)*3+(index%9)/3]
self.row.append(self)
self.col.append(self)
self.submatrix.append(self)
self.solutions = set(range(1,10))
def solve(self):
if self.solved: return
if len(self.solutions) == 1:
self.setSolution(self.solutions.pop())
else:
sol = set()
for cells in [self.row, self.col, self.submatrix]:
otherSolutions = self.cells2sols(cells, self)
sol |= (self.solutions - otherSolutions)
if len(sol) > 1: raise DeadEnd()
if sol: self.setSolution(sol.pop())
def skim(self):
submatrix = set(self.submatrix)
for r in (set(self.row), set(self.col)):
subset1 = submatrix - r
subset2 = r - submatrix
solsNotIn1 = set(range(1,10)) - self.cells2sols(subset2)
solsNotIn2 = set(range(1,10)) - self.cells2sols(subset1)
for c in subset1: c.delSolutions(solsNotIn1)
for c in subset2: c.delSolutions(solsNotIn2)
def setSolution(self, val):
self.solved = True
self.solutions = set((val,))
self.matrix.changed = True
for other in self.row+self.col+self.submatrix:
if other is self: continue
if other.solutions == self.solutions: raise DeadEnd()
other.delSolutions(self.solutions)
def delSolutions(self, val):
if not self.solved and val & self.solutions:
self.solutions -= val
self.matrix.changed = True
if not self.solutions: raise DeadEnd()
def __repr__(self):
return str(self.solved and list(self.solutions)[0] or list(self.solutions))
@staticmethod
def cells2sols(cells, exclude=None):
return set(s for c in cells for s in c.solutions if c is not exclude)
if __name__ == "__main__":
input ='''
0,0,0,0,9,6,8,0,0
0,0,1,0,0,0,0,7,0
0,2,0,0,0,0,0,0,3
0,3,0,0,0,8,0,0,6
0,0,4,0,2,0,3,0,0
6,0,0,5,0,0,0,8,0
9,0,0,0,0,0,0,5,0
0,7,0,0,0,0,1,0,0
0,0,5,9,4,0,0,0,0
'''
matrix = Matrix(input)
matrix.solve()
print matrix