I've made a simple Sudoku class (in Python). Its main attributes are the following:
def __init__(self, it: iter = None):
self.grid = [[None for _ in range(9)] for _ in range(9)]
self.usedInRow = [[False for _ in range(10)] for _ in range(9)]
self.usedInCol = [[False for _ in range(10)] for _ in range(9)]
self.usedInBox = [[False for _ in range(10)] for _ in range(9)]
def writeCell(self, pos: tuple, val: int) -> bool: [...]
def eraseCell(self, pos: tuple): [...]
self.grid keeps track of the actual numbers in the Sudoku. self.usedInRow, self.usedInCol, and self.usedInBox are boolean arrays that keep track of which numbers have been used in each row/column/square. writeCell is a method that tries to write a number in an empty cell and returns whether it succeeded (according to the rules of Sudoku). Finally, eraseCell replaces a cell's content with None.
Inside the class I've written a recursive-backtracking solver method. When implementing the solver, I tried different approaches for selecting which cell to fill in next. The first thing I tried was simply starting from (0, 0) and incrementing the position left-to-right and top-to-bottom (with columns as the 'small' unit and rows as the 'big' unit).
But that's not general enough (if for some reason the solver started solving from a position other than (0, 0) every empty cell before that would be skipped). So I added a member to keep track of empty cells:
def __init__(self, it: iter = None):
[...]
self.emptyCells = [(i, j) for i in range(9) for j in range(9)]
I modified the writeCell and eraseCell methods to mantain the correct record. At this point the solver looks more or less like this:
def solve(self) -> bool:
[...]
def _solve() -> bool:
if not self.emptyCells: return True
pos = self.emptyCells[0]
for n in range(1, 10):
if self.writeCell(pos, n):
if _solve(): return True
self.eraseCell(pos)
else: return False
success = _solve()
[...]
return success
At this point I thought that maybe it would be better if the self.emptyCells member was a set instead of a list: I didn't want duplicates, didn't need indexed access (I thought that it wouldn't matter the order in which cells were filled in), and wanted fast deletion. So I modified the self.emptyCells attribute
self.emptyCells = set((i, j) for i in range(9) for j in range(9))
and the methods that referred to it (for example I used .discard() instead of .remove() in writeCell). The solver method now looked like this:
def solve(self) -> bool:
[...]
def _solve() -> bool:
if not self.emptyCells: return True
pos = self.emptyCells.pop() # Get arbitrary position
for n in range(1, 10):
if self.writeCell(pos, n):
if _solve(): return True
self.eraseCell(pos)
else:
self.emptyCells.add(pos) # Backtrack
return False
success = _solve()
[...]
return success
To my surprise, the second approach (using a set) took about one minute to solve an empty Sudoku, whilst the first approach (using a list) took about two seconds. Why is that? Does the arbitrarily chosen position conferred by set.pop() affect the efficiency of the algorithm, compared to filling cells in order?
Full code
Using a list
import queue
import threading
import colorama
colorama.init() # For ANSI escape codes
class Sudoku:
@staticmethod
def isSudoku(S: iter):
return len(S) == 9 \
and all(len(row) == 9 for row in S) \
and all(num in range(1, 10) for row in S for num in row)
def __init__(self, it: iter = None):
self.grid = [[None for _ in range(9)] for _ in range(9)]
self.usedInRow = [[False for _ in range(10)] for _ in range(9)]
self.usedInCol = [[False for _ in range(10)] for _ in range(9)]
self.usedInBox = [[False for _ in range(10)] for _ in range(9)]
self.emptyCells = [(i, j) for i in range(9) for j in range(9)]
if it is not None:
for i in range(9):
for j, num in zip(range(9), it):
if num is not None:
if not self.writeCell((i, j), num): raise ValueError("Invalid Sudoku")
def __iter__(self):
return iter(self.grid)
def __len__(self):
return 9
def __str__(self):
def numsToStr(row):
for num in row: yield num if num is not None else ' '
def rowsBlock(i):
yield ''
for row in self.grid[3 * i:3 * (i + 1)]:
yield '|{} {} {}|{} {} {}|{} {} {}|'.format(*numsToStr(row))
yield ''
def blocks():
yield ''
for i in range(3): yield '\n'.join(rowsBlock(i))
yield ''
return ('-' * 19).join(blocks())
def __getitem__(self, key: tuple):
if not len(key) == 2: raise TypeError("Two indices needed")
r, c = key
return self.grid[r][c]
def __setitem__(self, key: tuple, value: int):
if not len(key) == 2: raise TypeError("Two indices needed")
self.eraseCell(key)
self.writeCell(key, value)
def _canWrite(self, r: int, c: int, b: int, val: int) -> bool:
if self.usedInRow[r][val] or self.usedInCol[c][val] or self.usedInBox[b][val] \
or self.grid[r][c] is not None: return False
self.usedInRow[r][val] = self.usedInCol[c][val] = self.usedInBox[b][val] = True
return True
def writeCell(self, pos: tuple, val: int) -> bool:
if val is None: return True # Ignore writing none
r, c = pos
b = 3*(r//3) + c//3
if not self._canWrite(r, c, b, val): return False
# noinspection PyTypeChecker
self.grid[r][c] = val
self.emptyCells.remove(pos)
return True
def eraseCell(self, pos: tuple):
r, c = pos
b = 3*(r//3) + c//3
val = self.grid[r][c] # Get old value for reference
if val is None: return # If there wasn't anything in the first place
self.grid[r][c] = None # Actually erase
self.emptyCells.append(pos)
# noinspection PyTypeChecker
self.usedInRow[r][val] = self.usedInCol[c][val] = self.usedInBox[b][val] = False
def solve(self) -> bool:
printQueue = queue.PriorityQueue()
def printer():
while True:
priority, item = printQueue.get()
if item is StopAsyncIteration:
break
print(item)
print('\x1b[13A', end='')
printQueue.task_done()
printThread = threading.Thread(target=printer, name="PrintThread")
printThread.start()
def _solve() -> bool:
if not self.emptyCells: return True
pos = self.emptyCells[0]
for n in range(1, 10):
if self.writeCell(pos, n):
if _solve(): return True
printQueue.put((1, self))
self.eraseCell(pos)
else:
return False
success = _solve()
printQueue.put((0, StopAsyncIteration))
printThread.join()
return success
Using a set
The only things that change are the following.
_solve() internal method:
def _solve():
if not self.emptyCells: return True
pos = self.emptyCells.pop()
for n in range(1, 10):
if self.writeCell(pos, n):
if _solve(): return True
printQueue.put((1, self))
self.eraseCell(pos)
else:
self.emptyCells.add(pos)
return False
eraseCell and writeCell:
def writeCell(self, pos: tuple, val: int):
if val is None: return True # Ignore writing none
r, c = pos
b = 3*(r//3) + c//3
if not self._canWrite(r, c, b, val): return False
# noinspection PyTypeChecker
self.grid[r][c] = val
self.emptyCells.discard(pos)
return True
def eraseCell(self, pos: tuple):
r, c = pos
b = 3*(r//3) + c//3
val = self.grid[r][c] # Get old value for reference
if val is None: return # If there wasn't anything in the first place
self.grid[r][c] = None # Actually erase
self.emptyCells.add(pos)
# noinspection PyTypeChecker
self.usedInRow[r][val] = self.usedInCol[c][val] = self.usedInBox[b][val] = False
Note
The reason I don't put back pos in the list version—and also why I get pos as self.emptyCells[0] instead of self.emptyCells.pop(0)—is because writeCell and eraseCell already take care of that: if writeCell succeeds in writing a (previously empty) cell, then it will call self.emptyCells.remove(pos)—and here's another reason why I'm not popping the list: otherwise I would be removeing a non-existent element in the list, and would have to catch the ValueError.
After that, if backtracking occurs (i.e. the _solve() recursive call returns False), eraseCell will put pos back in the list. Thus, if the for loop "fails"—i.e., the else branch is entered—, the last eraseCell will already have put pos back.
On the other hand, when using a set, there is no way of accessing an element without removing it from the set (to my knowledge). Thus, I'm forced to pop from it and put it back in manually. That's why discard is used instead of remove in the set-version of writeCell, since pos may already have been popped by solve_.
An alternative maybe would be to save pos = self.emptyCells.pop() and add it straight back (self.emptyCells.add(pos)), and then proceed with solve_. Then I could change discard to remove.
Usage example
sudoku = Sudoku()
sudoku.solve()
print(sudoku)
Profiling results
I profiled both versions but didn't find any obvious culprit—that is, the proportions of cumulative time taken by each method were very similar, although the actual absolute time they took was just scaled up in the set version with respect to the list version.
Update — pos counts
I added an array to keep track of how many times each pos was selected to be explored, and I printed it at the end of the solve. Here are the results.
List version:
[[ 1 1 1 1 1 1 1 1 1]
[ 1 1 1 1 145 671 7169 9869 7606]
[ 774 1115 645 7 2066 1240 451 463 554]
[ 511 466 504 424 554 422 358 502 566]
[ 545 224 507 539 578 404 487 357 574]
[ 421 421 452 436 465 406 326 621 483]
[ 436 519 284 462 545 534 410 563 730]
[ 433 561 311 397 365 254 308 708 384]
[ 445 559 558 413 513 572 564 511 686]]
Set version (as far as I can tell, results are consistent):
[[ 19895 1 91239 66620 1 1 76794 1 1]
[ 1 1 77617 43061 95435 1 1 49617 77772]
[ 54441 1 1 96081 79660 1 1 1 89575]
[101233 1 1 85645 55172 1 1 1 54613]
[ 1 1 71512 61978 5531 1 76794 82692 74311]
[ 96268 1 57136 92052 1 1 86104 91911 1]
[ 1 98119 95706 10680 1 97232 67210 1 39094]
[ 53447 1 1 1 89846 1 1 88072 1]
[ 74468 19103 1 39934 75698 1 1 90778 53260]]
