⚠️ Warning: This is a draft ⚠️

This means it might contain formatting issues, incorrect code, conceptual problems, or other severe issues.

If you want to help to improve and eventually enable this page, please fork RosettaGit's repository and open a merge request on GitHub.

==Original Python exhaustive search== I was experimenting with various things when doing the Python. This is the original:

Exhaustive search.

```from itertools import combinations, count
from functools import lru_cache, reduce

# n-by-n board
n = 5

def _2d(n=n):
for i in range(n):
print('  '.join(f'{i},{j}' for j in range(n)))

def _1d(n=n):
for i in range(0, n*n, n):
print(',  '.join(f'{i+j:2}' for j in range(n)))

_bbullet, _wbullet = '\u2022\u25E6'
#_bqueen, _wqueen = 'BW'
_bqueen, _wqueen = '\u265B\u2655'
_bqueenh, _wqueenh = '&#x265b;', '<font color="green">&#x2655;</font>'
_or = set.__or__

def place(m, n):
"Place m black and white queens, peacefully, on an n-by-n board"

# 2-D Board as 1-D array:  2D(x, y) == 1D(t%n, t//n)
board = set(range(n*n))

#placements = list(combinations(board, m))
placements = {frozenset(c) for c in combinations(board, m)}
for blacks in placements:
black_attacks = reduce(_or,
(queen_attacks_from(pos, n) for pos in blacks),
set())
#for whites in placements:
for whites in {frozenset(c) for c in combinations(board - black_attacks, m)}:
if not black_attacks & whites:
return blacks, whites
return set(), set()

@lru_cache(maxsize=None)
def queen_attacks_from(pos, n=n):
a = set([pos])    # Its position
a.update(range(pos//n*n, pos//n*n+n))    # Its row
a.update(range(pos%n, n*n, n))           # Its column
# Diagonals
x0, y0 = pos%n, pos//n
for x1 in range(n):
# l-to-r diag
y1 = y0 -x0 +x1
if 0 <= y1 < n:
# r-to-l diag
y1 = y0 +x0 -x1
if 0 <= y1 < n:
return a

def pboard(black_white=None, n=n):
if black_white is None:
blk, wht = set(), set()
else:
blk, wht = black_white
print(f"## {len(blk)} black and {len(wht)} white queens "
f"on a {n}-by-{n} board:", end='')
for xy in range(n*n):
if xy %n == 0:
print()
ch = ('?' if xy in blk and xy in wht
else _bqueen if xy in blk
else _wqueen if xy in wht
else _bbullet if (xy%n + xy//n)%2 else _wbullet)
print('%s' % ch, end='')
print()

def hboard(black_white=None, n=n):
if black_white is None:
blk, wht = set(), set()
else:
blk, wht = black_white
out = (f"
<b>## {len(blk)} black and {len(wht)} white queens "
f"on a {n}-by-{n} board:</b>
\n")
out += "<table>\n  "
tbl = ''
for xy in range(n*n):
if xy %n == 0:
tbl += '</tr>\n  <tr>\n'
ch = ('<span style="color:red">?</span>' if xy in blk and xy in wht
else _bqueenh if xy in blk
else _wqueenh if xy in wht
else "")
bg = "" if (xy%n + xy//n)%2 else ' bgcolor="silver"'
tbl += f'    <td style="width:16pt; height:16pt;"{bg}>{ch}</td>\n'
out += tbl[7:]
out += '</tr>\n</table>\n
\n'
return out

if __name__ == '__main__':
n=2
html = ''
for n in range(2, 7):
print()
queen_attacks_from.cache_clear()    # memoization cache
#
for m in count(1):
ans = place(m, n)
if ans:
pboard(ans, n)
html += hboard(ans, n)
else:
comment = f"# Can't place {m}+ queens on a {n}-by-{n} board"
print (comment)
html += f"<b>{comment}</b>

\n\n"
break
print('\n')
html += '
\n'
#
m, n = 5, 7
queen_attacks_from.cache_clear()
ans = place(m, n)
pboard(ans, n)
html += hboard(ans, n)
with open('peaceful_queen_armies.htm', 'w') as f:
f.write(html)
```

{{out}} The console output Unicode queen characters display wider than other characters in monospace font so the alternative HTML output is shown below.

# Can't place 1+ queens on a 2-by-2 board

## 1 black and 1 white queens on a 3-by-3 board:

 ♛ ♕

# Can't place 2+ queens on a 3-by-3 board

## 1 black and 1 white queens on a 4-by-4 board:

 ♛ ♕

## 2 black and 2 white queens on a 4-by-4 board:

 ♛ ♛ ♕ ♕

# Can't place 3+ queens on a 4-by-4 board

## 1 black and 1 white queens on a 5-by-5 board:

 ♛ ♕

## 2 black and 2 white queens on a 5-by-5 board:

 ♛ ♛ ♕ ♕

## 3 black and 3 white queens on a 5-by-5 board:

 ♕ ♕ ♛ ♕ ♛ ♛

## 4 black and 4 white queens on a 5-by-5 board:

 ♕ ♕ ♕ ♛ ♛ ♕ ♛ ♛

# Can't place 5+ queens on a 5-by-5 board

## 1 black and 1 white queens on a 6-by-6 board:

 ♕ ♛

## 2 black and 2 white queens on a 6-by-6 board:

 ♛ ♕ ♕ ♛

## 3 black and 3 white queens on a 6-by-6 board:

 ♛ ♕ ♛ ♛ ♕ ♕

## 4 black and 4 white queens on a 6-by-6 board:

 ♕ ♕ ♛ ♕ ♕ ♛ ♛ ♛

## 5 black and 5 white queens on a 6-by-6 board:

 ♛ ♛ ♛ ♛ ♛ ♕ ♕ ♕ ♕ ♕

# Can't place 6+ queens on a 6-by-6 board

## 5 black and 5 white queens on a 7-by-7 board:

 ♕ ♕ ♛ ♛ ♛ ♛ ♛ ♕ ♕ ♕