FA-82421 / Chess move legality / Open access
Black en passant rejected on its fifth rank · case 01
A black pawn on d4 cannot capture en passant on e3.
ROOT CAUSE
The capturer rank is hard-coded to white's fifth rank for both colours.
VERIFIED REPAIR
White captures from rank index 4, black from rank index 3.
Unsuccessful approach: Swapping the indices breaks both colours.
Case contract
Input: {"board", "move": [from, to], "ep": target square or null}. Return true when the move is a legal en passant capture: a pawn on its fifth rank moves diagonally forward onto the ep square, an enemy pawn stands beside it on the origin rank, and after removing both pawns from their squares and placing the capturer, no enemy rook, bishop or queen attacks the mover's king along an open line.
Why this case matters
Move validators gate every move in engines, servers and arbiters; accepting an illegal move or rejecting a legal one corrupts the game record.
1 / The failure
Exit 1"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(x):
b=dict(x['board']); fr,to=x['move']; p=b.get(fr)
if p not in ('P','p') or to!=x['ep']: return False
white=p=='P'
d=1 if white else -1
f0,r0=ord(fr[0])-97,int(fr[1])-1
f1,r1=ord(to[0])-97,int(to[1])-1
if r1-r0!=d or abs(f1-f0)!=1: return False
if r0!=4: return False
victim=to[0]+fr[1]
if b.get(victim)!=('p' if white else 'P'): return False
del b[fr]; del b[victim]; b[to]=p
ks=[s for s,v in b.items() if v==('K' if white else 'k')][0]
kf,kr=ord(ks[0])-97,int(ks[1])-1
for df,dr in ((1,0),(-1,0),(0,1),(0,-1),(1,1),(1,-1),(-1,1),(-1,-1)):
cf,cr=kf+df,kr+dr
while 0<=cf<8 and 0<=cr<8:
v=b.get(chr(97+cf)+str(cr+1))
if v:
if v.isupper()!=white and v.lower() in (('r','q') if df==0 or dr==0 else ('b','q')): return False
break
cf+=df; cr+=dr
return True
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('plain ep',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
True),
('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('rank pin both pawns',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
False),
('rank pin black side',
{'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
'ep': 'e3',
'move': ['d4', 'e3']},
False),
('no ep square',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': None, 'move': ['e5', 'd6']},
False),
('wrong target',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
'ep': 'd6',
'move': ['e5', 'f6']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('same file ep target',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd3', 'move': ['e5', 'd6']},
False)],
[('plain ep',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
True),
('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('bishop diagonal after ep',
{'board': {'c3': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p', 'f6': 'n', 'h8': 'b'},
'ep': 'f6',
'move': ['e5', 'f6']},
True),
('wrong target',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
'ep': 'd6',
'move': ['e5', 'f6']},
False),
('wrong rank for white',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('same file ep target',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd3', 'move': ['e5', 'd6']},
False),
('rook on file of ep square',
{'board': {'a1': 'K', 'd1': 'r', 'd5': 'p', 'e5': 'P', 'e8': 'k'},
'ep': 'd6',
'move': ['e5', 'd6']},
False)],
[('plain ep',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
True),
('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('bishop diagonal after ep',
{'board': {'c3': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p', 'f6': 'n', 'h8': 'b'},
'ep': 'f6',
'move': ['e5', 'f6']},
True),
('wrong rank for white',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('file rook behind pawn',
{'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('pin by own rook harmless',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('bishop on the rank',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
'ep': 'd6',
'move': ['e5', 'd6']},
True)],
[('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('rank pin both pawns',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
False),
('wrong rank for white',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('file rook behind pawn',
{'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('pin by own rook harmless',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('bishop on the rank',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('rook on the diagonal',
{'board': {'c3': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'g7': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
True)],
[('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('rank pin both pawns',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
False),
('rank pin black side',
{'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
'ep': 'e3',
'move': ['d4', 'e3']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('file rook behind pawn',
{'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('pin by own rook harmless',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('bishop on the rank',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('rook on the diagonal',
{'board': {'c3': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'g7': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
True)]]
for label, args, expected in fixtures[N-1]:
try:
actual = solve(args)
except Exception as exc:
actual = 'error:' + type(exc).__name__
check(label, actual, expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
| Boundary fixture | Actual | Expected | Outcome |
|---|---|---|---|
| plain ep | True | True | Passed |
| black ep | False | True | Failed |
| rank pin both pawns | False | False | Passed |
| rank pin black side | False | False | Passed |
| no ep square | False | False | Passed |
| wrong target | False | False | Passed |
| black wrong rank | True | False | Failed |
| same file ep target | False | False | Passed |
SHA-256 / 4c5ae6a8d653ccb17a93137b1d8b77d942c2c8730fa36a12354932df737b538f
2 / The unsuccessful fix
Exit 1"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(x):
b=dict(x['board']); fr,to=x['move']; p=b.get(fr)
if p not in ('P','p') or to!=x['ep']: return False
white=p=='P'
d=1 if white else -1
f0,r0=ord(fr[0])-97,int(fr[1])-1
f1,r1=ord(to[0])-97,int(to[1])-1
if r1-r0!=d or abs(f1-f0)!=1: return False
if r0!=(3 if white else 4): return False
victim=to[0]+fr[1]
if b.get(victim)!=('p' if white else 'P'): return False
del b[fr]; del b[victim]; b[to]=p
ks=[s for s,v in b.items() if v==('K' if white else 'k')][0]
kf,kr=ord(ks[0])-97,int(ks[1])-1
for df,dr in ((1,0),(-1,0),(0,1),(0,-1),(1,1),(1,-1),(-1,1),(-1,-1)):
cf,cr=kf+df,kr+dr
while 0<=cf<8 and 0<=cr<8:
v=b.get(chr(97+cf)+str(cr+1))
if v:
if v.isupper()!=white and v.lower() in (('r','q') if df==0 or dr==0 else ('b','q')): return False
break
cf+=df; cr+=dr
return True
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('plain ep',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
True),
('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('rank pin both pawns',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
False),
('rank pin black side',
{'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
'ep': 'e3',
'move': ['d4', 'e3']},
False),
('no ep square',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': None, 'move': ['e5', 'd6']},
False),
('wrong target',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
'ep': 'd6',
'move': ['e5', 'f6']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('same file ep target',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd3', 'move': ['e5', 'd6']},
False)],
[('plain ep',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
True),
('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('bishop diagonal after ep',
{'board': {'c3': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p', 'f6': 'n', 'h8': 'b'},
'ep': 'f6',
'move': ['e5', 'f6']},
True),
('wrong target',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
'ep': 'd6',
'move': ['e5', 'f6']},
False),
('wrong rank for white',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('same file ep target',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd3', 'move': ['e5', 'd6']},
False),
('rook on file of ep square',
{'board': {'a1': 'K', 'd1': 'r', 'd5': 'p', 'e5': 'P', 'e8': 'k'},
'ep': 'd6',
'move': ['e5', 'd6']},
False)],
[('plain ep',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
True),
('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('bishop diagonal after ep',
{'board': {'c3': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p', 'f6': 'n', 'h8': 'b'},
'ep': 'f6',
'move': ['e5', 'f6']},
True),
('wrong rank for white',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('file rook behind pawn',
{'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('pin by own rook harmless',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('bishop on the rank',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
'ep': 'd6',
'move': ['e5', 'd6']},
True)],
[('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('rank pin both pawns',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
False),
('wrong rank for white',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('file rook behind pawn',
{'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('pin by own rook harmless',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('bishop on the rank',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('rook on the diagonal',
{'board': {'c3': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'g7': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
True)],
[('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('rank pin both pawns',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
False),
('rank pin black side',
{'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
'ep': 'e3',
'move': ['d4', 'e3']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('file rook behind pawn',
{'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('pin by own rook harmless',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('bishop on the rank',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('rook on the diagonal',
{'board': {'c3': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'g7': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
True)]]
for label, args, expected in fixtures[N-1]:
try:
actual = solve(args)
except Exception as exc:
actual = 'error:' + type(exc).__name__
check(label, actual, expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
| Boundary fixture | Actual | Expected | Outcome |
|---|---|---|---|
| plain ep | False | True | Failed |
| black ep | False | True | Failed |
| rank pin both pawns | False | False | Passed |
| rank pin black side | False | False | Passed |
| no ep square | False | False | Passed |
| wrong target | False | False | Passed |
| black wrong rank | True | False | Failed |
| same file ep target | False | False | Passed |
SHA-256 / dc9bab643ca459b41baae7dc81bc7f525fe17028515f49f99d58df2801b3f8ba
3 / The verified repair
Exit 0"""Failure Map reference implementation. Python standard library only."""
import json
N = 1
observations = []
def solve(x):
b=dict(x['board']); fr,to=x['move']; p=b.get(fr)
if p not in ('P','p') or to!=x['ep']: return False
white=p=='P'
d=1 if white else -1
f0,r0=ord(fr[0])-97,int(fr[1])-1
f1,r1=ord(to[0])-97,int(to[1])-1
if r1-r0!=d or abs(f1-f0)!=1: return False
if r0!=(4 if white else 3): return False
victim=to[0]+fr[1]
if b.get(victim)!=('p' if white else 'P'): return False
del b[fr]; del b[victim]; b[to]=p
ks=[s for s,v in b.items() if v==('K' if white else 'k')][0]
kf,kr=ord(ks[0])-97,int(ks[1])-1
for df,dr in ((1,0),(-1,0),(0,1),(0,-1),(1,1),(1,-1),(-1,1),(-1,-1)):
cf,cr=kf+df,kr+dr
while 0<=cf<8 and 0<=cr<8:
v=b.get(chr(97+cf)+str(cr+1))
if v:
if v.isupper()!=white and v.lower() in (('r','q') if df==0 or dr==0 else ('b','q')): return False
break
cf+=df; cr+=dr
return True
def check(label, actual, expected):
observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('plain ep',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
True),
('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('rank pin both pawns',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
False),
('rank pin black side',
{'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
'ep': 'e3',
'move': ['d4', 'e3']},
False),
('no ep square',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': None, 'move': ['e5', 'd6']},
False),
('wrong target',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
'ep': 'd6',
'move': ['e5', 'f6']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('same file ep target',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd3', 'move': ['e5', 'd6']},
False)],
[('plain ep',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
True),
('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('bishop diagonal after ep',
{'board': {'c3': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p', 'f6': 'n', 'h8': 'b'},
'ep': 'f6',
'move': ['e5', 'f6']},
True),
('wrong target',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p'},
'ep': 'd6',
'move': ['e5', 'f6']},
False),
('wrong rank for white',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('same file ep target',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd3', 'move': ['e5', 'd6']},
False),
('rook on file of ep square',
{'board': {'a1': 'K', 'd1': 'r', 'd5': 'p', 'e5': 'P', 'e8': 'k'},
'ep': 'd6',
'move': ['e5', 'd6']},
False)],
[('plain ep',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'd6', 'move': ['e5', 'd6']},
True),
('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('bishop diagonal after ep',
{'board': {'c3': 'K', 'e5': 'P', 'e8': 'k', 'f5': 'p', 'f6': 'n', 'h8': 'b'},
'ep': 'f6',
'move': ['e5', 'f6']},
True),
('wrong rank for white',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('file rook behind pawn',
{'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('pin by own rook harmless',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('bishop on the rank',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
'ep': 'd6',
'move': ['e5', 'd6']},
True)],
[('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('rank pin both pawns',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
False),
('wrong rank for white',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'd5', 'move': ['e4', 'd5']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('file rook behind pawn',
{'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('pin by own rook harmless',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('bishop on the rank',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('rook on the diagonal',
{'board': {'c3': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'g7': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
True)],
[('black ep',
{'board': {'d4': 'p', 'e1': 'K', 'e4': 'P', 'e8': 'k'}, 'ep': 'e3', 'move': ['d4', 'e3']},
True),
('rank pin both pawns',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
False),
('rank pin black side',
{'board': {'a4': 'Q', 'd4': 'p', 'e1': 'K', 'e4': 'P', 'h4': 'k'},
'ep': 'e3',
'move': ['d4', 'e3']},
False),
('black wrong rank',
{'board': {'d5': 'p', 'e1': 'K', 'e5': 'P', 'e8': 'k'}, 'ep': 'e4', 'move': ['d5', 'e4']},
False),
('file rook behind pawn',
{'board': {'d1': 'K', 'd5': 'p', 'd8': 'r', 'e5': 'P', 'e8': 'k'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('pin by own rook harmless',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'R'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('bishop on the rank',
{'board': {'a5': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'h5': 'b'},
'ep': 'd6',
'move': ['e5', 'd6']},
True),
('rook on the diagonal',
{'board': {'c3': 'K', 'd5': 'p', 'e5': 'P', 'e8': 'k', 'g7': 'r'},
'ep': 'd6',
'move': ['e5', 'd6']},
True)]]
for label, args, expected in fixtures[N-1]:
try:
actual = solve(args)
except Exception as exc:
actual = 'error:' + type(exc).__name__
check(label, actual, expected)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)
| Boundary fixture | Actual | Expected | Outcome |
|---|---|---|---|
| plain ep | True | True | Passed |
| black ep | True | True | Passed |
| rank pin both pawns | False | False | Passed |
| rank pin black side | False | False | Passed |
| no ep square | False | False | Passed |
| wrong target | False | False | Passed |
| black wrong rank | False | False | Passed |
| same file ep target | False | False | Passed |
SHA-256 / ae8cd98d949bb798012f76f8562082afd42c7f71c01220abad0c3216bc3a9925
Verification & scope
A deterministic toy model with an explicitly stated contract; boards are {square: piece} with white upper-case and black lower-case pieces; no claim of full FIDE Laws conformance. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.
Observations recorded using Python 3.12.14 at 2026-09-29T14:50:12.053920+00:00.
Case digest / e70f5a0d2af0bd0acc6302433bdcee583f9c423e757c91d5dfe55aeec29001c1