FA-82661 / Chess move legality / Open access

Enemy piece of the same type forces a disambiguator · case 01

With a black knight on f3, the white move Nb1-d2 is written "Nbd2".

Verified by executionVariant 1 · 8 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Competing pieces are matched by type regardless of colour.

VERIFIED REPAIR

Only same-colour pieces of the same type can make a move ambiguous.

Unsuccessful approach: Matching only the upper-case letter never finds competing black pieces.

Case contract

Input: {"board", "from", "to"} for a knight, bishop, rook, queen or king move. Return SAN (no check suffix): upper-case piece letter, then a disambiguator only if another same-colour piece of the same type could also reach the target (sliders need a clear path): the origin file if it is unique among those pieces, else the origin rank if unique, else the full origin square; then "x" if the target is occupied; then the target.

Why this case matters

Exported PGN must be re-readable; a wrong disambiguator makes the game unreplayable.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    b=x['board']; fr,to=x['from'],x['to']; p=b[fr]; white=p.isupper()
    def c(q): return ord(q[0])-97,int(q[1])-1
    def reach(q):
        f,r=c(q); tf,tr=c(to); df,dr=tf-f,tr-r
        t=p.upper()
        if t=='N': return (abs(df),abs(dr)) in ((1,2),(2,1))
        if t=='K': return max(abs(df),abs(dr))==1
        orth=(df==0)!=(dr==0); diag=abs(df)==abs(dr) and df!=0
        if not ((t=='R' and orth) or (t=='B' and diag) or (t=='Q' and (orth or diag))): return False
        sf=(df>0)-(df<0); sr=(dr>0)-(dr<0)
        cf,cr=f+sf,r+sr
        while (cf,cr)!=(tf,tr):
            if chr(97+cf)+str(cr+1) in b: return False
            cf+=sf; cr+=sr
        return True
    others=[q for q,v in b.items() if v.upper()==p.upper() and q!=fr and reach(q)]
    dis=''
    if others:
        if all(q[0]!=fr[0] for q in others): dis=fr[0]
        elif all(q[1]!=fr[1] for q in others): dis=fr[1]
        else: dis=fr
    cap='x' if to in b else ''
    return p.upper()+dis+cap+to
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('two knights different files',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'N'}, 'from': 'b1', 'to': 'd2'}, 'Nbd2'),
  ('two rooks same file',
   {'board': {'a1': 'R', 'a5': 'R', 'e1': 'K', 'e8': 'k'}, 'from': 'a1', 'to': 'a3'}, 'R1a3'),
  ('three queens need square',
   {'board': {'a1': 'K', 'a8': 'k', 'e4': 'Q', 'h1': 'Q', 'h4': 'Q'}, 'from': 'h4', 'to': 'e1'},
   'Qh4e1'),
  ('blocked rook no disambiguation',
   {'board': {'a1': 'R', 'c1': 'B', 'e2': 'K', 'e8': 'k', 'h1': 'R'}, 'from': 'h1', 'to': 'd1'},
   'Rd1'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('black rook capture',
   {'board': {'a2': 'P', 'a8': 'r', 'e1': 'K', 'e8': 'k'}, 'from': 'a8', 'to': 'a2'}, 'Rxa2'),
  ('queen diagonal twin',
   {'board': {'a1': 'Q', 'g1': 'Q', 'h1': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd4'}, 'Qad4')],
 [('blocked rook no disambiguation',
   {'board': {'a1': 'R', 'c1': 'B', 'e2': 'K', 'e8': 'k', 'h1': 'R'}, 'from': 'h1', 'to': 'd1'},
   'Rd1'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('black rook capture',
   {'board': {'a2': 'P', 'a8': 'r', 'e1': 'K', 'e8': 'k'}, 'from': 'a8', 'to': 'a2'}, 'Rxa2'),
  ('queen diagonal twin',
   {'board': {'a1': 'Q', 'g1': 'Q', 'h1': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd4'}, 'Qad4'),
  ('capture with file',
   {'board': {'a1': 'K', 'a8': 'k', 'c3': 'N', 'e4': 'p', 'g3': 'N'}, 'from': 'c3', 'to': 'e4'},
   'Ncxe4'),
  ('simple bishop', {'board': {'c1': 'B', 'e1': 'K', 'e8': 'k'}, 'from': 'c1', 'to': 'g5'}, 'Bg5'),
  ('rooks on same rank',
   {'board': {'a1': 'R', 'f1': 'R', 'h2': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd1'}, 'Rad1')],
 [('two knights different files',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'N'}, 'from': 'b1', 'to': 'd2'}, 'Nbd2'),
  ('two rooks same file',
   {'board': {'a1': 'R', 'a5': 'R', 'e1': 'K', 'e8': 'k'}, 'from': 'a1', 'to': 'a3'}, 'R1a3'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('capture with file',
   {'board': {'a1': 'K', 'a8': 'k', 'c3': 'N', 'e4': 'p', 'g3': 'N'}, 'from': 'c3', 'to': 'e4'},
   'Ncxe4'),
  ('simple bishop', {'board': {'c1': 'B', 'e1': 'K', 'e8': 'k'}, 'from': 'c1', 'to': 'g5'}, 'Bg5'),
  ('rooks on same rank',
   {'board': {'a1': 'R', 'f1': 'R', 'h2': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd1'}, 'Rad1'),
  ('rook shielded by bishop',
   {'board': {'a4': 'R', 'a8': 'k', 'c4': 'B', 'h1': 'K', 'h4': 'R'}, 'from': 'h4', 'to': 'd4'},
   'Rd4')],
 [('two knights different files',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'N'}, 'from': 'b1', 'to': 'd2'}, 'Nbd2'),
  ('two rooks same file',
   {'board': {'a1': 'R', 'a5': 'R', 'e1': 'K', 'e8': 'k'}, 'from': 'a1', 'to': 'a3'}, 'R1a3'),
  ('three queens need square',
   {'board': {'a1': 'K', 'a8': 'k', 'e4': 'Q', 'h1': 'Q', 'h4': 'Q'}, 'from': 'h4', 'to': 'e1'},
   'Qh4e1'),
  ('blocked rook no disambiguation',
   {'board': {'a1': 'R', 'c1': 'B', 'e2': 'K', 'e8': 'k', 'h1': 'R'}, 'from': 'h1', 'to': 'd1'},
   'Rd1'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('black rook capture',
   {'board': {'a2': 'P', 'a8': 'r', 'e1': 'K', 'e8': 'k'}, 'from': 'a8', 'to': 'a2'}, 'Rxa2'),
  ('rook shielded by bishop',
   {'board': {'a4': 'R', 'a8': 'k', 'c4': 'B', 'h1': 'K', 'h4': 'R'}, 'from': 'h4', 'to': 'd4'},
   'Rd4')],
 [('two knights different files',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'N'}, 'from': 'b1', 'to': 'd2'}, 'Nbd2'),
  ('two rooks same file',
   {'board': {'a1': 'R', 'a5': 'R', 'e1': 'K', 'e8': 'k'}, 'from': 'a1', 'to': 'a3'}, 'R1a3'),
  ('three queens need square',
   {'board': {'a1': 'K', 'a8': 'k', 'e4': 'Q', 'h1': 'Q', 'h4': 'Q'}, 'from': 'h4', 'to': 'e1'},
   'Qh4e1'),
  ('blocked rook no disambiguation',
   {'board': {'a1': 'R', 'c1': 'B', 'e2': 'K', 'e8': 'k', 'h1': 'R'}, 'from': 'h1', 'to': 'd1'},
   'Rd1'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('black rook capture',
   {'board': {'a2': 'P', 'a8': 'r', 'e1': 'K', 'e8': 'k'}, 'from': 'a8', 'to': 'a2'}, 'Rxa2'),
  ('queen diagonal twin',
   {'board': {'a1': 'Q', 'g1': 'Q', 'h1': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd4'}, 'Qad4')]]
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
two knights different files	`Nbd2`	`Nbd2`	Passed
two rooks same file	`R1a3`	`R1a3`	Passed
three queens need square	`Qh4e1`	`Qh4e1`	Passed
blocked rook no disambiguation	`Rd1`	`Rd1`	Passed
enemy knight irrelevant	`Nbd2`	`Nd2`	Failed
black knights	`Nbd7`	`Nbd7`	Passed
black rook capture	`Rxa2`	`Rxa2`	Passed
queen diagonal twin	`Qad4`	`Qad4`	Passed

SHA-256 / 6ba38976b6086ab7219138e92c9ac68b01996c6e9eed66d8ca6f6f3a6845fd5e

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    b=x['board']; fr,to=x['from'],x['to']; p=b[fr]; white=p.isupper()
    def c(q): return ord(q[0])-97,int(q[1])-1
    def reach(q):
        f,r=c(q); tf,tr=c(to); df,dr=tf-f,tr-r
        t=p.upper()
        if t=='N': return (abs(df),abs(dr)) in ((1,2),(2,1))
        if t=='K': return max(abs(df),abs(dr))==1
        orth=(df==0)!=(dr==0); diag=abs(df)==abs(dr) and df!=0
        if not ((t=='R' and orth) or (t=='B' and diag) or (t=='Q' and (orth or diag))): return False
        sf=(df>0)-(df<0); sr=(dr>0)-(dr<0)
        cf,cr=f+sf,r+sr
        while (cf,cr)!=(tf,tr):
            if chr(97+cf)+str(cr+1) in b: return False
            cf+=sf; cr+=sr
        return True
    others=[q for q,v in b.items() if v==p.upper() and q!=fr and reach(q)]
    dis=''
    if others:
        if all(q[0]!=fr[0] for q in others): dis=fr[0]
        elif all(q[1]!=fr[1] for q in others): dis=fr[1]
        else: dis=fr
    cap='x' if to in b else ''
    return p.upper()+dis+cap+to
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('two knights different files',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'N'}, 'from': 'b1', 'to': 'd2'}, 'Nbd2'),
  ('two rooks same file',
   {'board': {'a1': 'R', 'a5': 'R', 'e1': 'K', 'e8': 'k'}, 'from': 'a1', 'to': 'a3'}, 'R1a3'),
  ('three queens need square',
   {'board': {'a1': 'K', 'a8': 'k', 'e4': 'Q', 'h1': 'Q', 'h4': 'Q'}, 'from': 'h4', 'to': 'e1'},
   'Qh4e1'),
  ('blocked rook no disambiguation',
   {'board': {'a1': 'R', 'c1': 'B', 'e2': 'K', 'e8': 'k', 'h1': 'R'}, 'from': 'h1', 'to': 'd1'},
   'Rd1'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('black rook capture',
   {'board': {'a2': 'P', 'a8': 'r', 'e1': 'K', 'e8': 'k'}, 'from': 'a8', 'to': 'a2'}, 'Rxa2'),
  ('queen diagonal twin',
   {'board': {'a1': 'Q', 'g1': 'Q', 'h1': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd4'}, 'Qad4')],
 [('blocked rook no disambiguation',
   {'board': {'a1': 'R', 'c1': 'B', 'e2': 'K', 'e8': 'k', 'h1': 'R'}, 'from': 'h1', 'to': 'd1'},
   'Rd1'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('black rook capture',
   {'board': {'a2': 'P', 'a8': 'r', 'e1': 'K', 'e8': 'k'}, 'from': 'a8', 'to': 'a2'}, 'Rxa2'),
  ('queen diagonal twin',
   {'board': {'a1': 'Q', 'g1': 'Q', 'h1': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd4'}, 'Qad4'),
  ('capture with file',
   {'board': {'a1': 'K', 'a8': 'k', 'c3': 'N', 'e4': 'p', 'g3': 'N'}, 'from': 'c3', 'to': 'e4'},
   'Ncxe4'),
  ('simple bishop', {'board': {'c1': 'B', 'e1': 'K', 'e8': 'k'}, 'from': 'c1', 'to': 'g5'}, 'Bg5'),
  ('rooks on same rank',
   {'board': {'a1': 'R', 'f1': 'R', 'h2': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd1'}, 'Rad1')],
 [('two knights different files',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'N'}, 'from': 'b1', 'to': 'd2'}, 'Nbd2'),
  ('two rooks same file',
   {'board': {'a1': 'R', 'a5': 'R', 'e1': 'K', 'e8': 'k'}, 'from': 'a1', 'to': 'a3'}, 'R1a3'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('capture with file',
   {'board': {'a1': 'K', 'a8': 'k', 'c3': 'N', 'e4': 'p', 'g3': 'N'}, 'from': 'c3', 'to': 'e4'},
   'Ncxe4'),
  ('simple bishop', {'board': {'c1': 'B', 'e1': 'K', 'e8': 'k'}, 'from': 'c1', 'to': 'g5'}, 'Bg5'),
  ('rooks on same rank',
   {'board': {'a1': 'R', 'f1': 'R', 'h2': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd1'}, 'Rad1'),
  ('rook shielded by bishop',
   {'board': {'a4': 'R', 'a8': 'k', 'c4': 'B', 'h1': 'K', 'h4': 'R'}, 'from': 'h4', 'to': 'd4'},
   'Rd4')],
 [('two knights different files',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'N'}, 'from': 'b1', 'to': 'd2'}, 'Nbd2'),
  ('two rooks same file',
   {'board': {'a1': 'R', 'a5': 'R', 'e1': 'K', 'e8': 'k'}, 'from': 'a1', 'to': 'a3'}, 'R1a3'),
  ('three queens need square',
   {'board': {'a1': 'K', 'a8': 'k', 'e4': 'Q', 'h1': 'Q', 'h4': 'Q'}, 'from': 'h4', 'to': 'e1'},
   'Qh4e1'),
  ('blocked rook no disambiguation',
   {'board': {'a1': 'R', 'c1': 'B', 'e2': 'K', 'e8': 'k', 'h1': 'R'}, 'from': 'h1', 'to': 'd1'},
   'Rd1'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('black rook capture',
   {'board': {'a2': 'P', 'a8': 'r', 'e1': 'K', 'e8': 'k'}, 'from': 'a8', 'to': 'a2'}, 'Rxa2'),
  ('rook shielded by bishop',
   {'board': {'a4': 'R', 'a8': 'k', 'c4': 'B', 'h1': 'K', 'h4': 'R'}, 'from': 'h4', 'to': 'd4'},
   'Rd4')],
 [('two knights different files',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'N'}, 'from': 'b1', 'to': 'd2'}, 'Nbd2'),
  ('two rooks same file',
   {'board': {'a1': 'R', 'a5': 'R', 'e1': 'K', 'e8': 'k'}, 'from': 'a1', 'to': 'a3'}, 'R1a3'),
  ('three queens need square',
   {'board': {'a1': 'K', 'a8': 'k', 'e4': 'Q', 'h1': 'Q', 'h4': 'Q'}, 'from': 'h4', 'to': 'e1'},
   'Qh4e1'),
  ('blocked rook no disambiguation',
   {'board': {'a1': 'R', 'c1': 'B', 'e2': 'K', 'e8': 'k', 'h1': 'R'}, 'from': 'h1', 'to': 'd1'},
   'Rd1'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('black rook capture',
   {'board': {'a2': 'P', 'a8': 'r', 'e1': 'K', 'e8': 'k'}, 'from': 'a8', 'to': 'a2'}, 'Rxa2'),
  ('queen diagonal twin',
   {'board': {'a1': 'Q', 'g1': 'Q', 'h1': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd4'}, 'Qad4')]]
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
two knights different files	`Nbd2`	`Nbd2`	Passed
two rooks same file	`R1a3`	`R1a3`	Passed
three queens need square	`Qh4e1`	`Qh4e1`	Passed
blocked rook no disambiguation	`Rd1`	`Rd1`	Passed
enemy knight irrelevant	`Nd2`	`Nd2`	Passed
black knights	`Nd7`	`Nbd7`	Failed
black rook capture	`Rxa2`	`Rxa2`	Passed
queen diagonal twin	`Qad4`	`Qad4`	Passed

SHA-256 / 7b6989b807ffb75fd4b3eef427b1be14e4bac3b8cc81252cbb1cc9fcbbf30137

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    b=x['board']; fr,to=x['from'],x['to']; p=b[fr]; white=p.isupper()
    def c(q): return ord(q[0])-97,int(q[1])-1
    def reach(q):
        f,r=c(q); tf,tr=c(to); df,dr=tf-f,tr-r
        t=p.upper()
        if t=='N': return (abs(df),abs(dr)) in ((1,2),(2,1))
        if t=='K': return max(abs(df),abs(dr))==1
        orth=(df==0)!=(dr==0); diag=abs(df)==abs(dr) and df!=0
        if not ((t=='R' and orth) or (t=='B' and diag) or (t=='Q' and (orth or diag))): return False
        sf=(df>0)-(df<0); sr=(dr>0)-(dr<0)
        cf,cr=f+sf,r+sr
        while (cf,cr)!=(tf,tr):
            if chr(97+cf)+str(cr+1) in b: return False
            cf+=sf; cr+=sr
        return True
    others=[q for q,v in b.items() if v==p and q!=fr and reach(q)]
    dis=''
    if others:
        if all(q[0]!=fr[0] for q in others): dis=fr[0]
        elif all(q[1]!=fr[1] for q in others): dis=fr[1]
        else: dis=fr
    cap='x' if to in b else ''
    return p.upper()+dis+cap+to
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[('two knights different files',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'N'}, 'from': 'b1', 'to': 'd2'}, 'Nbd2'),
  ('two rooks same file',
   {'board': {'a1': 'R', 'a5': 'R', 'e1': 'K', 'e8': 'k'}, 'from': 'a1', 'to': 'a3'}, 'R1a3'),
  ('three queens need square',
   {'board': {'a1': 'K', 'a8': 'k', 'e4': 'Q', 'h1': 'Q', 'h4': 'Q'}, 'from': 'h4', 'to': 'e1'},
   'Qh4e1'),
  ('blocked rook no disambiguation',
   {'board': {'a1': 'R', 'c1': 'B', 'e2': 'K', 'e8': 'k', 'h1': 'R'}, 'from': 'h1', 'to': 'd1'},
   'Rd1'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('black rook capture',
   {'board': {'a2': 'P', 'a8': 'r', 'e1': 'K', 'e8': 'k'}, 'from': 'a8', 'to': 'a2'}, 'Rxa2'),
  ('queen diagonal twin',
   {'board': {'a1': 'Q', 'g1': 'Q', 'h1': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd4'}, 'Qad4')],
 [('blocked rook no disambiguation',
   {'board': {'a1': 'R', 'c1': 'B', 'e2': 'K', 'e8': 'k', 'h1': 'R'}, 'from': 'h1', 'to': 'd1'},
   'Rd1'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('black rook capture',
   {'board': {'a2': 'P', 'a8': 'r', 'e1': 'K', 'e8': 'k'}, 'from': 'a8', 'to': 'a2'}, 'Rxa2'),
  ('queen diagonal twin',
   {'board': {'a1': 'Q', 'g1': 'Q', 'h1': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd4'}, 'Qad4'),
  ('capture with file',
   {'board': {'a1': 'K', 'a8': 'k', 'c3': 'N', 'e4': 'p', 'g3': 'N'}, 'from': 'c3', 'to': 'e4'},
   'Ncxe4'),
  ('simple bishop', {'board': {'c1': 'B', 'e1': 'K', 'e8': 'k'}, 'from': 'c1', 'to': 'g5'}, 'Bg5'),
  ('rooks on same rank',
   {'board': {'a1': 'R', 'f1': 'R', 'h2': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd1'}, 'Rad1')],
 [('two knights different files',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'N'}, 'from': 'b1', 'to': 'd2'}, 'Nbd2'),
  ('two rooks same file',
   {'board': {'a1': 'R', 'a5': 'R', 'e1': 'K', 'e8': 'k'}, 'from': 'a1', 'to': 'a3'}, 'R1a3'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('capture with file',
   {'board': {'a1': 'K', 'a8': 'k', 'c3': 'N', 'e4': 'p', 'g3': 'N'}, 'from': 'c3', 'to': 'e4'},
   'Ncxe4'),
  ('simple bishop', {'board': {'c1': 'B', 'e1': 'K', 'e8': 'k'}, 'from': 'c1', 'to': 'g5'}, 'Bg5'),
  ('rooks on same rank',
   {'board': {'a1': 'R', 'f1': 'R', 'h2': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd1'}, 'Rad1'),
  ('rook shielded by bishop',
   {'board': {'a4': 'R', 'a8': 'k', 'c4': 'B', 'h1': 'K', 'h4': 'R'}, 'from': 'h4', 'to': 'd4'},
   'Rd4')],
 [('two knights different files',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'N'}, 'from': 'b1', 'to': 'd2'}, 'Nbd2'),
  ('two rooks same file',
   {'board': {'a1': 'R', 'a5': 'R', 'e1': 'K', 'e8': 'k'}, 'from': 'a1', 'to': 'a3'}, 'R1a3'),
  ('three queens need square',
   {'board': {'a1': 'K', 'a8': 'k', 'e4': 'Q', 'h1': 'Q', 'h4': 'Q'}, 'from': 'h4', 'to': 'e1'},
   'Qh4e1'),
  ('blocked rook no disambiguation',
   {'board': {'a1': 'R', 'c1': 'B', 'e2': 'K', 'e8': 'k', 'h1': 'R'}, 'from': 'h1', 'to': 'd1'},
   'Rd1'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('black rook capture',
   {'board': {'a2': 'P', 'a8': 'r', 'e1': 'K', 'e8': 'k'}, 'from': 'a8', 'to': 'a2'}, 'Rxa2'),
  ('rook shielded by bishop',
   {'board': {'a4': 'R', 'a8': 'k', 'c4': 'B', 'h1': 'K', 'h4': 'R'}, 'from': 'h4', 'to': 'd4'},
   'Rd4')],
 [('two knights different files',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'N'}, 'from': 'b1', 'to': 'd2'}, 'Nbd2'),
  ('two rooks same file',
   {'board': {'a1': 'R', 'a5': 'R', 'e1': 'K', 'e8': 'k'}, 'from': 'a1', 'to': 'a3'}, 'R1a3'),
  ('three queens need square',
   {'board': {'a1': 'K', 'a8': 'k', 'e4': 'Q', 'h1': 'Q', 'h4': 'Q'}, 'from': 'h4', 'to': 'e1'},
   'Qh4e1'),
  ('blocked rook no disambiguation',
   {'board': {'a1': 'R', 'c1': 'B', 'e2': 'K', 'e8': 'k', 'h1': 'R'}, 'from': 'h1', 'to': 'd1'},
   'Rd1'),
  ('enemy knight irrelevant',
   {'board': {'b1': 'N', 'e1': 'K', 'e8': 'k', 'f3': 'n'}, 'from': 'b1', 'to': 'd2'}, 'Nd2'),
  ('black knights',
   {'board': {'b8': 'n', 'e1': 'K', 'e8': 'k', 'f6': 'n'}, 'from': 'b8', 'to': 'd7'}, 'Nbd7'),
  ('black rook capture',
   {'board': {'a2': 'P', 'a8': 'r', 'e1': 'K', 'e8': 'k'}, 'from': 'a8', 'to': 'a2'}, 'Rxa2'),
  ('queen diagonal twin',
   {'board': {'a1': 'Q', 'g1': 'Q', 'h1': 'K', 'h8': 'k'}, 'from': 'a1', 'to': 'd4'}, 'Qad4')]]
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
two knights different files	`Nbd2`	`Nbd2`	Passed
two rooks same file	`R1a3`	`R1a3`	Passed
three queens need square	`Qh4e1`	`Qh4e1`	Passed
blocked rook no disambiguation	`Rd1`	`Rd1`	Passed
enemy knight irrelevant	`Nd2`	`Nd2`	Passed
black knights	`Nbd7`	`Nbd7`	Passed
black rook capture	`Rxa2`	`Rxa2`	Passed
queen diagonal twin	`Qad4`	`Qad4`	Passed

SHA-256 / e0b9ec6393ca74dd6dfc5941583d59214b19822f9b0592a109b5511c416a6d36

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 2026-09-29T14:50:14.116797+00:00.

Case digest / af952dcbf0e373e27bebec8a0915f15e9cc68e7384fb732ca303f8a6eda77c30