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FA-83511 / Card game rule engines / Open access

Reverse in a two-player game hands the turn to the opponent · case 01

With two players a reverse does not let the player go again.

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

ROOT CAUSE

The two-player special case is missing.

VERIFIED REPAIR

With two players reverse acts as a skip.

Unsuccessful approach: Extending the skip behaviour to three players skips a player who should move.

Case contract

Input [players, current, direction(+1/-1), card]. Reverse flips direction (with two players it acts as skip); skip jumps one player; draw2/wild4 make the next player draw 2/4 and lose their turn. Return [next_player, direction, draw_count, victim_or_None].

Why this case matters

Card-game engines, scoring apps and online tables apply this rule automatically on every hand.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    n, cur, direction, card = x
    step = 1
    victim = None
    draw = 0
    if card == 'reverse':
        direction = -direction
    elif card == 'skip':
        step = 2
    elif card in ('draw2', 'wild4'):
        victim = (cur + direction) % n
        draw = 2 if card == 'draw2' else 4
        step = 2
    nxt = (cur + direction * step) % n
    return [nxt, direction, draw, victim]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[2, 1, 1, 'wild4'], [1, 1, 4, 0]], [[2, 1, -1, 'skip'], [1, -1, 0, None]], [[2, 1, -1, 'wild'], [0, -1, 0, None]], [[3, 0, 1, 'draw2'], [2, 1, 2, 1]], [[3, 0, 1, 'wild'], [1, 1, 0, None]], [[3, 0, 1, 'wild4'], [2, 1, 4, 1]], [[3, 0, -1, 'reverse'], [1, 1, 0, None]]], [[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[3, 0, -1, 'reverse'], [1, 1, 0, None]], [[3, 0, -1, 'wild'], [2, -1, 0, None]], [[3, 0, -1, 'wild4'], [1, -1, 4, 2]], [[3, 1, 1, 'num'], [2, 1, 0, None]], [[3, 1, 1, 'skip'], [0, 1, 0, None]], [[3, 1, 1, 'wild'], [2, 1, 0, None]], [[3, 2, -1, 'reverse'], [0, 1, 0, None]]], [[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[3, 1, -1, 'num'], [0, -1, 0, None]], [[3, 1, -1, 'skip'], [2, -1, 0, None]], [[3, 1, -1, 'reverse'], [2, 1, 0, None]], [[3, 1, -1, 'wild4'], [2, -1, 4, 0]], [[3, 2, 1, 'skip'], [1, 1, 0, None]], [[3, 2, 1, 'draw2'], [1, 1, 2, 0]], [[3, 2, 1, 'wild4'], [1, 1, 4, 0]]], [[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[3, 0, -1, 'reverse'], [1, 1, 0, None]], [[3, 2, -1, 'reverse'], [0, 1, 0, None]], [[3, 2, -1, 'wild4'], [0, -1, 4, 1]], [[4, 0, 1, 'num'], [1, 1, 0, None]], [[4, 0, 1, 'skip'], [2, 1, 0, None]], [[4, 0, 1, 'reverse'], [3, -1, 0, None]], [[4, 0, 1, 'wild'], [1, 1, 0, None]]], [[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[3, 2, -1, 'reverse'], [0, 1, 0, None]], [[4, 0, -1, 'num'], [3, -1, 0, None]], [[4, 0, -1, 'skip'], [2, -1, 0, None]], [[4, 0, -1, 'wild4'], [2, -1, 4, 3]], [[4, 1, 1, 'num'], [2, 1, 0, None]], [[4, 1, 1, 'skip'], [3, 1, 0, None]], [[4, 1, 1, 'reverse'], [0, -1, 0, None]]]]
for i, (args, expected) in enumerate(fixtures[N-1]):
    check("turn case %d" % i, solve(args), 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 fixtureActualExpectedOutcome
turn case 0[1, 1, 0, None][0, 1, 0, None]Failed
turn case 1[1, 1, 4, 0][1, 1, 4, 0]Passed
turn case 2[1, -1, 0, None][1, -1, 0, None]Passed
turn case 3[0, -1, 0, None][0, -1, 0, None]Passed
turn case 4[2, 1, 2, 1][2, 1, 2, 1]Passed
turn case 5[1, 1, 0, None][1, 1, 0, None]Passed
turn case 6[2, 1, 4, 1][2, 1, 4, 1]Passed
turn case 7[1, 1, 0, None][1, 1, 0, None]Passed

SHA-256 / aabafb024168a181147adf877333c49efc1bf07ecad2ebd7cd1abfbeeefaf38e

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    n, cur, direction, card = x
    step = 1
    victim = None
    draw = 0
    if card == 'reverse':
        direction = -direction
        if n <= 3:
            step = 2
    elif card == 'skip':
        step = 2
    elif card in ('draw2', 'wild4'):
        victim = (cur + direction) % n
        draw = 2 if card == 'draw2' else 4
        step = 2
    nxt = (cur + direction * step) % n
    return [nxt, direction, draw, victim]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[2, 1, 1, 'wild4'], [1, 1, 4, 0]], [[2, 1, -1, 'skip'], [1, -1, 0, None]], [[2, 1, -1, 'wild'], [0, -1, 0, None]], [[3, 0, 1, 'draw2'], [2, 1, 2, 1]], [[3, 0, 1, 'wild'], [1, 1, 0, None]], [[3, 0, 1, 'wild4'], [2, 1, 4, 1]], [[3, 0, -1, 'reverse'], [1, 1, 0, None]]], [[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[3, 0, -1, 'reverse'], [1, 1, 0, None]], [[3, 0, -1, 'wild'], [2, -1, 0, None]], [[3, 0, -1, 'wild4'], [1, -1, 4, 2]], [[3, 1, 1, 'num'], [2, 1, 0, None]], [[3, 1, 1, 'skip'], [0, 1, 0, None]], [[3, 1, 1, 'wild'], [2, 1, 0, None]], [[3, 2, -1, 'reverse'], [0, 1, 0, None]]], [[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[3, 1, -1, 'num'], [0, -1, 0, None]], [[3, 1, -1, 'skip'], [2, -1, 0, None]], [[3, 1, -1, 'reverse'], [2, 1, 0, None]], [[3, 1, -1, 'wild4'], [2, -1, 4, 0]], [[3, 2, 1, 'skip'], [1, 1, 0, None]], [[3, 2, 1, 'draw2'], [1, 1, 2, 0]], [[3, 2, 1, 'wild4'], [1, 1, 4, 0]]], [[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[3, 0, -1, 'reverse'], [1, 1, 0, None]], [[3, 2, -1, 'reverse'], [0, 1, 0, None]], [[3, 2, -1, 'wild4'], [0, -1, 4, 1]], [[4, 0, 1, 'num'], [1, 1, 0, None]], [[4, 0, 1, 'skip'], [2, 1, 0, None]], [[4, 0, 1, 'reverse'], [3, -1, 0, None]], [[4, 0, 1, 'wild'], [1, 1, 0, None]]], [[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[3, 2, -1, 'reverse'], [0, 1, 0, None]], [[4, 0, -1, 'num'], [3, -1, 0, None]], [[4, 0, -1, 'skip'], [2, -1, 0, None]], [[4, 0, -1, 'wild4'], [2, -1, 4, 3]], [[4, 1, 1, 'num'], [2, 1, 0, None]], [[4, 1, 1, 'skip'], [3, 1, 0, None]], [[4, 1, 1, 'reverse'], [0, -1, 0, None]]]]
for i, (args, expected) in enumerate(fixtures[N-1]):
    check("turn case %d" % i, solve(args), 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 fixtureActualExpectedOutcome
turn case 0[0, 1, 0, None][0, 1, 0, None]Passed
turn case 1[1, 1, 4, 0][1, 1, 4, 0]Passed
turn case 2[1, -1, 0, None][1, -1, 0, None]Passed
turn case 3[0, -1, 0, None][0, -1, 0, None]Passed
turn case 4[2, 1, 2, 1][2, 1, 2, 1]Passed
turn case 5[1, 1, 0, None][1, 1, 0, None]Passed
turn case 6[2, 1, 4, 1][2, 1, 4, 1]Passed
turn case 7[2, 1, 0, None][1, 1, 0, None]Failed

SHA-256 / f864cf7107c6b323af6912b988ad2cd1fceba1e017640c48ddd8302084162834

3 / The verified repair

Exit 0
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(x):
    n, cur, direction, card = x
    step = 1
    victim = None
    draw = 0
    if card == 'reverse':
        direction = -direction
        if n == 2:
            step = 2
    elif card == 'skip':
        step = 2
    elif card in ('draw2', 'wild4'):
        victim = (cur + direction) % n
        draw = 2 if card == 'draw2' else 4
        step = 2
    nxt = (cur + direction * step) % n
    return [nxt, direction, draw, victim]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[2, 1, 1, 'wild4'], [1, 1, 4, 0]], [[2, 1, -1, 'skip'], [1, -1, 0, None]], [[2, 1, -1, 'wild'], [0, -1, 0, None]], [[3, 0, 1, 'draw2'], [2, 1, 2, 1]], [[3, 0, 1, 'wild'], [1, 1, 0, None]], [[3, 0, 1, 'wild4'], [2, 1, 4, 1]], [[3, 0, -1, 'reverse'], [1, 1, 0, None]]], [[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[3, 0, -1, 'reverse'], [1, 1, 0, None]], [[3, 0, -1, 'wild'], [2, -1, 0, None]], [[3, 0, -1, 'wild4'], [1, -1, 4, 2]], [[3, 1, 1, 'num'], [2, 1, 0, None]], [[3, 1, 1, 'skip'], [0, 1, 0, None]], [[3, 1, 1, 'wild'], [2, 1, 0, None]], [[3, 2, -1, 'reverse'], [0, 1, 0, None]]], [[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[3, 1, -1, 'num'], [0, -1, 0, None]], [[3, 1, -1, 'skip'], [2, -1, 0, None]], [[3, 1, -1, 'reverse'], [2, 1, 0, None]], [[3, 1, -1, 'wild4'], [2, -1, 4, 0]], [[3, 2, 1, 'skip'], [1, 1, 0, None]], [[3, 2, 1, 'draw2'], [1, 1, 2, 0]], [[3, 2, 1, 'wild4'], [1, 1, 4, 0]]], [[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[3, 0, -1, 'reverse'], [1, 1, 0, None]], [[3, 2, -1, 'reverse'], [0, 1, 0, None]], [[3, 2, -1, 'wild4'], [0, -1, 4, 1]], [[4, 0, 1, 'num'], [1, 1, 0, None]], [[4, 0, 1, 'skip'], [2, 1, 0, None]], [[4, 0, 1, 'reverse'], [3, -1, 0, None]], [[4, 0, 1, 'wild'], [1, 1, 0, None]]], [[[2, 0, -1, 'reverse'], [0, 1, 0, None]], [[3, 2, -1, 'reverse'], [0, 1, 0, None]], [[4, 0, -1, 'num'], [3, -1, 0, None]], [[4, 0, -1, 'skip'], [2, -1, 0, None]], [[4, 0, -1, 'wild4'], [2, -1, 4, 3]], [[4, 1, 1, 'num'], [2, 1, 0, None]], [[4, 1, 1, 'skip'], [3, 1, 0, None]], [[4, 1, 1, 'reverse'], [0, -1, 0, None]]]]
for i, (args, expected) in enumerate(fixtures[N-1]):
    check("turn case %d" % i, solve(args), 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 fixtureActualExpectedOutcome
turn case 0[0, 1, 0, None][0, 1, 0, None]Passed
turn case 1[1, 1, 4, 0][1, 1, 4, 0]Passed
turn case 2[1, -1, 0, None][1, -1, 0, None]Passed
turn case 3[0, -1, 0, None][0, -1, 0, None]Passed
turn case 4[2, 1, 2, 1][2, 1, 2, 1]Passed
turn case 5[1, 1, 0, None][1, 1, 0, None]Passed
turn case 6[2, 1, 4, 1][2, 1, 4, 1]Passed
turn case 7[1, 1, 0, None][1, 1, 0, None]Passed

SHA-256 / 429497d49ba105987c3515a519cf24ab7b5933f7448ade66166ebf14734fd33b

Verification & scope

A bounded toy rule contract stated explicitly in the contract field; cards are two-character codes (rank, suit). Not a complete implementation of any published rulebook or casino table rules. 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 2026-09-29T14:50:22.210261+00:00.

Case digest / 1b511ca0625530c6947bf84297c289b25884dbf4cd8dc8f7deca5a5ef2d8343a