FA-5401 / Integer arithmetic / Open access

Clear lowest set bit · case 01

Subtracting one also sets trailing zero bits.

Verified by executionVariant 1 · 4 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Subtracting one also sets trailing zero bits.

VERIFIED REPAIR

Apply the specified mathematical contract directly, preserving all terms and boundary cases: return n & (n-1)

Unsuccessful approach: Incrementing clears a run of low one bits rather than one set bit.

Case contract

Integer arguments; denominators, steps, moduli, and bit widths are positive. Unsigned word inputs fit the specified width except where masking is explicitly the operation. Sequence sizes are nonnegative. n is nonnegative; clear exactly its least significant one, or return zero for zero. Exact operational definition: n & (n-1)

Why this case matters

Small exact fixtures expose this error without platform timing, external services, or probabilistic observations. Integer mathematics results depend on the stated convention.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import math
import calendar
import statistics
import itertools
from fractions import Fraction
from datetime import date, datetime, timedelta, timezone
from decimal import Decimal, ROUND_HALF_UP, ROUND_DOWN, ROUND_CEILING, ROUND_FLOOR

N = 1
observations = []
def solve(n):
    return n-1
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('fixture 1: (12,)', solve(*(12,)), 8)
check('fixture 2: (8,)', solve(*(8,)), 0)
check('fixture 3: (0,)', solve(*(0,)), 0)
check('fixture 4: (7,)', solve(*(7,)), 6)
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
fixture 1: (12,)	`11`	`8`	Failed
fixture 2: (8,)	`7`	`0`	Failed
fixture 3: (0,)	`-1`	`0`	Failed
fixture 4: (7,)	`6`	`6`	Passed

SHA-256 / 9df00322629c62f1cf1ce6fdbc3d61a39c0255ee9cf7bd61776406303027cdb9

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import math
import calendar
import statistics
import itertools
from fractions import Fraction
from datetime import date, datetime, timedelta, timezone
from decimal import Decimal, ROUND_HALF_UP, ROUND_DOWN, ROUND_CEILING, ROUND_FLOOR

N = 1
observations = []
def solve(n):
    return n & (n+1)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('fixture 1: (12,)', solve(*(12,)), 8)
check('fixture 2: (8,)', solve(*(8,)), 0)
check('fixture 3: (0,)', solve(*(0,)), 0)
check('fixture 4: (7,)', solve(*(7,)), 6)
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
fixture 1: (12,)	`12`	`8`	Failed
fixture 2: (8,)	`8`	`0`	Failed
fixture 3: (0,)	`0`	`0`	Passed
fixture 4: (7,)	`0`	`6`	Failed

SHA-256 / e247945ab2ddcbab29e682c8b134edd9688e57b28575692088c5e518b4821c24

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
import math
import calendar
import statistics
import itertools
from fractions import Fraction
from datetime import date, datetime, timedelta, timezone
from decimal import Decimal, ROUND_HALF_UP, ROUND_DOWN, ROUND_CEILING, ROUND_FLOOR

N = 1
observations = []
def solve(n):
    return n & (n-1)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('fixture 1: (12,)', solve(*(12,)), 8)
check('fixture 2: (8,)', solve(*(8,)), 0)
check('fixture 3: (0,)', solve(*(0,)), 0)
check('fixture 4: (7,)', solve(*(7,)), 6)
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
fixture 1: (12,)	`8`	`8`	Passed
fixture 2: (8,)	`0`	`0`	Passed
fixture 3: (0,)	`0`	`0`	Passed
fixture 4: (7,)	`6`	`6`	Passed

SHA-256 / 9f77c26742fbecf660dc5bce1079e41da12bdeab716a6f64e1f3b8267f0b7e1c

Verification & scope

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 2026-09-29T14:37:49.183235+00:00.

Case digest / cd536b92ec5306b82df5023afbf78b7aa3a7fadbb7cf65a8a8517573ead11859