FA-3681 / Binary formats / Open access

A three-byte field is padded on the wrong side · case 01

A three-byte field is padded on the wrong side.

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

ROOT CAUSE

The implementation applies an operation whose text or grammar semantics violate this contract: Decode a little-endian unsigned 24-bit field represented by exactly three bytes.

VERIFIED REPAIR

Implement the complete stated contract, including the boundary fixtures: Decode a little-endian unsigned 24-bit field represented by exactly three bytes.

Unsuccessful approach: The attempted repair handles the primary example but still violates a separate boundary of the same contract.

Case contract

Decode a little-endian unsigned 24-bit field represented by exactly three bytes.

Why this case matters

A local executable model for consumers of structured text; oracle values are authored literals, not outputs copied from the repaired implementation.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import re, unicodedata, json, csv, io, html, base64, binascii, codecs, struct
from urllib.parse import quote, unquote, unquote_to_bytes, urlencode, parse_qsl, urlsplit, urlunsplit
from email.header import decode_header, make_header
from email.utils import getaddresses
from xml.etree import ElementTree as ET
import shlex, string, textwrap

N = 1
observations = []
def solve(x):
    try:
        return int.from_bytes(bytes(x+[0]),"big")
    except Exception as exc:
        return {"error": type(exc).__name__}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('boundary 1', solve([1, 0, 0]), 1)
check('boundary 2', solve([0, 0, 1]), 65536)
check('boundary 3', solve([255, 255, 255]), 16777215)
check('boundary 4', solve([0, 0, 0]), 0)
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
boundary 1	`16777216`	`1`	Failed
boundary 2	`256`	`65536`	Failed
boundary 3	`4294967040`	`16777215`	Failed
boundary 4	`0`	`0`	Passed

SHA-256 / ab9aced69b043c5f8d62fbc9b38f443955e1af0d5a1e56f248df4f9afba383b7

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import re, unicodedata, json, csv, io, html, base64, binascii, codecs, struct
from urllib.parse import quote, unquote, unquote_to_bytes, urlencode, parse_qsl, urlsplit, urlunsplit
from email.header import decode_header, make_header
from email.utils import getaddresses
from xml.etree import ElementTree as ET
import shlex, string, textwrap

N = 1
observations = []
def solve(x):
    try:
        return int.from_bytes(bytes([0]+x),"little")
    except Exception as exc:
        return {"error": type(exc).__name__}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('boundary 1', solve([1, 0, 0]), 1)
check('boundary 2', solve([0, 0, 1]), 65536)
check('boundary 3', solve([255, 255, 255]), 16777215)
check('boundary 4', solve([0, 0, 0]), 0)
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
boundary 1	`256`	`1`	Failed
boundary 2	`16777216`	`65536`	Failed
boundary 3	`4294967040`	`16777215`	Failed
boundary 4	`0`	`0`	Passed

SHA-256 / 7b3ed67deb3fb186153097968c6ced1f2137aa7054e00fbff2719b32934acd40

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
import re, unicodedata, json, csv, io, html, base64, binascii, codecs, struct
from urllib.parse import quote, unquote, unquote_to_bytes, urlencode, parse_qsl, urlsplit, urlunsplit
from email.header import decode_header, make_header
from email.utils import getaddresses
from xml.etree import ElementTree as ET
import shlex, string, textwrap

N = 1
observations = []
def solve(x):
    try:
        return int.from_bytes(bytes(x),"little")
    except Exception as exc:
        return {"error": type(exc).__name__}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('boundary 1', solve([1, 0, 0]), 1)
check('boundary 2', solve([0, 0, 1]), 65536)
check('boundary 3', solve([255, 255, 255]), 16777215)
check('boundary 4', solve([0, 0, 0]), 0)
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
boundary 1	`1`	`1`	Passed
boundary 2	`65536`	`65536`	Passed
boundary 3	`16777215`	`16777215`	Passed
boundary 4	`0`	`0`	Passed

SHA-256 / b866fe0fc0aa34ae5e987bdde39469fc73710793a2c029ef4a5c9dfb73c3529f

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 2026-09-29T14:37:29.508143+00:00.

Case digest / dbd83fc0fa9a184d5fd58da96713e136ac9099c6ffff12f2d338d6c3f78c4539