FA-81366 / MIDI event timing / Open access

Variable-length quantity stream decoder: five-byte quantity accepted · case 01

A malformed five-byte quantity is decoded as a 35-bit value instead of being rejected.

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

ROOT CAUSE

The length guard fires only after a fifth continuation byte.

VERIFIED REPAIR

Restore the maximum length step so that it reads `count == 4`.

Unsuccessful approach: Firing at the third byte rejects legal four-byte quantities such as [255,255,255,127].

Case contract

Input a list of byte values. Decode consecutive variable-length quantities (7 data bits per byte, bit 7 = continuation, most significant first, at most 4 bytes each). Return the list of values; return None for a byte outside 0..255, a quantity whose fourth byte still has the continuation bit, or a truncated final quantity.

Why this case matters

MIDI sequencers, file readers and synth drivers depend on exact event ordering and tick/time arithmetic.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(x):
    if not isinstance(x, list):
        return None
    values = []
    cur = 0
    count = 0
    for b in x:
        if not isinstance(b, int) or b < 0 or b > 255:
            return None
        cur = (cur << 7) | (b & 0x7F)
        count += 1
        if b & 0x80:
            if count == 5:
                return None
            continue
        values.append(cur)
        cur = 0
        count = 0
    if count:
        return None
    return values
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[([0], [0]), ([127], [127]), ([129, 0], [128]), ([192, 0], [8192]), ([129, 128, 0], [16384]), ([255, 255, 255, 127], [268435455]), ([255, 255, 255, 255, 127], None), ([129], None)], [([192, 0], [8192]), ([129, 128, 0], [16384]), ([255, 255, 255, 255, 127], None), ([129], None), ([0, 129, 0, 5], [0, 128, 5]), ([129, 0, 0, 129, 0], [128, 0, 128]), ([200, 1, 2], [9217, 2]), ([128, 128, 128, 0], [0])], [([255, 255, 255, 255, 127], None), ([0, 129, 0, 5], [0, 128, 5]), ([129, 0, 0, 129, 0], [128, 0, 128]), ([200, 1, 2], [9217, 2]), ([], []), ([256], None), ([-1], None), ([143, 255, 255, 127], [33554431])], [([255, 255, 255, 127], [268435455]), ([255, 255, 255, 255, 127], None), ([], []), ([256], None), ([-1], None), ([131, 255, 127, 0], [65535, 0]), ([128, 0], [0]), ([129, 0, 129, 0, 129, 0], [128, 128, 128])], [([255, 255, 255, 127], [268435455]), ([255, 255, 255, 255, 127], None), ([131, 255, 127, 0], [65535, 0]), ([128, 128, 128, 0], [0]), ([128, 0], [0]), ([129, 0, 129, 0, 129, 0], [128, 128, 128]), ([1, 2, 3], [1, 2, 3]), ([134, 141, 32], [100000])]]
for i, (args, expected) in enumerate(fixtures[N-1]):
    check("oracle %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 fixture	Actual	Expected	Outcome
oracle 0	`[0]`	`[0]`	Passed
oracle 1	`[127]`	`[127]`	Passed
oracle 2	`[128]`	`[128]`	Passed
oracle 3	`[8192]`	`[8192]`	Passed
oracle 4	`[16384]`	`[16384]`	Passed
oracle 5	`[268435455]`	`[268435455]`	Passed
oracle 6	`[34359738367]`	`None`	Failed
oracle 7	`None`	`None`	Passed

SHA-256 / 7f5f44428c5586c3743f55c6c3d714a024cc0b58fc19bf7169e872c40bed7084

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(x):
    if not isinstance(x, list):
        return None
    values = []
    cur = 0
    count = 0
    for b in x:
        if not isinstance(b, int) or b < 0 or b > 255:
            return None
        cur = (cur << 7) | (b & 0x7F)
        count += 1
        if b & 0x80:
            if count == 3:
                return None
            continue
        values.append(cur)
        cur = 0
        count = 0
    if count:
        return None
    return values
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[([0], [0]), ([127], [127]), ([129, 0], [128]), ([192, 0], [8192]), ([129, 128, 0], [16384]), ([255, 255, 255, 127], [268435455]), ([255, 255, 255, 255, 127], None), ([129], None)], [([192, 0], [8192]), ([129, 128, 0], [16384]), ([255, 255, 255, 255, 127], None), ([129], None), ([0, 129, 0, 5], [0, 128, 5]), ([129, 0, 0, 129, 0], [128, 0, 128]), ([200, 1, 2], [9217, 2]), ([128, 128, 128, 0], [0])], [([255, 255, 255, 255, 127], None), ([0, 129, 0, 5], [0, 128, 5]), ([129, 0, 0, 129, 0], [128, 0, 128]), ([200, 1, 2], [9217, 2]), ([], []), ([256], None), ([-1], None), ([143, 255, 255, 127], [33554431])], [([255, 255, 255, 127], [268435455]), ([255, 255, 255, 255, 127], None), ([], []), ([256], None), ([-1], None), ([131, 255, 127, 0], [65535, 0]), ([128, 0], [0]), ([129, 0, 129, 0, 129, 0], [128, 128, 128])], [([255, 255, 255, 127], [268435455]), ([255, 255, 255, 255, 127], None), ([131, 255, 127, 0], [65535, 0]), ([128, 128, 128, 0], [0]), ([128, 0], [0]), ([129, 0, 129, 0, 129, 0], [128, 128, 128]), ([1, 2, 3], [1, 2, 3]), ([134, 141, 32], [100000])]]
for i, (args, expected) in enumerate(fixtures[N-1]):
    check("oracle %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 fixture	Actual	Expected	Outcome
oracle 0	`[0]`	`[0]`	Passed
oracle 1	`[127]`	`[127]`	Passed
oracle 2	`[128]`	`[128]`	Passed
oracle 3	`[8192]`	`[8192]`	Passed
oracle 4	`[16384]`	`[16384]`	Passed
oracle 5	`None`	`[268435455]`	Failed
oracle 6	`None`	`None`	Passed
oracle 7	`None`	`None`	Passed

SHA-256 / 82ea11815ed74cc9606577f36d3a7c15762768281d67091bddaaa405765d7ed4

3 / The verified repair

Exit 0

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

N = 1
observations = []
def solve(x):
    if not isinstance(x, list):
        return None
    values = []
    cur = 0
    count = 0
    for b in x:
        if not isinstance(b, int) or b < 0 or b > 255:
            return None
        cur = (cur << 7) | (b & 0x7F)
        count += 1
        if b & 0x80:
            if count == 4:
                return None
            continue
        values.append(cur)
        cur = 0
        count = 0
    if count:
        return None
    return values
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[([0], [0]), ([127], [127]), ([129, 0], [128]), ([192, 0], [8192]), ([129, 128, 0], [16384]), ([255, 255, 255, 127], [268435455]), ([255, 255, 255, 255, 127], None), ([129], None)], [([192, 0], [8192]), ([129, 128, 0], [16384]), ([255, 255, 255, 255, 127], None), ([129], None), ([0, 129, 0, 5], [0, 128, 5]), ([129, 0, 0, 129, 0], [128, 0, 128]), ([200, 1, 2], [9217, 2]), ([128, 128, 128, 0], [0])], [([255, 255, 255, 255, 127], None), ([0, 129, 0, 5], [0, 128, 5]), ([129, 0, 0, 129, 0], [128, 0, 128]), ([200, 1, 2], [9217, 2]), ([], []), ([256], None), ([-1], None), ([143, 255, 255, 127], [33554431])], [([255, 255, 255, 127], [268435455]), ([255, 255, 255, 255, 127], None), ([], []), ([256], None), ([-1], None), ([131, 255, 127, 0], [65535, 0]), ([128, 0], [0]), ([129, 0, 129, 0, 129, 0], [128, 128, 128])], [([255, 255, 255, 127], [268435455]), ([255, 255, 255, 255, 127], None), ([131, 255, 127, 0], [65535, 0]), ([128, 128, 128, 0], [0]), ([128, 0], [0]), ([129, 0, 129, 0, 129, 0], [128, 128, 128]), ([1, 2, 3], [1, 2, 3]), ([134, 141, 32], [100000])]]
for i, (args, expected) in enumerate(fixtures[N-1]):
    check("oracle %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 fixture	Actual	Expected	Outcome
oracle 0	`[0]`	`[0]`	Passed
oracle 1	`[127]`	`[127]`	Passed
oracle 2	`[128]`	`[128]`	Passed
oracle 3	`[8192]`	`[8192]`	Passed
oracle 4	`[16384]`	`[16384]`	Passed
oracle 5	`[268435455]`	`[268435455]`	Passed
oracle 6	`None`	`None`	Passed
oracle 7	`None`	`None`	Passed

SHA-256 / bbb58f425391c836381795fdb3fa10444bb46317d187b6c99b076e0ecb2519b8

Verification & scope

A deterministic bounded teaching model with a stipulated toy contract; it is not a complete Standard MIDI File or MIDI 1.0/2.0 implementation. 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 2026-09-29T14:50:02.448333+00:00.

Case digest / 4481ef34fee5c90285659e4a06919191e67fe92350bd8d7d1f86909e35f56e1b