FA-81376 / MIDI event timing / Open access

Variable-length quantity stream decoder: truncated final quantity silently dropped · case 01

A stream ending in a byte with the continuation bit set, like [129], returns [] instead of None.

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

ROOT CAUSE

The decoder does not check for an unfinished quantity after the last byte.

THE FAILURE

The decoder does not check for an unfinished quantity after the last byte.

Unsuccessful approach: Emitting the partial value invents a delta from a truncated stream.

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 == 4:
                return None
            continue
        values.append(cur)
        cur = 0
        count = 0
    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, 127], [268435455]), ([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])], [([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]), ([], []), ([256], None), ([-1], None)], [([129], None), ([200, 1, 2], [9217, 2]), ([], []), ([256], None), ([-1], None), ([131, 255, 127, 0], [65535, 0]), ([128, 128, 128, 0], [0]), ([128, 0], [0])], [([129], None), ([-1], 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]), ([143, 255, 255, 127], [33554431])]]
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`	Failed

SHA-256 / 19b72e653580901491e104352a758c37f78f2f25f268671117e1d3348637cba9

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 == 4:
                return None
            continue
        values.append(cur)
        cur = 0
        count = 0
    if count:
        values.append(cur)
    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, 127], [268435455]), ([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])], [([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]), ([], []), ([256], None), ([-1], None)], [([129], None), ([200, 1, 2], [9217, 2]), ([], []), ([256], None), ([-1], None), ([131, 255, 127, 0], [65535, 0]), ([128, 128, 128, 0], [0]), ([128, 0], [0])], [([129], None), ([-1], 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]), ([143, 255, 255, 127], [33554431])]]
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	`[1]`	`None`	Failed

SHA-256 / 71bb5c96d38b59e18e78f2e9ecc051989cf86d9da259faf5ad753d8166f2ee8a

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 8 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

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 2026-09-29T14:50:02.492251+00:00.

Case digest / 71d8e80ff29efc716a90cfada1e6011a6933d2fe1b90189dd91bb7d84351d2ce