FA-81421 / MIDI event timing / Open access

Track event parser with running status: meta payload skip off by one · case 01

After a meta event the parser lands on the last payload byte and reads it as the next delta.

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

ROOT CAUSE

The meta skip counts FF and type but forgets the length byte itself.

VERIFIED REPAIR

Restore the meta length skip step so that it reads `i += 3 + x[i + 2]`.

Unsuccessful approach: Skipping only the three header bytes ignores the payload, so payload bytes are parsed as events.

Case contract

Input the bytes of a track body: each event is a variable-length delta then either a meta event (FF type len data...), a status byte (bit 7 set) with its data bytes, or data bytes reusing the previous channel status (running status). Program change (Cx) and channel pressure (Dx) take one data byte, other channel messages two. Meta events cancel running status. Return [absolute tick, status, data...] for channel events and [tick, "meta", type] for meta events, or None on truncation or data without a running status.

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
    out = []
    i = 0
    tick = 0
    running = None
    while i < len(x):
        delta = 0
        while True:
            if i >= len(x):
                return None
            b = x[i]
            i += 1
            delta = (delta << 7) | (b & 0x7F)
            if not b & 0x80:
                break
        tick += delta
        if i >= len(x):
            return None
        status = x[i]
        if status == 0xFF:
            if i + 2 >= len(x):
                return None
            kind = x[i + 1]
            i += 2 + x[i + 2]
            if i > len(x):
                return None
            running = None
            out.append([tick, 'meta', kind])
            continue
        if status & 0x80:
            i += 1
            running = status
        elif running is None:
            return None
        else:
            status = running
        size = 1 if (status & 0xF0) in (0xC0, 0xD0) else 2
        if i + size > len(x):
            return None
        out.append([tick, status] + x[i:i + size])
        i += size
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[([0, 144, 60, 100, 10, 60, 0], [[0, 144, 60, 100], [10, 144, 60, 0]]), ([0, 192, 5, 0, 6], [[0, 192, 5], [0, 192, 6]]), ([0, 208, 64, 0, 50], [[0, 208, 64], [0, 208, 50]]), ([0, 144, 60, 100, 0, 255, 1, 0, 0, 62, 100], None), ([0, 144, 60, 100, 0, 255, 1, 1, 65, 0, 62, 100], None), ([0, 176, 7, 100, 20, 10, 90], [[0, 176, 7, 100], [20, 176, 10, 90]]), ([0, 255, 47, 0], [[0, 'meta', 47]]), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]])], [([0, 144, 60, 100, 0, 255, 1, 0, 0, 62, 100], None), ([0, 144, 60, 100, 0, 255, 1, 1, 65, 0, 62, 100], None), ([0, 176, 7, 100, 20, 10, 90], [[0, 176, 7, 100], [20, 176, 10, 90]]), ([127, 144, 60, 1], [[127, 144, 60, 1]]), ([128, 0, 144, 60, 1], [[0, 144, 60, 1]]), ([129, 0, 144, 60, 1], [[128, 144, 60, 1]]), ([0, 144, 60], None), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]])], [([127, 144, 60, 1], [[127, 144, 60, 1]]), ([128, 0, 144, 60, 1], [[0, 144, 60, 1]]), ([129, 0, 144, 60, 1], [[128, 144, 60, 1]]), ([0, 144, 60], None), ([0, 62, 100], None), ([0, 192, 5, 7, 6, 0, 144, 64, 90], [[0, 192, 5], [7, 192, 6], [7, 144, 64, 90]]), ([0, 255, 47, 0], [[0, 'meta', 47]]), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]])], [([0, 144, 60], None), ([0, 62, 100], None), ([0, 192, 5, 7, 6, 0, 144, 64, 90], [[0, 192, 5], [7, 192, 6], [7, 144, 64, 90]]), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]]), ([0, 224, 0, 64, 16, 0, 65], [[0, 224, 0, 64], [16, 224, 0, 65]]), ([5, 144, 60, 100, 5, 128, 60, 0, 5, 62, 0], [[5, 144, 60, 100], [10, 128, 60, 0], [15, 128, 62, 0]]), ([0, 208, 10, 0, 20], [[0, 208, 10], [0, 208, 20]]), ([0, 255, 3], None)], [([0, 255, 47, 0], [[0, 'meta', 47]]), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]]), ([0, 224, 0, 64, 16, 0, 65], [[0, 224, 0, 64], [16, 224, 0, 65]]), ([5, 144, 60, 100, 5, 128, 60, 0, 5, 62, 0], [[5, 144, 60, 100], [10, 128, 60, 0], [15, 128, 62, 0]]), ([0, 208, 10, 0, 20], [[0, 208, 10], [0, 208, 20]]), ([0, 255, 3], None), ([0, 255, 3, 5, 65], None), ([0, 144, 60, 100, 0, 192, 9, 3, 11], [[0, 144, 60, 100], [0, 192, 9], [3, 192, 11]])]]
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, 144, 60, 100], [10, 144, 60, 0]]`	`[[0, 144, 60, 100], [10, 144, 60, 0]]`	Passed
oracle 1	`[[0, 192, 5], [0, 192, 6]]`	`[[0, 192, 5], [0, 192, 6]]`	Passed
oracle 2	`[[0, 208, 64], [0, 208, 50]]`	`[[0, 208, 64], [0, 208, 50]]`	Passed
oracle 3	`None`	`None`	Passed
oracle 4	`None`	`None`	Passed
oracle 5	`[[0, 176, 7, 100], [20, 176, 10, 90]]`	`[[0, 176, 7, 100], [20, 176, 10, 90]]`	Passed
oracle 6	`None`	`[[0, 'meta', 47]]`	Failed
oracle 7	`None`	`[[0, 'meta', 81], [0, 144, 60, 64]]`	Failed

SHA-256 / 61ca285224d9f986f892c161d26a32be0cae56cdbb932b24efa26cbfdd8cfe65

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
    out = []
    i = 0
    tick = 0
    running = None
    while i < len(x):
        delta = 0
        while True:
            if i >= len(x):
                return None
            b = x[i]
            i += 1
            delta = (delta << 7) | (b & 0x7F)
            if not b & 0x80:
                break
        tick += delta
        if i >= len(x):
            return None
        status = x[i]
        if status == 0xFF:
            if i + 2 >= len(x):
                return None
            kind = x[i + 1]
            i += 3
            if i > len(x):
                return None
            running = None
            out.append([tick, 'meta', kind])
            continue
        if status & 0x80:
            i += 1
            running = status
        elif running is None:
            return None
        else:
            status = running
        size = 1 if (status & 0xF0) in (0xC0, 0xD0) else 2
        if i + size > len(x):
            return None
        out.append([tick, status] + x[i:i + size])
        i += size
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[([0, 144, 60, 100, 10, 60, 0], [[0, 144, 60, 100], [10, 144, 60, 0]]), ([0, 192, 5, 0, 6], [[0, 192, 5], [0, 192, 6]]), ([0, 208, 64, 0, 50], [[0, 208, 64], [0, 208, 50]]), ([0, 144, 60, 100, 0, 255, 1, 0, 0, 62, 100], None), ([0, 144, 60, 100, 0, 255, 1, 1, 65, 0, 62, 100], None), ([0, 176, 7, 100, 20, 10, 90], [[0, 176, 7, 100], [20, 176, 10, 90]]), ([0, 255, 47, 0], [[0, 'meta', 47]]), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]])], [([0, 144, 60, 100, 0, 255, 1, 0, 0, 62, 100], None), ([0, 144, 60, 100, 0, 255, 1, 1, 65, 0, 62, 100], None), ([0, 176, 7, 100, 20, 10, 90], [[0, 176, 7, 100], [20, 176, 10, 90]]), ([127, 144, 60, 1], [[127, 144, 60, 1]]), ([128, 0, 144, 60, 1], [[0, 144, 60, 1]]), ([129, 0, 144, 60, 1], [[128, 144, 60, 1]]), ([0, 144, 60], None), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]])], [([127, 144, 60, 1], [[127, 144, 60, 1]]), ([128, 0, 144, 60, 1], [[0, 144, 60, 1]]), ([129, 0, 144, 60, 1], [[128, 144, 60, 1]]), ([0, 144, 60], None), ([0, 62, 100], None), ([0, 192, 5, 7, 6, 0, 144, 64, 90], [[0, 192, 5], [7, 192, 6], [7, 144, 64, 90]]), ([0, 255, 47, 0], [[0, 'meta', 47]]), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]])], [([0, 144, 60], None), ([0, 62, 100], None), ([0, 192, 5, 7, 6, 0, 144, 64, 90], [[0, 192, 5], [7, 192, 6], [7, 144, 64, 90]]), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]]), ([0, 224, 0, 64, 16, 0, 65], [[0, 224, 0, 64], [16, 224, 0, 65]]), ([5, 144, 60, 100, 5, 128, 60, 0, 5, 62, 0], [[5, 144, 60, 100], [10, 128, 60, 0], [15, 128, 62, 0]]), ([0, 208, 10, 0, 20], [[0, 208, 10], [0, 208, 20]]), ([0, 255, 3], None)], [([0, 255, 47, 0], [[0, 'meta', 47]]), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]]), ([0, 224, 0, 64, 16, 0, 65], [[0, 224, 0, 64], [16, 224, 0, 65]]), ([5, 144, 60, 100, 5, 128, 60, 0, 5, 62, 0], [[5, 144, 60, 100], [10, 128, 60, 0], [15, 128, 62, 0]]), ([0, 208, 10, 0, 20], [[0, 208, 10], [0, 208, 20]]), ([0, 255, 3], None), ([0, 255, 3, 5, 65], None), ([0, 144, 60, 100, 0, 192, 9, 3, 11], [[0, 144, 60, 100], [0, 192, 9], [3, 192, 11]])]]
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, 144, 60, 100], [10, 144, 60, 0]]`	`[[0, 144, 60, 100], [10, 144, 60, 0]]`	Passed
oracle 1	`[[0, 192, 5], [0, 192, 6]]`	`[[0, 192, 5], [0, 192, 6]]`	Passed
oracle 2	`[[0, 208, 64], [0, 208, 50]]`	`[[0, 208, 64], [0, 208, 50]]`	Passed
oracle 3	`None`	`None`	Passed
oracle 4	`None`	`None`	Passed
oracle 5	`[[0, 176, 7, 100], [20, 176, 10, 90]]`	`[[0, 176, 7, 100], [20, 176, 10, 90]]`	Passed
oracle 6	`[[0, 'meta', 47]]`	`[[0, 'meta', 47]]`	Passed
oracle 7	`None`	`[[0, 'meta', 81], [0, 144, 60, 64]]`	Failed

SHA-256 / de903577c5a910bb4bb00eeef549c3f936cdeac44d05288d6a8e99a2ef277589

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
    out = []
    i = 0
    tick = 0
    running = None
    while i < len(x):
        delta = 0
        while True:
            if i >= len(x):
                return None
            b = x[i]
            i += 1
            delta = (delta << 7) | (b & 0x7F)
            if not b & 0x80:
                break
        tick += delta
        if i >= len(x):
            return None
        status = x[i]
        if status == 0xFF:
            if i + 2 >= len(x):
                return None
            kind = x[i + 1]
            i += 3 + x[i + 2]
            if i > len(x):
                return None
            running = None
            out.append([tick, 'meta', kind])
            continue
        if status & 0x80:
            i += 1
            running = status
        elif running is None:
            return None
        else:
            status = running
        size = 1 if (status & 0xF0) in (0xC0, 0xD0) else 2
        if i + size > len(x):
            return None
        out.append([tick, status] + x[i:i + size])
        i += size
    return out
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
fixtures = [[([0, 144, 60, 100, 10, 60, 0], [[0, 144, 60, 100], [10, 144, 60, 0]]), ([0, 192, 5, 0, 6], [[0, 192, 5], [0, 192, 6]]), ([0, 208, 64, 0, 50], [[0, 208, 64], [0, 208, 50]]), ([0, 144, 60, 100, 0, 255, 1, 0, 0, 62, 100], None), ([0, 144, 60, 100, 0, 255, 1, 1, 65, 0, 62, 100], None), ([0, 176, 7, 100, 20, 10, 90], [[0, 176, 7, 100], [20, 176, 10, 90]]), ([0, 255, 47, 0], [[0, 'meta', 47]]), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]])], [([0, 144, 60, 100, 0, 255, 1, 0, 0, 62, 100], None), ([0, 144, 60, 100, 0, 255, 1, 1, 65, 0, 62, 100], None), ([0, 176, 7, 100, 20, 10, 90], [[0, 176, 7, 100], [20, 176, 10, 90]]), ([127, 144, 60, 1], [[127, 144, 60, 1]]), ([128, 0, 144, 60, 1], [[0, 144, 60, 1]]), ([129, 0, 144, 60, 1], [[128, 144, 60, 1]]), ([0, 144, 60], None), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]])], [([127, 144, 60, 1], [[127, 144, 60, 1]]), ([128, 0, 144, 60, 1], [[0, 144, 60, 1]]), ([129, 0, 144, 60, 1], [[128, 144, 60, 1]]), ([0, 144, 60], None), ([0, 62, 100], None), ([0, 192, 5, 7, 6, 0, 144, 64, 90], [[0, 192, 5], [7, 192, 6], [7, 144, 64, 90]]), ([0, 255, 47, 0], [[0, 'meta', 47]]), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]])], [([0, 144, 60], None), ([0, 62, 100], None), ([0, 192, 5, 7, 6, 0, 144, 64, 90], [[0, 192, 5], [7, 192, 6], [7, 144, 64, 90]]), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]]), ([0, 224, 0, 64, 16, 0, 65], [[0, 224, 0, 64], [16, 224, 0, 65]]), ([5, 144, 60, 100, 5, 128, 60, 0, 5, 62, 0], [[5, 144, 60, 100], [10, 128, 60, 0], [15, 128, 62, 0]]), ([0, 208, 10, 0, 20], [[0, 208, 10], [0, 208, 20]]), ([0, 255, 3], None)], [([0, 255, 47, 0], [[0, 'meta', 47]]), ([0, 255, 81, 3, 7, 161, 32, 0, 144, 60, 64], [[0, 'meta', 81], [0, 144, 60, 64]]), ([0, 224, 0, 64, 16, 0, 65], [[0, 224, 0, 64], [16, 224, 0, 65]]), ([5, 144, 60, 100, 5, 128, 60, 0, 5, 62, 0], [[5, 144, 60, 100], [10, 128, 60, 0], [15, 128, 62, 0]]), ([0, 208, 10, 0, 20], [[0, 208, 10], [0, 208, 20]]), ([0, 255, 3], None), ([0, 255, 3, 5, 65], None), ([0, 144, 60, 100, 0, 192, 9, 3, 11], [[0, 144, 60, 100], [0, 192, 9], [3, 192, 11]])]]
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, 144, 60, 100], [10, 144, 60, 0]]`	`[[0, 144, 60, 100], [10, 144, 60, 0]]`	Passed
oracle 1	`[[0, 192, 5], [0, 192, 6]]`	`[[0, 192, 5], [0, 192, 6]]`	Passed
oracle 2	`[[0, 208, 64], [0, 208, 50]]`	`[[0, 208, 64], [0, 208, 50]]`	Passed
oracle 3	`None`	`None`	Passed
oracle 4	`None`	`None`	Passed
oracle 5	`[[0, 176, 7, 100], [20, 176, 10, 90]]`	`[[0, 176, 7, 100], [20, 176, 10, 90]]`	Passed
oracle 6	`[[0, 'meta', 47]]`	`[[0, 'meta', 47]]`	Passed
oracle 7	`[[0, 'meta', 81], [0, 144, 60, 64]]`	`[[0, 'meta', 81], [0, 144, 60, 64]]`	Passed

SHA-256 / 13c11958d76562886b22d1d3df9eeb5bf38abc94ab124b42964415c7f3192c0e

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 2026-09-29T14:50:02.845612+00:00.

Case digest / cb9532f54a8c5e594e1ab8fd53d7f11932992a8938111ad2774c268c14050631