FA-81406 / MIDI event timing / Open access
Track event parser with running status: one-byte messages recognised only on channel 1 · case 01
A program change on channel 2 (C1) swallows the next delta byte as a second data byte and desynchronises the track.
ROOT CAUSE
The one-data-byte test compares the whole status byte instead of its message-type nibble, so only channel 1 matches.
VERIFIED REPAIR
Restore the data length classification step so that it reads `(status & 0xF0) in (0xC0, 0xD0)`.
Unsuccessful approach: Parenthesising the mask but listing only program change leaves channel pressure with two data bytes.
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 += 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 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, 193, 5, 0, 6], [[0, 193, 5], [0, 193, 6]]), ([0, 194, 1, 4, 144, 60, 1], [[0, 194, 1], [4, 144, 60, 1]])], [([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, 208, 10, 0, 20], [[0, 208, 10], [0, 208, 20]]), ([0, 193, 5, 0, 6], [[0, 193, 5], [0, 193, 6]]), ([0, 209, 64, 0, 50], [[0, 209, 64], [0, 209, 50]])], [([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, 209, 64, 0, 50], [[0, 209, 64], [0, 209, 50]]), ([0, 194, 1, 4, 144, 60, 1], [[0, 194, 1], [4, 144, 60, 1]])], [([0, 208, 64, 0, 50], [[0, 208, 64], [0, 208, 50]]), ([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, 224, 0, 64, 16, 0, 65], [[0, 224, 0, 64], [16, 224, 0, 65]]), ([0, 193, 5, 0, 6], [[0, 193, 5], [0, 193, 6]]), ([0, 194, 1, 4, 144, 60, 1], [[0, 194, 1], [4, 144, 60, 1]])], [([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, 193, 5, 0, 6], [[0, 193, 5], [0, 193, 6]]), ([0, 209, 64, 0, 50], [[0, 209, 64], [0, 209, 50]])]]
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, 193, 5], [0, 193, 6]] | Failed |
| oracle 7 | None | [[0, 194, 1], [4, 144, 60, 1]] | Failed |
SHA-256 / 0ccbc3e78df655687fc5d8fc21bb95a3aa46cd2890a87a6fddfaa3c75cb0e8e8
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 + 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,) 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, 193, 5, 0, 6], [[0, 193, 5], [0, 193, 6]]), ([0, 194, 1, 4, 144, 60, 1], [[0, 194, 1], [4, 144, 60, 1]])], [([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, 208, 10, 0, 20], [[0, 208, 10], [0, 208, 20]]), ([0, 193, 5, 0, 6], [[0, 193, 5], [0, 193, 6]]), ([0, 209, 64, 0, 50], [[0, 209, 64], [0, 209, 50]])], [([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, 209, 64, 0, 50], [[0, 209, 64], [0, 209, 50]]), ([0, 194, 1, 4, 144, 60, 1], [[0, 194, 1], [4, 144, 60, 1]])], [([0, 208, 64, 0, 50], [[0, 208, 64], [0, 208, 50]]), ([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, 224, 0, 64, 16, 0, 65], [[0, 224, 0, 64], [16, 224, 0, 65]]), ([0, 193, 5, 0, 6], [[0, 193, 5], [0, 193, 6]]), ([0, 194, 1, 4, 144, 60, 1], [[0, 194, 1], [4, 144, 60, 1]])], [([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, 193, 5, 0, 6], [[0, 193, 5], [0, 193, 6]]), ([0, 209, 64, 0, 50], [[0, 209, 64], [0, 209, 50]])]]
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 | None | [[0, 208, 64], [0, 208, 50]] | Failed |
| 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, 193, 5], [0, 193, 6]] | [[0, 193, 5], [0, 193, 6]] | Passed |
| oracle 7 | [[0, 194, 1], [4, 144, 60, 1]] | [[0, 194, 1], [4, 144, 60, 1]] | Passed |
SHA-256 / 8f04f6138d6dfee3997b8fb9f054f80c4ed9ddb2eeef73bfbef029f5cba71c27
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, 193, 5, 0, 6], [[0, 193, 5], [0, 193, 6]]), ([0, 194, 1, 4, 144, 60, 1], [[0, 194, 1], [4, 144, 60, 1]])], [([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, 208, 10, 0, 20], [[0, 208, 10], [0, 208, 20]]), ([0, 193, 5, 0, 6], [[0, 193, 5], [0, 193, 6]]), ([0, 209, 64, 0, 50], [[0, 209, 64], [0, 209, 50]])], [([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, 209, 64, 0, 50], [[0, 209, 64], [0, 209, 50]]), ([0, 194, 1, 4, 144, 60, 1], [[0, 194, 1], [4, 144, 60, 1]])], [([0, 208, 64, 0, 50], [[0, 208, 64], [0, 208, 50]]), ([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, 224, 0, 64, 16, 0, 65], [[0, 224, 0, 64], [16, 224, 0, 65]]), ([0, 193, 5, 0, 6], [[0, 193, 5], [0, 193, 6]]), ([0, 194, 1, 4, 144, 60, 1], [[0, 194, 1], [4, 144, 60, 1]])], [([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, 193, 5, 0, 6], [[0, 193, 5], [0, 193, 6]]), ([0, 209, 64, 0, 50], [[0, 209, 64], [0, 209, 50]])]]
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, 193, 5], [0, 193, 6]] | [[0, 193, 5], [0, 193, 6]] | Passed |
| oracle 7 | [[0, 194, 1], [4, 144, 60, 1]] | [[0, 194, 1], [4, 144, 60, 1]] | Passed |
SHA-256 / bea6bae44a41cdc322807fc869748824b68650157c2a823aa680cf0d16772fa7
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 2026-09-29T14:50:02.547104+00:00.
Case digest / 109602483920ea854e070268229ae67b8602ffdc7a2e1054aef852e106238886