FAILURE MAP
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FA-90461 / Garbage collector invariants / Open access

Free-list sweep: slivers below the minimum block size created · case 01

Allocations leave 8-byte free fragments that can never hold a block header.

Verified by executionVariant 1 · 6 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

Any positive remainder is split off.

THE FAILURE

Any positive remainder is split off.

Unsuccessful approach: Requiring more than 16 bytes refuses to split an exactly minimum-sized remainder.

Case contract

blocks are [address, size, marked] covering the heap in any order. Sweep in address order: unmarked blocks become free and physically adjacent free blocks coalesce. Then serve requests first-fit in address order: round the request up to 8 bytes; split the chosen block when the remainder is at least 16 bytes (the allocation takes the low end), otherwise hand out the whole block; no fit yields None. Return the swept free list (as it was right after sweeping), the allocation addresses and the final free list.

Why this case matters

Sweepers and free-list allocators must keep block boundaries exact or they hand out overlapping memory.

1 / The failure

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(blocks, requests):
    free = []
    for addr, size, marked in sorted(blocks):
        if marked:
            continue
        if free and free[-1][0] + free[-1][1] == addr:
            free[-1][1] += size
        else:
            free.append([addr, size])
    swept = [list(b) for b in free]
    got = []
    for req in requests:
        need = (req + 7) // 8 * 8
        choice = None
        for b in free:
            if b[1] >= need:
                choice = b
                break
        if choice is None:
            got.append(None)
            continue
        got.append(choice[0])
        rem = choice[1] - need
        if rem > 0:
            choice[0] += need
            choice[1] = rem
        else:
            free.remove(choice)
    return {'swept': swept, 'alloc': got, 'free': free}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
cases = [[('regression: sweep, split and exhaust',
   ([[1088, 8, True],
     [1040, 32, False],
     [1024, 16, True],
     [1072, 16, False],
     [1096, 24, False],
     [1120, 40, False],
     [1160, 16, True]],
    [20, 24, 40, 10, 8]),
   {'alloc': [1040, 1064, 1096, 1136, None], 'free': [], 'swept': [[1040, 48], [1096, 64]]}),
  ('exact fit uses the first block',
   ([[1088, 8, True],
     [1040, 32, False],
     [1024, 16, True],
     [1072, 16, False],
     [1096, 24, False],
     [1120, 40, False],
     [1160, 16, True]],
    [48, 64]),
   {'alloc': [1040, 1096], 'free': [], 'swept': [[1040, 48], [1096, 64]]}),
  ('remainder of exactly the minimum block size',
   ([[1088, 8, True],
     [1040, 32, False],
     [1024, 16, True],
     [1072, 16, False],
     [1096, 24, False],
     [1120, 40, False],
     [1160, 16, True]],
    [32, 40]),
   {'alloc': [1040, 1096], 'free': [[1072, 16], [1136, 24]], 'swept': [[1040, 48], [1096, 64]]}),
  ('remainder too small to split',
   ([[1088, 8, True],
     [1040, 32, False],
     [1024, 16, True],
     [1072, 16, False],
     [1096, 24, False],
     [1120, 40, False],
     [1160, 16, True]],
    [44, 50, 1]),
   {'alloc': [1040, 1096, None], 'free': [], 'swept': [[1040, 48], [1096, 64]]}),
  ('sweep snapshot is independent of allocation',
   ([[1088, 8, True],
     [1040, 32, False],
     [1024, 16, True],
     [1072, 16, False],
     [1096, 24, False],
     [1120, 40, False],
     [1160, 16, True]],
    [1]),
   {'alloc': [1040], 'free': [[1048, 40], [1096, 64]], 'swept': [[1040, 48], [1096, 64]]}),
  ('control: fully marked heap',
   ([[1024, 32, True], [1056, 16, True]], [8]),
   {'alloc': [None], 'free': [], 'swept': []})],
 [('regression: sweep, split and exhaust',
   ([[2112, 8, True],
     [2064, 32, False],
     [2048, 16, True],
     [2096, 16, False],
     [2120, 24, False],
     [2144, 40, False],
     [2184, 16, True]],
    [20, 24, 40, 10, 8]),
   {'alloc': [2064, 2088, 2120, 2160, None], 'free': [], 'swept': [[2064, 48], [2120, 64]]}),
  ('exact fit uses the first block',
   ([[2112, 8, True],
     [2064, 32, False],
     [2048, 16, True],
     [2096, 16, False],
     [2120, 24, False],
     [2144, 40, False],
     [2184, 16, True]],
    [48, 64]),
   {'alloc': [2064, 2120], 'free': [], 'swept': [[2064, 48], [2120, 64]]}),
  ('remainder of exactly the minimum block size',
   ([[2112, 8, True],
     [2064, 32, False],
     [2048, 16, True],
     [2096, 16, False],
     [2120, 24, False],
     [2144, 40, False],
     [2184, 16, True]],
    [32, 40]),
   {'alloc': [2064, 2120], 'free': [[2096, 16], [2160, 24]], 'swept': [[2064, 48], [2120, 64]]}),
  ('remainder too small to split',
   ([[2112, 8, True],
     [2064, 32, False],
     [2048, 16, True],
     [2096, 16, False],
     [2120, 24, False],
     [2144, 40, False],
     [2184, 16, True]],
    [44, 50, 1]),
   {'alloc': [2064, 2120, None], 'free': [], 'swept': [[2064, 48], [2120, 64]]}),
  ('sweep snapshot is independent of allocation',
   ([[2112, 8, True],
     [2064, 32, False],
     [2048, 16, True],
     [2096, 16, False],
     [2120, 24, False],
     [2144, 40, False],
     [2184, 16, True]],
    [1]),
   {'alloc': [2064], 'free': [[2072, 40], [2120, 64]], 'swept': [[2064, 48], [2120, 64]]}),
  ('control: fully marked heap',
   ([[2048, 32, True], [2080, 16, True]], [8]),
   {'alloc': [None], 'free': [], 'swept': []})],
 [('regression: sweep, split and exhaust',
   ([[3136, 8, True],
     [3088, 32, False],
     [3072, 16, True],
     [3120, 16, False],
     [3144, 24, False],
     [3168, 40, False],
     [3208, 16, True]],
    [20, 24, 40, 10, 8]),
   {'alloc': [3088, 3112, 3144, 3184, None], 'free': [], 'swept': [[3088, 48], [3144, 64]]}),
  ('exact fit uses the first block',
   ([[3136, 8, True],
     [3088, 32, False],
     [3072, 16, True],
     [3120, 16, False],
     [3144, 24, False],
     [3168, 40, False],
     [3208, 16, True]],
    [48, 64]),
   {'alloc': [3088, 3144], 'free': [], 'swept': [[3088, 48], [3144, 64]]}),
  ('remainder of exactly the minimum block size',
   ([[3136, 8, True],
     [3088, 32, False],
     [3072, 16, True],
     [3120, 16, False],
     [3144, 24, False],
     [3168, 40, False],
     [3208, 16, True]],
    [32, 40]),
   {'alloc': [3088, 3144], 'free': [[3120, 16], [3184, 24]], 'swept': [[3088, 48], [3144, 64]]}),
  ('remainder too small to split',
   ([[3136, 8, True],
     [3088, 32, False],
     [3072, 16, True],
     [3120, 16, False],
     [3144, 24, False],
     [3168, 40, False],
     [3208, 16, True]],
    [44, 50, 1]),
   {'alloc': [3088, 3144, None], 'free': [], 'swept': [[3088, 48], [3144, 64]]}),
  ('sweep snapshot is independent of allocation',
   ([[3136, 8, True],
     [3088, 32, False],
     [3072, 16, True],
     [3120, 16, False],
     [3144, 24, False],
     [3168, 40, False],
     [3208, 16, True]],
    [1]),
   {'alloc': [3088], 'free': [[3096, 40], [3144, 64]], 'swept': [[3088, 48], [3144, 64]]}),
  ('control: fully marked heap',
   ([[3072, 32, True], [3104, 16, True]], [8]),
   {'alloc': [None], 'free': [], 'swept': []})],
 [('regression: sweep, split and exhaust',
   ([[4160, 8, True],
     [4112, 32, False],
     [4096, 16, True],
     [4144, 16, False],
     [4168, 24, False],
     [4192, 40, False],
     [4232, 16, True]],
    [20, 24, 40, 10, 8]),
   {'alloc': [4112, 4136, 4168, 4208, None], 'free': [], 'swept': [[4112, 48], [4168, 64]]}),
  ('exact fit uses the first block',
   ([[4160, 8, True],
     [4112, 32, False],
     [4096, 16, True],
     [4144, 16, False],
     [4168, 24, False],
     [4192, 40, False],
     [4232, 16, True]],
    [48, 64]),
   {'alloc': [4112, 4168], 'free': [], 'swept': [[4112, 48], [4168, 64]]}),
  ('remainder of exactly the minimum block size',
   ([[4160, 8, True],
     [4112, 32, False],
     [4096, 16, True],
     [4144, 16, False],
     [4168, 24, False],
     [4192, 40, False],
     [4232, 16, True]],
    [32, 40]),
   {'alloc': [4112, 4168], 'free': [[4144, 16], [4208, 24]], 'swept': [[4112, 48], [4168, 64]]}),
  ('remainder too small to split',
   ([[4160, 8, True],
     [4112, 32, False],
     [4096, 16, True],
     [4144, 16, False],
     [4168, 24, False],
     [4192, 40, False],
     [4232, 16, True]],
    [44, 50, 1]),
   {'alloc': [4112, 4168, None], 'free': [], 'swept': [[4112, 48], [4168, 64]]}),
  ('sweep snapshot is independent of allocation',
   ([[4160, 8, True],
     [4112, 32, False],
     [4096, 16, True],
     [4144, 16, False],
     [4168, 24, False],
     [4192, 40, False],
     [4232, 16, True]],
    [1]),
   {'alloc': [4112], 'free': [[4120, 40], [4168, 64]], 'swept': [[4112, 48], [4168, 64]]}),
  ('control: fully marked heap',
   ([[4096, 32, True], [4128, 16, True]], [8]),
   {'alloc': [None], 'free': [], 'swept': []})],
 [('regression: sweep, split and exhaust',
   ([[5184, 8, True],
     [5136, 32, False],
     [5120, 16, True],
     [5168, 16, False],
     [5192, 24, False],
     [5216, 40, False],
     [5256, 16, True]],
    [20, 24, 40, 10, 8]),
   {'alloc': [5136, 5160, 5192, 5232, None], 'free': [], 'swept': [[5136, 48], [5192, 64]]}),
  ('exact fit uses the first block',
   ([[5184, 8, True],
     [5136, 32, False],
     [5120, 16, True],
     [5168, 16, False],
     [5192, 24, False],
     [5216, 40, False],
     [5256, 16, True]],
    [48, 64]),
   {'alloc': [5136, 5192], 'free': [], 'swept': [[5136, 48], [5192, 64]]}),
  ('remainder of exactly the minimum block size',
   ([[5184, 8, True],
     [5136, 32, False],
     [5120, 16, True],
     [5168, 16, False],
     [5192, 24, False],
     [5216, 40, False],
     [5256, 16, True]],
    [32, 40]),
   {'alloc': [5136, 5192], 'free': [[5168, 16], [5232, 24]], 'swept': [[5136, 48], [5192, 64]]}),
  ('remainder too small to split',
   ([[5184, 8, True],
     [5136, 32, False],
     [5120, 16, True],
     [5168, 16, False],
     [5192, 24, False],
     [5216, 40, False],
     [5256, 16, True]],
    [44, 50, 1]),
   {'alloc': [5136, 5192, None], 'free': [], 'swept': [[5136, 48], [5192, 64]]}),
  ('sweep snapshot is independent of allocation',
   ([[5184, 8, True],
     [5136, 32, False],
     [5120, 16, True],
     [5168, 16, False],
     [5192, 24, False],
     [5216, 40, False],
     [5256, 16, True]],
    [1]),
   {'alloc': [5136], 'free': [[5144, 40], [5192, 64]], 'swept': [[5136, 48], [5192, 64]]}),
  ('control: fully marked heap',
   ([[5120, 32, True], [5152, 16, True]], [8]),
   {'alloc': [None], 'free': [], 'swept': []})]]
for label, args, expected in cases[N - 1]:
    check(label, 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 fixtureActualExpectedOutcome
regression: sweep, split and exhaust{'alloc': [1040, 1064, 1096, 1136, 1152], 'free': [], 'swept': [[1040, 48], [1096, 64]]}{'alloc': [1040, 1064, 1096, 1136, None], 'free': [], 'swept': [[1040, 48], [1096, 64]]}Failed
exact fit uses the first block{'alloc': [1040, 1096], 'free': [], 'swept': [[1040, 48], [1096, 64]]}{'alloc': [1040, 1096], 'free': [], 'swept': [[1040, 48], [1096, 64]]}Passed
remainder of exactly the minimum block size{'alloc': [1040, 1096], 'free': [[1072, 16], [1136, 24]], 'swept': [[1040, 48], [1096, 64]]}{'alloc': [1040, 1096], 'free': [[1072, 16], [1136, 24]], 'swept': [[1040, 48], [1096, 64]]}Passed
remainder too small to split{'alloc': [1040, 1096, 1152], 'free': [], 'swept': [[1040, 48], [1096, 64]]}{'alloc': [1040, 1096, None], 'free': [], 'swept': [[1040, 48], [1096, 64]]}Failed
sweep snapshot is independent of allocation{'alloc': [1040], 'free': [[1048, 40], [1096, 64]], 'swept': [[1040, 48], [1096, 64]]}{'alloc': [1040], 'free': [[1048, 40], [1096, 64]], 'swept': [[1040, 48], [1096, 64]]}Passed
control: fully marked heap{'alloc': [None], 'free': [], 'swept': []}{'alloc': [None], 'free': [], 'swept': []}Passed

SHA-256 / 8e940b4af549713ae55655cffcf8b9125c4af3625410f2ad0287457eed91afa5

2 / The unsuccessful fix

Exit 1
"""Failure Map reference implementation. Python standard library only."""
import json

N = 1
observations = []
def solve(blocks, requests):
    free = []
    for addr, size, marked in sorted(blocks):
        if marked:
            continue
        if free and free[-1][0] + free[-1][1] == addr:
            free[-1][1] += size
        else:
            free.append([addr, size])
    swept = [list(b) for b in free]
    got = []
    for req in requests:
        need = (req + 7) // 8 * 8
        choice = None
        for b in free:
            if b[1] >= need:
                choice = b
                break
        if choice is None:
            got.append(None)
            continue
        got.append(choice[0])
        rem = choice[1] - need
        if rem > 16:
            choice[0] += need
            choice[1] = rem
        else:
            free.remove(choice)
    return {'swept': swept, 'alloc': got, 'free': free}
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
cases = [[('regression: sweep, split and exhaust',
   ([[1088, 8, True],
     [1040, 32, False],
     [1024, 16, True],
     [1072, 16, False],
     [1096, 24, False],
     [1120, 40, False],
     [1160, 16, True]],
    [20, 24, 40, 10, 8]),
   {'alloc': [1040, 1064, 1096, 1136, None], 'free': [], 'swept': [[1040, 48], [1096, 64]]}),
  ('exact fit uses the first block',
   ([[1088, 8, True],
     [1040, 32, False],
     [1024, 16, True],
     [1072, 16, False],
     [1096, 24, False],
     [1120, 40, False],
     [1160, 16, True]],
    [48, 64]),
   {'alloc': [1040, 1096], 'free': [], 'swept': [[1040, 48], [1096, 64]]}),
  ('remainder of exactly the minimum block size',
   ([[1088, 8, True],
     [1040, 32, False],
     [1024, 16, True],
     [1072, 16, False],
     [1096, 24, False],
     [1120, 40, False],
     [1160, 16, True]],
    [32, 40]),
   {'alloc': [1040, 1096], 'free': [[1072, 16], [1136, 24]], 'swept': [[1040, 48], [1096, 64]]}),
  ('remainder too small to split',
   ([[1088, 8, True],
     [1040, 32, False],
     [1024, 16, True],
     [1072, 16, False],
     [1096, 24, False],
     [1120, 40, False],
     [1160, 16, True]],
    [44, 50, 1]),
   {'alloc': [1040, 1096, None], 'free': [], 'swept': [[1040, 48], [1096, 64]]}),
  ('sweep snapshot is independent of allocation',
   ([[1088, 8, True],
     [1040, 32, False],
     [1024, 16, True],
     [1072, 16, False],
     [1096, 24, False],
     [1120, 40, False],
     [1160, 16, True]],
    [1]),
   {'alloc': [1040], 'free': [[1048, 40], [1096, 64]], 'swept': [[1040, 48], [1096, 64]]}),
  ('control: fully marked heap',
   ([[1024, 32, True], [1056, 16, True]], [8]),
   {'alloc': [None], 'free': [], 'swept': []})],
 [('regression: sweep, split and exhaust',
   ([[2112, 8, True],
     [2064, 32, False],
     [2048, 16, True],
     [2096, 16, False],
     [2120, 24, False],
     [2144, 40, False],
     [2184, 16, True]],
    [20, 24, 40, 10, 8]),
   {'alloc': [2064, 2088, 2120, 2160, None], 'free': [], 'swept': [[2064, 48], [2120, 64]]}),
  ('exact fit uses the first block',
   ([[2112, 8, True],
     [2064, 32, False],
     [2048, 16, True],
     [2096, 16, False],
     [2120, 24, False],
     [2144, 40, False],
     [2184, 16, True]],
    [48, 64]),
   {'alloc': [2064, 2120], 'free': [], 'swept': [[2064, 48], [2120, 64]]}),
  ('remainder of exactly the minimum block size',
   ([[2112, 8, True],
     [2064, 32, False],
     [2048, 16, True],
     [2096, 16, False],
     [2120, 24, False],
     [2144, 40, False],
     [2184, 16, True]],
    [32, 40]),
   {'alloc': [2064, 2120], 'free': [[2096, 16], [2160, 24]], 'swept': [[2064, 48], [2120, 64]]}),
  ('remainder too small to split',
   ([[2112, 8, True],
     [2064, 32, False],
     [2048, 16, True],
     [2096, 16, False],
     [2120, 24, False],
     [2144, 40, False],
     [2184, 16, True]],
    [44, 50, 1]),
   {'alloc': [2064, 2120, None], 'free': [], 'swept': [[2064, 48], [2120, 64]]}),
  ('sweep snapshot is independent of allocation',
   ([[2112, 8, True],
     [2064, 32, False],
     [2048, 16, True],
     [2096, 16, False],
     [2120, 24, False],
     [2144, 40, False],
     [2184, 16, True]],
    [1]),
   {'alloc': [2064], 'free': [[2072, 40], [2120, 64]], 'swept': [[2064, 48], [2120, 64]]}),
  ('control: fully marked heap',
   ([[2048, 32, True], [2080, 16, True]], [8]),
   {'alloc': [None], 'free': [], 'swept': []})],
 [('regression: sweep, split and exhaust',
   ([[3136, 8, True],
     [3088, 32, False],
     [3072, 16, True],
     [3120, 16, False],
     [3144, 24, False],
     [3168, 40, False],
     [3208, 16, True]],
    [20, 24, 40, 10, 8]),
   {'alloc': [3088, 3112, 3144, 3184, None], 'free': [], 'swept': [[3088, 48], [3144, 64]]}),
  ('exact fit uses the first block',
   ([[3136, 8, True],
     [3088, 32, False],
     [3072, 16, True],
     [3120, 16, False],
     [3144, 24, False],
     [3168, 40, False],
     [3208, 16, True]],
    [48, 64]),
   {'alloc': [3088, 3144], 'free': [], 'swept': [[3088, 48], [3144, 64]]}),
  ('remainder of exactly the minimum block size',
   ([[3136, 8, True],
     [3088, 32, False],
     [3072, 16, True],
     [3120, 16, False],
     [3144, 24, False],
     [3168, 40, False],
     [3208, 16, True]],
    [32, 40]),
   {'alloc': [3088, 3144], 'free': [[3120, 16], [3184, 24]], 'swept': [[3088, 48], [3144, 64]]}),
  ('remainder too small to split',
   ([[3136, 8, True],
     [3088, 32, False],
     [3072, 16, True],
     [3120, 16, False],
     [3144, 24, False],
     [3168, 40, False],
     [3208, 16, True]],
    [44, 50, 1]),
   {'alloc': [3088, 3144, None], 'free': [], 'swept': [[3088, 48], [3144, 64]]}),
  ('sweep snapshot is independent of allocation',
   ([[3136, 8, True],
     [3088, 32, False],
     [3072, 16, True],
     [3120, 16, False],
     [3144, 24, False],
     [3168, 40, False],
     [3208, 16, True]],
    [1]),
   {'alloc': [3088], 'free': [[3096, 40], [3144, 64]], 'swept': [[3088, 48], [3144, 64]]}),
  ('control: fully marked heap',
   ([[3072, 32, True], [3104, 16, True]], [8]),
   {'alloc': [None], 'free': [], 'swept': []})],
 [('regression: sweep, split and exhaust',
   ([[4160, 8, True],
     [4112, 32, False],
     [4096, 16, True],
     [4144, 16, False],
     [4168, 24, False],
     [4192, 40, False],
     [4232, 16, True]],
    [20, 24, 40, 10, 8]),
   {'alloc': [4112, 4136, 4168, 4208, None], 'free': [], 'swept': [[4112, 48], [4168, 64]]}),
  ('exact fit uses the first block',
   ([[4160, 8, True],
     [4112, 32, False],
     [4096, 16, True],
     [4144, 16, False],
     [4168, 24, False],
     [4192, 40, False],
     [4232, 16, True]],
    [48, 64]),
   {'alloc': [4112, 4168], 'free': [], 'swept': [[4112, 48], [4168, 64]]}),
  ('remainder of exactly the minimum block size',
   ([[4160, 8, True],
     [4112, 32, False],
     [4096, 16, True],
     [4144, 16, False],
     [4168, 24, False],
     [4192, 40, False],
     [4232, 16, True]],
    [32, 40]),
   {'alloc': [4112, 4168], 'free': [[4144, 16], [4208, 24]], 'swept': [[4112, 48], [4168, 64]]}),
  ('remainder too small to split',
   ([[4160, 8, True],
     [4112, 32, False],
     [4096, 16, True],
     [4144, 16, False],
     [4168, 24, False],
     [4192, 40, False],
     [4232, 16, True]],
    [44, 50, 1]),
   {'alloc': [4112, 4168, None], 'free': [], 'swept': [[4112, 48], [4168, 64]]}),
  ('sweep snapshot is independent of allocation',
   ([[4160, 8, True],
     [4112, 32, False],
     [4096, 16, True],
     [4144, 16, False],
     [4168, 24, False],
     [4192, 40, False],
     [4232, 16, True]],
    [1]),
   {'alloc': [4112], 'free': [[4120, 40], [4168, 64]], 'swept': [[4112, 48], [4168, 64]]}),
  ('control: fully marked heap',
   ([[4096, 32, True], [4128, 16, True]], [8]),
   {'alloc': [None], 'free': [], 'swept': []})],
 [('regression: sweep, split and exhaust',
   ([[5184, 8, True],
     [5136, 32, False],
     [5120, 16, True],
     [5168, 16, False],
     [5192, 24, False],
     [5216, 40, False],
     [5256, 16, True]],
    [20, 24, 40, 10, 8]),
   {'alloc': [5136, 5160, 5192, 5232, None], 'free': [], 'swept': [[5136, 48], [5192, 64]]}),
  ('exact fit uses the first block',
   ([[5184, 8, True],
     [5136, 32, False],
     [5120, 16, True],
     [5168, 16, False],
     [5192, 24, False],
     [5216, 40, False],
     [5256, 16, True]],
    [48, 64]),
   {'alloc': [5136, 5192], 'free': [], 'swept': [[5136, 48], [5192, 64]]}),
  ('remainder of exactly the minimum block size',
   ([[5184, 8, True],
     [5136, 32, False],
     [5120, 16, True],
     [5168, 16, False],
     [5192, 24, False],
     [5216, 40, False],
     [5256, 16, True]],
    [32, 40]),
   {'alloc': [5136, 5192], 'free': [[5168, 16], [5232, 24]], 'swept': [[5136, 48], [5192, 64]]}),
  ('remainder too small to split',
   ([[5184, 8, True],
     [5136, 32, False],
     [5120, 16, True],
     [5168, 16, False],
     [5192, 24, False],
     [5216, 40, False],
     [5256, 16, True]],
    [44, 50, 1]),
   {'alloc': [5136, 5192, None], 'free': [], 'swept': [[5136, 48], [5192, 64]]}),
  ('sweep snapshot is independent of allocation',
   ([[5184, 8, True],
     [5136, 32, False],
     [5120, 16, True],
     [5168, 16, False],
     [5192, 24, False],
     [5216, 40, False],
     [5256, 16, True]],
    [1]),
   {'alloc': [5136], 'free': [[5144, 40], [5192, 64]], 'swept': [[5136, 48], [5192, 64]]}),
  ('control: fully marked heap',
   ([[5120, 32, True], [5152, 16, True]], [8]),
   {'alloc': [None], 'free': [], 'swept': []})]]
for label, args, expected in cases[N - 1]:
    check(label, 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 fixtureActualExpectedOutcome
regression: sweep, split and exhaust{'alloc': [1040, 1064, 1096, 1136, None], 'free': [], 'swept': [[1040, 48], [1096, 64]]}{'alloc': [1040, 1064, 1096, 1136, None], 'free': [], 'swept': [[1040, 48], [1096, 64]]}Passed
exact fit uses the first block{'alloc': [1040, 1096], 'free': [], 'swept': [[1040, 48], [1096, 64]]}{'alloc': [1040, 1096], 'free': [], 'swept': [[1040, 48], [1096, 64]]}Passed
remainder of exactly the minimum block size{'alloc': [1040, 1096], 'free': [[1136, 24]], 'swept': [[1040, 48], [1096, 64]]}{'alloc': [1040, 1096], 'free': [[1072, 16], [1136, 24]], 'swept': [[1040, 48], [1096, 64]]}Failed
remainder too small to split{'alloc': [1040, 1096, None], 'free': [], 'swept': [[1040, 48], [1096, 64]]}{'alloc': [1040, 1096, None], 'free': [], 'swept': [[1040, 48], [1096, 64]]}Passed
sweep snapshot is independent of allocation{'alloc': [1040], 'free': [[1048, 40], [1096, 64]], 'swept': [[1040, 48], [1096, 64]]}{'alloc': [1040], 'free': [[1048, 40], [1096, 64]], 'swept': [[1040, 48], [1096, 64]]}Passed
control: fully marked heap{'alloc': [None], 'free': [], 'swept': []}{'alloc': [None], 'free': [], 'swept': []}Passed

SHA-256 / 173a90e9cb2cb5d64095cb85e7bd8cfad97819bf6ee8d430fdc6077d30814d0b

HELD IN THE MEMBER ARCHIVE

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

This mechanism has 6 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.

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Verification & scope

A deterministic, bounded teaching model of one garbage-collector mechanism with stipulated rules; it is not a production collector and claims no conformance to any particular runtime. 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 2026-09-29T14:51:27.078906+00:00.

Case digest / e9e6cff36e8208a96f2cdc036af04c9b34bc953b73330531f25bcde20e31dd74