FA-10391 / Caching / Open access

Shrinking cache capacity evicts only one excess entry · case 01

Shrinking cache capacity evicts only one excess entry.

Verified by executionVariant 1 · 4 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

A capacity reduction is handled as a single insertion overflow.

VERIFIED REPAIR

Preserve the cache-state invariant: Capacity is a nonnegative integer. Retain at most capacity keys from the most-recent end of the ordered cache.

Unsuccessful approach: Keeping the prefix retains the least recent entries instead of the most recent.

Case contract

Capacity is a nonnegative integer. Retain at most capacity keys from the most-recent end of the ordered cache.

Why this case matters

A deterministic cache state transformation. Inputs are copied or treated as immutable; no remote storage, real clock, or concurrent interleaving is simulated.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(order, capacity):
    return order[1:] if len(order)>capacity else list(order)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('fixture 1', solve(*(['a', 'b', 'c', 'd'], 1)), ['d'])
check('fixture 2', solve(*(['a', 'b', 'c'], 2)), ['b', 'c'])
check('fixture 3', solve(*(['a'], 0)), [])
check('fixture 4', solve(*(['a', 'b'], 5)), ['a', 'b'])
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
fixture 1	`['b', 'c', 'd']`	`['d']`	Failed
fixture 2	`['b', 'c']`	`['b', 'c']`	Passed
fixture 3	`[]`	`[]`	Passed
fixture 4	`['a', 'b']`	`['a', 'b']`	Passed

SHA-256 / 06b7b2d4a169450c91fe42217b0bb2e4096e6c8362beaab6b19f5a1748d9e724

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(order, capacity):
    return list(order[:capacity])
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('fixture 1', solve(*(['a', 'b', 'c', 'd'], 1)), ['d'])
check('fixture 2', solve(*(['a', 'b', 'c'], 2)), ['b', 'c'])
check('fixture 3', solve(*(['a'], 0)), [])
check('fixture 4', solve(*(['a', 'b'], 5)), ['a', 'b'])
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
fixture 1	`['a']`	`['d']`	Failed
fixture 2	`['a', 'b']`	`['b', 'c']`	Failed
fixture 3	`[]`	`[]`	Passed
fixture 4	`['a', 'b']`	`['a', 'b']`	Passed

SHA-256 / c7c5767f091c465e688a9b93bd9242e55f5fc99fe51946c83599b12bd05c1ae2

3 / The verified repair

Exit 0

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

N = 1
observations = []
def solve(order, capacity):
    return list(order[-capacity:]) if capacity else []
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('fixture 1', solve(*(['a', 'b', 'c', 'd'], 1)), ['d'])
check('fixture 2', solve(*(['a', 'b', 'c'], 2)), ['b', 'c'])
check('fixture 3', solve(*(['a'], 0)), [])
check('fixture 4', solve(*(['a', 'b'], 5)), ['a', 'b'])
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
fixture 1	`['d']`	`['d']`	Passed
fixture 2	`['b', 'c']`	`['b', 'c']`	Passed
fixture 3	`[]`	`[]`	Passed
fixture 4	`['a', 'b']`	`['a', 'b']`	Passed

SHA-256 / d7eb55881303d126512ec8bb6a1f1f5bf21eb65150114b2ccf8e5d6f0e630274

Verification & scope

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 2026-09-29T14:38:38.632910+00:00.

Case digest / d66b4b857cf8be554345ef9ae16ebe3b6e21ff098e894b0a085741a1a02efb81