FA-4796 / Bounded deques / Open access

Deque reconstruction retains newest suffix · case 01

The operation returns a result or retained state that violates this contract: Construct a bounded deque from a sequence, retaining its newest suffix when the initial sequence exceeds capacity.

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

ROOT CAUSE

Keeping the prefix retains the oldest values instead of the newest.

VERIFIED REPAIR

Construct a bounded deque from a sequence, retaining its newest suffix when the initial sequence exceeds capacity.

Unsuccessful approach: Negative-zero slicing fails the zero-capacity boundary.

Case contract

Construct a bounded deque from a sequence, retaining its newest suffix when the initial sequence exceeds capacity. Inputs are the finite Python values shown by the executable fixtures; no concurrent execution is assumed.

Why this case matters

A controlled local-runtime regression for collection APIs, language semantics, or ownership wrappers. Fixtures include boundary and interaction cases.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
from collections import Counter, ChainMap, deque
import heapq
N = 1
observations = []
def solve(x, y=None):
    return x[:y]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('truncates left', solve(['a', 'b', 'c'], 2), ['b', 'c'])
check('zero retains none', solve(['a'], 0), [])
check('under capacity', solve(['a'], 3), ['a'])
check('empty', solve([], 1), [])
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
truncates left	`['a', 'b']`	`['b', 'c']`	Failed
zero retains none	`[]`	`[]`	Passed
under capacity	`['a']`	`['a']`	Passed
empty	`[]`	`[]`	Passed

SHA-256 / 9db6f925d82ee58f2723bf96c24a391e85055f24a971ca9cdde3c3feb7f9c545

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
from collections import Counter, ChainMap, deque
import heapq
N = 1
observations = []
def solve(x, y=None):
    return x[-y:]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('truncates left', solve(['a', 'b', 'c'], 2), ['b', 'c'])
check('zero retains none', solve(['a'], 0), [])
check('under capacity', solve(['a'], 3), ['a'])
check('empty', solve([], 1), [])
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
truncates left	`['b', 'c']`	`['b', 'c']`	Passed
zero retains none	`['a']`	`[]`	Failed
under capacity	`['a']`	`['a']`	Passed
empty	`[]`	`[]`	Passed

SHA-256 / 37b645082b90d77daace59174bb6955f1224e1d6bdc6ca585f5e51f08509728e

3 / The verified repair

Exit 0

"""Failure Map reference implementation. Python standard library only."""
import json
from collections import Counter, ChainMap, deque
import heapq
N = 1
observations = []
def solve(x, y=None):
    return list(deque(x,maxlen=y))
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('truncates left', solve(['a', 'b', 'c'], 2), ['b', 'c'])
check('zero retains none', solve(['a'], 0), [])
check('under capacity', solve(['a'], 3), ['a'])
check('empty', solve([], 1), [])
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
truncates left	`['b', 'c']`	`['b', 'c']`	Passed
zero retains none	`[]`	`[]`	Passed
under capacity	`['a']`	`['a']`	Passed
empty	`[]`	`[]`	Passed

SHA-256 / c4ce392d9cf634bb88c122fd0204e3fe6537ebc9b015dde1bc9d265199e247a4

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 2026-09-29T14:37:42.887775+00:00.

Case digest / 73800e18f20871d0589bf315d3f9db23c73314dd66de9e39a5997f333a3002ca