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Breadcrumb ancestor rendering: Collapsed breadcrumbs omit the root context · case 01

Collapsed breadcrumbs omit the root context.

Verified by executionVariant 1 · 7 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The ellipsis operation uses `['…']` where the contract requires `[chain[0], '…']`.

THE FAILURE

The ellipsis operation uses `['…']` where the contract requires `[chain[0], '…']`.

Unsuccessful approach: Using current as root duplicates the current page.

Case contract

Append current after ancestors; optionally hide a distinct root; collapse long paths preserving root and recent tail within budget; only ancestors are links. Budget is at least two.

Why this case matters

A deterministic model of breadcrumb ancestor rendering; this isolates one interface invariant without requiring a browser.

1 / The failure

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

N = 1
observations = []
def solve(ancestors, current, max_items, include_root):
    chain = list(ancestors) + [current]
    if not include_root and len(chain) > 1: chain = chain[1:]
    if len(chain) > max_items:
        chain = ['…'] + chain[-(max_items-2):] if max_items > 2 else chain[-max_items:]
    return [(x, i != len(chain)-1) for i, x in enumerate(chain)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('normal', solve(['root','parent'], 'current', 5, True), [('root', True), ('parent', True), ('current', False)])
check('only current', solve([], 'current', 5, False), [('current', False)])
check('hide root', solve(['root','parent'], 'current', 5, False), [('parent', True), ('current', False)])
check('collapsed', solve(['root','a','b','c'], 'current', 4, True), [('root', True), ('…', True), ('c', True), ('current', False)])
check('small budget', solve(['root','a','b'], 'current', 2, True), [('b', True), ('current', False)])
check('parameterized ancestor depth', solve(list(range(N)),N,N+2,True), [(i,i!=N) for i in range(N+1)])
for repetition in range(N):
    check('repeat small budget', solve(['root','a','b'], 'current', 2, True), [('b', True), ('current', False)])
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
normal[['root', True], ['parent', True], ['current', False]][['root', True], ['parent', True], ['current', False]]Passed
only current[['current', False]][['current', False]]Passed
hide root[['parent', True], ['current', False]][['parent', True], ['current', False]]Passed
collapsed[['…', True], ['c', True], ['current', False]][['root', True], ['…', True], ['c', True], ['current', False]]Failed
small budget[['b', True], ['current', False]][['b', True], ['current', False]]Passed
parameterized ancestor depth[[0, True], [1, False]][[0, True], [1, False]]Passed
repeat small budget[['b', True], ['current', False]][['b', True], ['current', False]]Passed

SHA-256 / d91596fdce315b792d64be6dc3b536dac9b17e9de8b0b37234d041a06dce2d31

2 / The unsuccessful fix

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

N = 1
observations = []
def solve(ancestors, current, max_items, include_root):
    chain = list(ancestors) + [current]
    if not include_root and len(chain) > 1: chain = chain[1:]
    if len(chain) > max_items:
        chain = [chain[-1], '…'] + chain[-(max_items-2):] if max_items > 2 else chain[-max_items:]
    return [(x, i != len(chain)-1) for i, x in enumerate(chain)]
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
check('normal', solve(['root','parent'], 'current', 5, True), [('root', True), ('parent', True), ('current', False)])
check('only current', solve([], 'current', 5, False), [('current', False)])
check('hide root', solve(['root','parent'], 'current', 5, False), [('parent', True), ('current', False)])
check('collapsed', solve(['root','a','b','c'], 'current', 4, True), [('root', True), ('…', True), ('c', True), ('current', False)])
check('small budget', solve(['root','a','b'], 'current', 2, True), [('b', True), ('current', False)])
check('parameterized ancestor depth', solve(list(range(N)),N,N+2,True), [(i,i!=N) for i in range(N+1)])
for repetition in range(N):
    check('repeat small budget', solve(['root','a','b'], 'current', 2, True), [('b', True), ('current', False)])
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
normal[['root', True], ['parent', True], ['current', False]][['root', True], ['parent', True], ['current', False]]Passed
only current[['current', False]][['current', False]]Passed
hide root[['parent', True], ['current', False]][['parent', True], ['current', False]]Passed
collapsed[['current', True], ['…', True], ['c', True], ['current', False]][['root', True], ['…', True], ['c', True], ['current', False]]Failed
small budget[['b', True], ['current', False]][['b', True], ['current', False]]Passed
parameterized ancestor depth[[0, True], [1, False]][[0, True], [1, False]]Passed
repeat small budget[['b', True], ['current', False]][['b', True], ['current', False]]Passed

SHA-256 / e2b63a12e2fa59fc09e7b0aa6797d1d2b14a7c1bffdd745e9c7489ffb9b5deb1

HELD IN THE MEMBER ARCHIVE

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

This mechanism has 7 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

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 2026-09-29T14:38:23.239034+00:00.

Case digest / 1c27d63a0df3098f5edb81df28aaf10334007386820899688ad4bf50ff514104