FA-76146 / Chat ordering and read receipts / Open access

Render who is typing from start and stop events: plural form · case 01

With three typers the indicator silently drops the third name.

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

ROOT CAUSE

The plural form decision evaluates `if len(who) >= 2: return who[0] + ' and ' + who[1] + ' are typing'` where the contract requires `if len(who) == 2: return who[0] + ' and ' + who[1] + ' are typing'`.

VERIFIED REPAIR

Use `if len(who) == 2: return who[0] + ' and ' + who[1] + ' are typing'` for the plural form decision and keep every other rule of the model unchanged.

Unsuccessful approach: Joining every name produces an unbounded list instead of the collective phrase. The attempted `if len(who) >= 2: return ' and '.join(who) + ' are typing'` still disagrees with a fixture.

Case contract

events [ts_ms, user, kind] with kind start, stop or message are applied in (ts, user, kind) order. A user is typing when their latest start has no later stop or message and now - start < 6000 ms (a start stamped in the future by a skewed clock stays active). The viewer is excluded. Typers are listed by start time: one -> "A is typing", two -> "A and B are typing", more -> "Several people are typing", none -> "". names maps ids to display names.

Why this case matters

Typing indicators are ephemeral presence; stale or misordered indicators mislead users about who is replying.

1 / The failure

Exit 1

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

N = 1
observations = []
def solve(events, me, now, names):
    last = {}
    for ts, user, kind in sorted(events):
        if kind == 'start':
            last[user] = ts
        else:
            last.pop(user, None)
    active = sorted((ts, user) for user, ts in last.items() if user != me and now - ts < 6000)
    who = [names.get(u, u) for _, u in active]
    if not who:
        return ''
    if len(who) == 1:
        return who[0] + ' is typing'
    if len(who) >= 2:
        return who[0] + ' and ' + who[1] + ' are typing'
    return 'Several people are typing'
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('exact expiry', ([[1000, 'bo', 'start']], 'me', 7000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50001, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [301, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [701, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 11, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2501, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 21, {}), '')], 2: [('exact expiry', ([[2000, 'bo', 'start']], 'me', 8000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50002, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [302, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [702, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 12, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2502, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 22, {}), '')], 3: [('exact expiry', ([[3000, 'bo', 'start']], 'me', 9000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50003, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [303, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [703, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 13, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2503, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 23, {}), '')], 4: [('exact expiry', ([[4000, 'bo', 'start']], 'me', 10000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50004, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [304, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [704, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 14, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2504, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 24, {}), '')], 5: [('exact expiry', ([[5000, 'bo', 'start']], 'me', 11000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50005, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [305, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [705, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 15, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2505, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 25, {}), '')]}
for _label, _args, _expected in _CASES[N]:
    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 fixture	Actual	Expected	Outcome
exact expiry			Passed
future stamped start	`bo is typing`	`bo is typing`	Passed
message clears typing			Passed
start order	`Zed and Amy are typing`	`Zed and Amy are typing`	Passed
three typers	`A and B are typing`	`Several people are typing`	Failed
stop overtaken in transit	`Cy is typing`	`Cy is typing`	Passed
self typing hidden			Passed

SHA-256 / 5ba4ebd5eba989f515d4d954d7f8b1b761aa24c8fadc589a20f76a18eb7d237d

2 / The unsuccessful fix

Exit 1

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

N = 1
observations = []
def solve(events, me, now, names):
    last = {}
    for ts, user, kind in sorted(events):
        if kind == 'start':
            last[user] = ts
        else:
            last.pop(user, None)
    active = sorted((ts, user) for user, ts in last.items() if user != me and now - ts < 6000)
    who = [names.get(u, u) for _, u in active]
    if not who:
        return ''
    if len(who) == 1:
        return who[0] + ' is typing'
    if len(who) >= 2:
        return ' and '.join(who) + ' are typing'
    return 'Several people are typing'
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('exact expiry', ([[1000, 'bo', 'start']], 'me', 7000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50001, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [301, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [701, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 11, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2501, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 21, {}), '')], 2: [('exact expiry', ([[2000, 'bo', 'start']], 'me', 8000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50002, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [302, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [702, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 12, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2502, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 22, {}), '')], 3: [('exact expiry', ([[3000, 'bo', 'start']], 'me', 9000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50003, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [303, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [703, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 13, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2503, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 23, {}), '')], 4: [('exact expiry', ([[4000, 'bo', 'start']], 'me', 10000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50004, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [304, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [704, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 14, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2504, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 24, {}), '')], 5: [('exact expiry', ([[5000, 'bo', 'start']], 'me', 11000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50005, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [305, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [705, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 15, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2505, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 25, {}), '')]}
for _label, _args, _expected in _CASES[N]:
    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 fixture	Actual	Expected	Outcome
exact expiry			Passed
future stamped start	`bo is typing`	`bo is typing`	Passed
message clears typing			Passed
start order	`Zed and Amy are typing`	`Zed and Amy are typing`	Passed
three typers	`A and B and C are typing`	`Several people are typing`	Failed
stop overtaken in transit	`Cy is typing`	`Cy is typing`	Passed
self typing hidden			Passed

SHA-256 / 6c48e44d5841bb3423ed41dffe609d1943dfec725571e7bd777be7395df72da8

3 / The verified repair

Exit 0

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

N = 1
observations = []
def solve(events, me, now, names):
    last = {}
    for ts, user, kind in sorted(events):
        if kind == 'start':
            last[user] = ts
        else:
            last.pop(user, None)
    active = sorted((ts, user) for user, ts in last.items() if user != me and now - ts < 6000)
    who = [names.get(u, u) for _, u in active]
    if not who:
        return ''
    if len(who) == 1:
        return who[0] + ' is typing'
    if len(who) == 2:
        return who[0] + ' and ' + who[1] + ' are typing'
    return 'Several people are typing'
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_CASES = {1: [('exact expiry', ([[1000, 'bo', 'start']], 'me', 7000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50001, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [301, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [701, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 11, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2501, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 21, {}), '')], 2: [('exact expiry', ([[2000, 'bo', 'start']], 'me', 8000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50002, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [302, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [702, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 12, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2502, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 22, {}), '')], 3: [('exact expiry', ([[3000, 'bo', 'start']], 'me', 9000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50003, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [303, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [703, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 13, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2503, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 23, {}), '')], 4: [('exact expiry', ([[4000, 'bo', 'start']], 'me', 10000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50004, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [304, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [704, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 14, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2504, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 24, {}), '')], 5: [('exact expiry', ([[5000, 'bo', 'start']], 'me', 11000, {'bo': 'Bo'}), ''), ('future stamped start', ([[50005, 'bo', 'start']], 'me', 40000, {}), 'bo is typing'), ('message clears typing', ([[100, 'bo', 'start'], [305, 'bo', 'message']], 'me', 1000, {'bo': 'Bo'}), ''), ('start order', ([[500, 'zed', 'start'], [705, 'amy', 'start']], 'me', 2000, {'zed': 'Zed', 'amy': 'Amy'}), 'Zed and Amy are typing'), ('three typers', ([[1, 'a', 'start'], [2, 'b', 'start'], [3, 'c', 'start']], 'me', 15, {'a': 'A', 'b': 'B', 'c': 'C'}), 'Several people are typing'), ('stop overtaken in transit', ([[2505, 'bo', 'stop'], [2100, 'bo', 'start'], [2600, 'cy', 'start']], 'me', 3000, {'cy': 'Cy'}), 'Cy is typing'), ('self typing hidden', ([[10, 'me', 'start']], 'me', 25, {}), '')]}
for _label, _args, _expected in _CASES[N]:
    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 fixture	Actual	Expected	Outcome
exact expiry			Passed
future stamped start	`bo is typing`	`bo is typing`	Passed
message clears typing			Passed
start order	`Zed and Amy are typing`	`Zed and Amy are typing`	Passed
three typers	`Several people are typing`	`Several people are typing`	Passed
stop overtaken in transit	`Cy is typing`	`Cy is typing`	Passed
self typing hidden			Passed

SHA-256 / 436320fe63e8d4316a628d05523e6fa46b0984afaf1fae68c8bbd49e810ac9df

Verification & scope

Stipulated offline chat model; not a complete messaging protocol, client or server 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 2026-09-29T14:49:13.529930+00:00.

Case digest / fbb22469a7a7b3a033382f81acd88381c03055781a2eb7ea40e3207eeb596338