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park(vehicle) {\n const spot = inventory.find(s => !s.ticketId && fits[vehicle.type]?.includes(s.type));\n if (!spot) return null;\n // Claim synchronously before any await: atomic within this single event loop.\n const ticket = { id: 'T' + (++sequence), spotId: spot.id, status: 'ACTIVE', enteredAt: clock() };\n spot.ticketId = ticket.id;\n tickets.set(ticket.id, ticket);\n return { ...ticket };\n },\n async exit(ticketId) {\n const ticket = tickets.get(ticketId);\n if (!ticket) throw new Error('Unknown ticket');\n if (ticket.status === 'CLOSED') return { ...ticket.receipt };\n if (inFlight.has(ticketId)) return inFlight.get(ticketId);\n const operation = (async () => {\n if (ticket.quote === undefined) ticket.quote = price({ ...ticket }, clock());\n ticket.paymentId ??= 'pay-' + ticket.id;\n ticket.status = 'PAYMENT_PENDING';\n let outcome;\n try { outcome = await charge(ticket.paymentId, ticket.quote); }\n catch { return { status: 'PENDING' }; }\n if (outcome === 'UNKNOWN') return { status: 'PENDING' };\n if (outcome !== 'CONFIRMED') return { status: 'FAILED' };\n const spot = inventory.find(s => s.id === ticket.spotId);\n if (spot.ticketId !== ticket.id) throw new Error('Spot ownership mismatch');\n spot.ticketId = null;\n ticket.status = 'CLOSED';\n ticket.receipt = { status: 'CONFIRMED', ticketId: ticket.id, amount: ticket.quote };\n return { ...ticket.receipt };\n })();\n inFlight.set(ticketId, operation);\n try { return await operation; } finally { inFlight.delete(ticketId); }\n },\n snapshot() {\n return { spots: inventory.map(s => ({ ...s })), tickets: [...tickets.values()].map(t => ({ ...t })) };\n }\n };\n}", "scenario": "race" }, "updatedAt": "2026-10-19T17:00:56.750Z" } ], "studioAttempts": [ { "id": "89fc4236-d88a-4fae-bded-6bf49f6348b8", "lab": "url-shortener", "value": { "report": { "kind": "hld", "checks": [ { "name": "Durable redirect path", "pass": true, "detail": "Client reaches the API and the API reaches durable storage." }, { "name": "API 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motorcycle arrival", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Unknown payment and stable retry", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Unknown payment and stable retry", "calls": 0, "label": "Ticket issued", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Unknown payment and stable retry", "calls": 1, "label": "Payment outcome unknown", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "CLOSED" } ] }, { "test": "Failed payment retains ownership", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Failed payment retains ownership", "calls": 0, "label": "Ticket issued", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Failed payment retains ownership", "calls": 1, "label": "Payment failed", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "CLOSED" } ] }, { "test": "Concurrent and repeated exit", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Concurrent and repeated exit", "calls": 0, "label": "Ticket issued", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Concurrent and repeated exit", "calls": 3, "label": "Concurrent exits and later retry", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "CLOSED" } ] }, { "test": "Unknown ticket changes nothing", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Unknown ticket changes nothing", "calls": 0, "label": "Unknown ticket rejected", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] } ], "metrics": { "total": 6, "passed": 1 }, "version": 1 }, "workspace": { "lesson": "", "source": "// Implement createParkingLot. This runnable starter has deliberate race and retry bugs.\n// Vehicle types: MOTORCYCLE fits MOTORCYCLE/CAR; CAR fits CAR; TRUCK fits TRUCK.\n// park(vehicle) -> ticket | null. exit(ticketId) -> receipt | {status:'PENDING'|'FAILED'}.\n// charge(paymentId, quote) -> 'CONFIRMED' | 'FAILED' | 'UNKNOWN'. Reuse payment identity.\n// snapshot() -> {spots: [{id,type,ticketId}], tickets: [{id,spotId,status,...}]}.\nfunction createParkingLot({ spots, charge, clock, price }) {\n const inventory = spots.map(s => ({ ...s, ticketId: null }));\n const tickets = new Map();\n let sequence = 0;\n return {\n async park(vehicle) {\n const spot = inventory.find(s => !s.ticketId && s.type === vehicle.type);\n if (!spot) return null;\n await Promise.resolve(); // Other arrivals can choose this same spot.\n const ticket = { id: 'T' + (++sequence), spotId: spot.id, status: 'ACTIVE', enteredAt: clock() };\n spot.ticketId = ticket.id;\n tickets.set(ticket.id, ticket);\n return { ...ticket };\n },\n async exit(ticketId) {\n const ticket = tickets.get(ticketId);\n if (!ticket) throw new Error('Unknown ticket');\n const paymentId = ticket.id + '-' + clock(); // Unstable retry identity.\n const outcome = await charge(paymentId, price(ticket, clock()));\n inventory.find(s => s.id === ticket.spotId).ticketId = null;\n ticket.status = 'CLOSED';\n return { status: outcome, ticketId };\n },\n snapshot() { return { spots: inventory.map(s => ({...s})), tickets: [...tickets.values()].map(t => ({...t})) }; }\n };\n}", "scenario": "all" } }, "createdAt": "2026-10-16T15:09:02.718Z" }, { "id": "cfc9b865-3fcf-40d1-bf80-73caea76a08f", "lab": "parking-lot", "value": { "report": { "kind": "lld", "checks": [ { "name": "Two arrivals, one spot", "pass": true, "detail": "Observed behavior satisfies this scenario’s contract." }, { "name": "Vehicle compatibility", "pass": true, "detail": "Observed behavior satisfies this scenario’s contract." }, { "name": "Unknown payment and stable retry", "pass": true, "detail": "Observed behavior satisfies this scenario’s contract." }, { "name": "Failed payment retains ownership", "pass": true, "detail": "Observed behavior satisfies this scenario’s contract." }, { "name": "Concurrent and repeated exit", "pass": true, "detail": "Observed behavior satisfies this scenario’s contract." }, { "name": "Unknown ticket changes nothing", "pass": true, "detail": "Observed behavior satisfies this scenario’s contract." } ], "frames": [ { "test": "Two arrivals, one spot", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Two arrivals, one spot", "calls": 0, "label": "Concurrent arrivals settled", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Vehicle compatibility", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Vehicle compatibility", "calls": 0, "label": "Truck rejected; motorcycle arrival", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Unknown payment and stable retry", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Unknown payment and stable retry", "calls": 0, "label": "Ticket issued", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Unknown payment and stable retry", "calls": 1, "label": "Payment outcome unknown", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "PAYMENT_PENDING" } ] }, { "test": "Unknown payment and stable retry", "calls": 2, "label": "Same payment reconciled", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "CLOSED" } ] }, { "test": "Failed payment retains ownership", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Failed payment retains ownership", "calls": 0, "label": "Ticket issued", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Failed payment retains ownership", "calls": 1, "label": "Payment failed", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "PAYMENT_PENDING" } ] }, { "test": "Concurrent and repeated exit", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Concurrent and repeated exit", "calls": 0, "label": "Ticket issued", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Concurrent and repeated exit", "calls": 1, "label": "Concurrent exits and later retry", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "CLOSED" } ] }, { "test": "Concurrent and repeated exit", "calls": 1, "label": "Old ticket retried after spot reuse", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T2" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "CLOSED" }, { "id": "T2", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Unknown ticket changes nothing", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Unknown ticket changes nothing", "calls": 0, "label": "Unknown ticket rejected", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] } ], "metrics": { "total": 6, "passed": 6 }, "version": 1 }, "workspace": { "lesson": "", "source": "function createParkingLot({ spots, charge, clock, price }) {\n const inventory = spots.map(s => ({ ...s, ticketId: null }));\n const tickets = new Map(), inFlight = new Map();\n let sequence = 0;\n const fits = { MOTORCYCLE: ['MOTORCYCLE', 'CAR'], CAR: ['CAR'], TRUCK: ['TRUCK'] };\n return {\n async park(vehicle) {\n const spot = inventory.find(s => !s.ticketId && fits[vehicle.type]?.includes(s.type));\n if (!spot) return null;\n // Claim synchronously before any await: atomic within this single event loop.\n const ticket = { id: 'T' + (++sequence), spotId: spot.id, status: 'ACTIVE', enteredAt: clock() };\n spot.ticketId = ticket.id;\n tickets.set(ticket.id, ticket);\n return { ...ticket };\n },\n async exit(ticketId) {\n const ticket = tickets.get(ticketId);\n if (!ticket) throw new Error('Unknown ticket');\n if (ticket.status === 'CLOSED') return { ...ticket.receipt };\n if (inFlight.has(ticketId)) return inFlight.get(ticketId);\n const operation = (async () => {\n if (ticket.quote === undefined) ticket.quote = price({ ...ticket }, clock());\n ticket.paymentId ??= 'pay-' + ticket.id;\n ticket.status = 'PAYMENT_PENDING';\n let outcome;\n try { outcome = await charge(ticket.paymentId, ticket.quote); }\n catch { return { status: 'PENDING' }; }\n if (outcome === 'UNKNOWN') return { status: 'PENDING' };\n if (outcome !== 'CONFIRMED') return { status: 'FAILED' };\n const spot = inventory.find(s => s.id === ticket.spotId);\n if (spot.ticketId !== ticket.id) throw new Error('Spot ownership mismatch');\n spot.ticketId = null;\n ticket.status = 'CLOSED';\n ticket.receipt = { status: 'CONFIRMED', ticketId: ticket.id, amount: ticket.quote };\n return { ...ticket.receipt };\n })();\n inFlight.set(ticketId, operation);\n try { return await operation; } finally { inFlight.delete(ticketId); }\n },\n snapshot() {\n return { spots: inventory.map(s => ({ ...s })), tickets: [...tickets.values()].map(t => ({ ...t })) };\n }\n };\n}", "scenario": "all" } }, "createdAt": "2026-10-16T15:09:10.060Z" }, { "id": "08d02f32-9a5c-411c-bdbc-4ac0e309a3ba", "lab": "parking-lot", "value": { "report": { "kind": "lld", "checks": [ { "name": "Two arrivals, one spot", "pass": true, "detail": "Observed behavior satisfies this scenario’s contract." }, { "name": "Vehicle compatibility", "pass": true, "detail": "Observed behavior satisfies this scenario’s contract." }, { "name": "Unknown payment and stable retry", "pass": true, "detail": "Observed behavior satisfies this scenario’s contract." }, { "name": "Failed payment retains ownership", "pass": true, "detail": "Observed behavior satisfies this scenario’s contract." }, { "name": "Concurrent and repeated exit", "pass": true, "detail": "Observed behavior satisfies this scenario’s contract." }, { "name": "Unknown ticket changes nothing", "pass": true, "detail": "Observed behavior satisfies this scenario’s contract." } ], "frames": [ { "test": "Two arrivals, one spot", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Two arrivals, one spot", "calls": 0, "label": "Concurrent arrivals settled", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Vehicle compatibility", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Vehicle compatibility", "calls": 0, "label": "Truck rejected; motorcycle arrival", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Unknown payment and stable retry", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Unknown payment and stable retry", "calls": 0, "label": "Ticket issued", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Unknown payment and stable retry", "calls": 1, "label": "Payment outcome unknown", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "PAYMENT_PENDING" } ] }, { "test": "Unknown payment and stable retry", "calls": 2, "label": "Same payment reconciled", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "CLOSED" } ] }, { "test": "Failed payment retains ownership", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Failed payment retains ownership", "calls": 0, "label": "Ticket issued", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Failed payment retains ownership", "calls": 1, "label": "Payment failed", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "PAYMENT_PENDING" } ] }, { "test": "Concurrent and repeated exit", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Concurrent and repeated exit", "calls": 0, "label": "Ticket issued", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T1" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Concurrent and repeated exit", "calls": 1, "label": "Concurrent exits and later retry", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "CLOSED" } ] }, { "test": "Concurrent and repeated exit", "calls": 1, "label": "Old ticket retried after spot reuse", "spots": [ { "id": "C1", "type": "CAR", "ticketId": "T2" } ], "tickets": [ { "id": "T1", "spotId": "C1", "status": "CLOSED" }, { "id": "T2", "spotId": "C1", "status": "ACTIVE" } ] }, { "test": "Unknown ticket changes nothing", "calls": 0, "label": "Empty lot", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] }, { "test": "Unknown ticket changes nothing", "calls": 0, "label": "Unknown ticket rejected", "spots": [ { "id": "C1", "type": "CAR", "ticketId": null } ], "tickets": [] } ], "metrics": { "total": 6, "passed": 6 }, "version": 1 }, "workspace": { "lesson": "", "source": "function createParkingLot({ spots, charge, clock, price }) {\n const inventory = spots.map(s => ({ ...s, ticketId: null }));\n const tickets = new Map(), inFlight = new Map();\n let sequence = 0;\n const fits = { MOTORCYCLE: ['MOTORCYCLE', 'CAR'], CAR: ['CAR'], TRUCK: ['TRUCK'] };\n return {\n async park(vehicle) {\n const spot = inventory.find(s => !s.ticketId && fits[vehicle.type]?.includes(s.type));\n if (!spot) return null;\n // Claim synchronously before any await: atomic within this single event loop.\n const ticket = { id: 'T' + (++sequence), spotId: spot.id, status: 'ACTIVE', enteredAt: clock() };\n spot.ticketId = ticket.id;\n tickets.set(ticket.id, ticket);\n return { ...ticket };\n },\n async exit(ticketId) {\n const ticket = tickets.get(ticketId);\n if (!ticket) throw new Error('Unknown ticket');\n if (ticket.status === 'CLOSED') return { ...ticket.receipt };\n if (inFlight.has(ticketId)) return inFlight.get(ticketId);\n const operation = (async () => {\n if (ticket.quote === undefined) ticket.quote = price({ ...ticket }, clock());\n ticket.paymentId ??= 'pay-' + ticket.id;\n ticket.status = 'PAYMENT_PENDING';\n let outcome;\n try { outcome = await charge(ticket.paymentId, ticket.quote); 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