Multi-Branch Pharmacy Fulfilment

Problem

Design PharmacyFulfilment to receive medicine stock, reserve one unit from an ordered route of pharmacy branches, and cancel active reservations. Calls are sequential on one shared instance.

Requirements

Receive stock. receiveStock increases the available quantity of the named medicine at the named branch by exactly units.
Follow route priority. reserve selects the first branch in constructor order with at least one available unit of the requested medicine.
Hold one unit. A successful reserve decreases that branch's available quantity by one and records one active reservation under reservationId.
Leave failed searches unchanged. If no branch has an available unit, reserve returns null without changing any branch-medicine quantity.
Cancel a hold. cancel restores exactly one unit of the reserved medicine to the branch that supplied it.

API

Reservation has string fields reservationId, medicineId, and branchId.

Signature	Returns	Behavior
`PharmacyFulfilment(branchIds: string[])`	Not applicable	Creates zero inventory for each distinct branch and fixes search priority to array order.
`receiveStock(branchId: string, medicineId: string, units: integer)`	`void`	Applies Receive stock.
`reserve(reservationId: string, medicineId: string)`	`Reservation` or `null`	Applies Follow route priority, Hold one unit, and Leave failed searches unchanged.
`cancel(reservationId: string)`	`void`	Applies Cancel a hold.

Examples

Starting with branches ["area", "additional", "main"]:

Step	Operation	Result
1	`receiveStock("main", "aspirin", 2)`	No return value
2	`receiveStock("additional", "aspirin", 1)`	No return value
3	`reserve("R-1", "aspirin")`	`{reservationId: "R-1", medicineId: "aspirin", branchId: "additional"}`
4	`reserve("R-2", "aspirin")`	`{reservationId: "R-2", medicineId: "aspirin", branchId: "main"}`
5	`cancel("R-1")`	No return value
6	`reserve("R-3", "aspirin")`	`{reservationId: "R-3", medicineId: "aspirin", branchId: "additional"}`

Constraints

1 <= branchIds.length <= 20
Each identifier has 1 through 64 ASCII letters, digits, periods, underscores, or hyphens.
branchIds contains distinct values.
1 <= units <= 1,000,000
At most 5,000 distinct medicines, 15,000 successful reservations, and 30,000 total method calls occur.
Each branch-medicine available quantity and total received quantity fit a signed 64-bit integer.
Every receiveStock branch belongs to the constructor route.
Each reservationId passed to reserve has never appeared in an earlier reserve call, including a failed call.
Each cancel is called exactly once for an active successful reservation.

Notes

All calls satisfy the constraints. Behavior outside these preconditions is unspecified and is not judged.