Recoverable Local Store

Problem

Design RecoverableLocalStore to serve string key-value reads from memory while preserving every completed write through an injected DurableStorage. A new store constructed with retained durable data must rebuild the same completed state. All calls are sequential.

Requirements

Read current values. get returns the current string value for a key, including an empty string, or null when the key has never been successfully written.
Complete durable writes. put replaces any prior value, returns only after DurableStorage atomically commits the pair, and makes the new value immediately visible to get.
Recover completed data. Construction loads the complete durable snapshot, so a fresh store returns the latest value for every completed write represented in that snapshot.

API

Signature	Returns	Behavior
`RecoverableLocalStore(storage: DurableStorage)`	Not applicable	Loads `storage.loadAll()` as the initial in-memory mapping. Raises `InvalidArgumentError` for null storage.
`put(key: string, value: string)`	`void`	Durably and visibly replaces the key's value. Raises `InvalidArgumentError` for an empty key or null value before changing local or durable state.
`get(key: string)`	`string or null`	Returns the current value or `null` when absent. Raises `InvalidArgumentError` for an empty key.

DurableStorage is a provided collaborator with loadAll() -> object<string, string> and write(key: string, value: string) -> void. Its contents outlive local-store objects. loadAll returns a copy of all completed pairs, and write atomically replaces one pair before returning. Implement RecoverableLocalStore, not the storage collaborator.

Examples

Step	Operation	Result
1	Construct with empty durable storage.	The local store is empty.
2	`put("mode", "draft")`	`get("mode")` returns `"draft"`; durable storage contains the same pair.
3	`put("mode", "ready")`	`get("mode")` returns `"ready"`; the durable value is replaced.
4	`get("missing")`	Returns `null`.
5	Construct a fresh store from retained `{"mode":"ready","region":"west"}`.	Reads return `"ready"` and `"west"`.

Constraints

Keys are nonempty strings containing at most 200 Unicode scalar values.
Values are non-null strings, may be empty, and contain at most 10,000 Unicode scalar values.
At most 10,000 distinct keys and 100,000 method calls occur per testcase.
Durable snapshots contain only valid keys and non-null string values.
One active store owns its provided storage, which changes only through that store.

Notes

A simulated crash or reboot is represented by constructing a testcase's single store from a retained durable snapshot containing only completed writes. Storage failures, deletion, checkpoints, logs, replay, public restart operations, and concurrent calls are out of scope.