Writing a ledger

tilttek/points

An application that keeps its points in its own storage implements Tilttek\Points\Ledger\Ledger:

Method	Contract
`append(Movement, DateTimeImmutable $at): Recorded`	Writes the movement at the next position, or returns the entry already there under its key (`new` false); the same key for another movement raises `MovementConflict`. Two processes writing the same key at once write it once.
`appendCovered(Movement, DateTimeImmutable $at): Recorded`	Like `append()`, but only when the debited account's balance covers the amount, checked and written as one step; otherwise `InsufficientPoints`. A duplicate returns the entry without checking again.
`find(MovementKey): ?LedgerEntry`	The entry of the key.
`causedBy(string $cause): list<LedgerEntry>`	The entries of an activity key, by position.
`balance(Account, string $currency): int`	Credits less debits of the account in the currency.
`net(Account $to, Account $from, string $currency, ?since, ?until): int`	What `to` received from `from` less what it gave back, over the moments from `since`, included, to `until`, excluded.
`statement(Account, ?string $currency, int $after, int $limit): list<LedgerEntry>`	The account's entries after a position, by position; `Reading::check()` refuses a negative position or a limit below 1.
`membersCreditedBefore(string $currency, DateTimeImmutable $cutoff): list<Account>`	The members' accounts credited at or before the cutoff, each once, sorted.
`forgetDerived(?DateTimeImmutable $since): int`	Deletes the earned, reversed and expired movements from `since` on, or all of them.
`eraseMember(ResourceReference): Erasure`	Deletes the member's movements with the ledger's own accounts, writes `erased:member` in place of the member in the others.

Positions only grow; they need not follow each other. The test case Tilttek\Points\Tests\Unit\Ledger\LedgerTestCase states the contract in tests: extend it, build your ledger in ledger(), and it runs against your storage.