The Spirit-Circle of Kelmorrow Tarn

Before every expedition into the marshes, the summoners of the Ashgrove Conclave chalk a binding-circle on the shore of Kelmorrow Tarn. The circle can hold only so many spirits at once. The Conclave offers a bounty to any keeper who can say, at every moment, which spirits remain bound and which are lost.

Implement the class SpiritCircle. Each spirit has a unique name (a non-empty string) and a rank (an integer). A bound spirit may also be sealed.

Methods

• constructor(capacity): capacity is a non-negative integer, the maximum number of spirits bound at once. The circle starts empty.
• summon(name, rank) returns a string or null.
• If name is already bound, its rank becomes rank and this counts as a use. It stays sealed or unsealed as it was. Return null. This case never evicts anyone.
• Otherwise, if fewer than capacity spirits are bound, bind it unsealed with rank rank. This counts as a use. Return null.
• Otherwise the circle is full. The candidate is the unsealed bound spirit with the lowest rank. If several share that lowest rank, the candidate is the one whose most recent use is earliest.
• If no bound spirit is unsealed, or rank is lower than the candidate's rank, the newcomer is turned away. Nothing changes, and the method returns name (the newcomer's own name).
• Otherwise, remove the candidate and bind the newcomer unsealed. This counts as a use. Return the candidate's name.
• call(name) returns an integer.
• If name is bound, return its rank. This counts as a use, and the call removes the spirit's seal if it had one.
• If name is not bound, return -1 and change nothing.
• seal(name) returns a boolean.
• If name is bound, it becomes sealed (it stays sealed if it already was), and the method returns true.
• Otherwise, return false.
• dismiss(name) returns a boolean.
• If name is bound, remove it whether sealed or not, and return true.
• Otherwise, return false.
• roster() returns a list of the names currently bound, sorted in ascending order by character code.

Recency

Only the summon and call outcomes marked above as counting as a use update a spirit's most recent use. Each such use is later than every use before it. Sealing, dismissing, listing the roster, a turned-away summon and a call on an unbound name are not uses. A spirit that is dismissed or removed and later summoned again is a fresh binding: unsealed, with its new rank.