The Lattice-Glass of Magister Orsolde

The Collegium of Wrenfall posts a commission for a binding-spirit to keep Magister Orsolde's lattice-glass, the crystal in which she carries her prepared battle-spells into the field. The glass holds only so many spells. When it is full and a new spell is pressed in, one of the old ones is lost. Spells the Magister has sealed are never lost this way. The Collegium wants the keeping written down exactly, so that no spell is ever lost by mistake.

Write the class SpellLattice.

Constructor: takes capacity, an integer >= 0. This is the largest number of spells the glass may hold at once. The glass starts empty.

Each held spell has: - a name, a non-empty case-sensitive string, - a number of charges, a positive integer, - a sealed flag, - a place in the order of recent use.

A use of a spell is one of two things: a successful inscribe of that spell, or a cast of it that leaves it still held. Nothing else counts as a use.

Methods:

- inscribe(name, charges) takes an integer charges >= 1 and returns a string or null (None in Python). The first rule that applies decides the result: 1. If name is already held, set its charges to charges. This counts as a use, and its sealed flag is unchanged. Return null. 2. Otherwise, if fewer than capacity spells are held, add the spell, unsealed, as a use. Return null. 3. Otherwise, if at least one held spell is unsealed, remove one unsealed spell. Choose the one with the fewest charges. If several are tied, choose the one whose most recent use is earliest. Then add the new spell, unsealed, as a use. Return the removed spell's name. 4. Otherwise every held spell is sealed. This includes the case capacity = 0. The glass is unchanged and the new spell is not held. Return name.

• cast(name) returns an integer.
• If name is not held, return -1 and change nothing.
• Otherwise reduce its charges by 1.
• If its charges reach 0, remove the spell entirely, whether it is sealed or not, and return 0.
• If its charges do not reach 0, the cast counts as a use. Return the remaining charges.

• seal(name) marks the spell as sealed and returns true. If the spell is not held, it returns false. This is not a use.

• unseal(name) marks the spell as unsealed and returns true. If the spell is not held, it returns false. This is not a use.

• charges(name) returns the spell's current charges, or -1 if it is not held. This is not a use.

A spell that has been removed and is later inscribed again comes back as a new, unsealed spell.