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Joined 2026-09-21 Message
Incoming links: Persistent data structure
Version snapshots simplify correctness: a complicated command restores the complete prior state, so a bespoke inverse cannot accidentally omit an auxiliary field. Redo restores the recorded result without re-executing behavior that might depend on time, randomness or external input. Restoring an immutable root takes constant time, though refreshing the display can cost more.
The storage comparison depends on the representation. A change-specific record for these elementary edits retains only a few indices and song references. Copying a length- array for each of edits can require storage and copying per edit. A persistent data structure with structural sharing can preserve many versions much more cheaply; balanced path copying can make small edits allocate new entries. Retaining versions also keeps their reachable data alive. Snapshot references are only constant-sized roots, not a guarantee of constant total retained memory. Neither scheme automatically reverses external effects such as playing audio or writing files. The desired undo boundary must be specified.
If sequence versions are immutable, replace The unused inverse argument can subsequently be removed from
perform by the following method; the insert, delete and move entry points can stay as written:private def perform(forward: () => Unit,
backward: () => Unit): Change = {
val before = songs
forward()
val after = songs
new Change {
def undo(): Unit = { songs = before }
def redo(): Unit = { songs = after }
}
}perform and its callers. Each history record now captures two complete versions. Undo and redo restore the respective roots without replaying an operation, an instance of the Memento pattern. The same chronological history discipline still applies.The supplied method signatures alone do not establish immutability or persistence. If
insert or delete changes a shared mutable software object, merely storing its reference fails to freeze the earlier state. Then independent copies of the relevant state are needed, including any mutable song state intended to be undoable, or the implementation must provide immutable persistent data structure versions. This is a representation requirement, not a reason to invent a particular unavailable copying API.Not necessarily: structural sharing can make snapshots cheap, but the trait alone gives no storage guarantee. With a persistent data structure, unchanged portions of old and new sequences can be shared safely. Saving two root references costs additional space per edit, and adjacent history entries commonly refer to the same intermediate version. With an appropriate balanced persistent tree, each positional edit may add only new vertices, giving approximately storage for retained edits of comparable presentation size, plus references to slides.
There are important qualifications. A fully copied immutable array may allocate per edit and use across its history. A persistent linked list shares suffixes but may copy a long prefix for a late positional edit. Moreover, snapshot history keeps old versions alive that an edit-specific inverse representation might otherwise release. Consequently a significant increase is possible with some implementations or workloads; it is not forced by saving
Sequence references, and it cannot be ruled out from the interface alone.