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Edit operations + undo/redo — ED.2

Every timeline edit is an EditOp applied to the project — a small, validated, invariant-preserving mutation. Undo/redo wraps them in a History.

The operations

classDiagram
  class EditOp {
    <<enum>>
    Split { at }
    Trim { index, edge, to }
    RippleDelete { start, end }
    SetSpeed { index, timescale }
    AddZoom { zoom }
    RemoveZoom { id }
    MoveZoom { id, start, end }
  }
  • Split cuts the segment under a project frame into two (a no-op on a boundary). Trim moves a segment's in/out point, clamped so it stays non-empty and inside the source. RippleDelete removes a project range and closes the gap — the surviving pieces simply concatenate. SetSpeed sets a segment's timescale (sanitized to a finite, positive multiplier).
  • AddZoom / RemoveZoom / MoveZoom edit the zoom list, which stays sorted by start frame; ids are assigned fresh on insert so move/remove can address a zoom stably as the list changes.

Undo without fragile inverses

History::apply doesn't derive a per-operation inverse (which is error-prone for splits and ripples). Instead it applies the operation to a clone and commits only if the project actually changed:

sequenceDiagram
  participant C as caller
  participant H as History
  participant P as EditProject
  C->>H: apply(op)
  H->>P: next = current.clone()
  H->>P: next.apply(op)
  alt next != current
    H->>H: push current onto undo stack, clear redo
    H->>H: current = next
  else unchanged (no-op / error)
    H->>H: discard the clone, record nothing
  end

apply+undo == identity, by construction

Because the prior state is snapshotted verbatim, undo restores it exactly — no inverse-operation bugs are possible. No-op and failed operations record nothing, so the undo stack only holds real changes. This property is proved by proptest: random operation sequences always preserve the project's invariants, and undoing them all returns to the starting project.

The lift-delete variant (delete a range but leave a black gap) needs a timeline "gap" item the segment model doesn't yet have; it is deferred (see _docs/ISSUES.md). Ripple-delete is the primary delete, which is what the timeline UI (ED.11) leads with.