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Edit History Model (Simplified & Complete)

We model edit history using two primitives:

  • Timeline (H): an ordered list of history items [0…N-1] (oldest → newest).
    • H[0] is always a sentinel placeholder representing the initial design / upload.
    • All other items are completed generations with {input, output, prompt}.
  • Pointer (p): either an integer index 0…N-1 or ⊥ (null) representing the live cursor (N+1).

Together, (H, p) fully describe the history state.

Navigation Rules

  • Rewind

    • If p = ⊥ → set p = max(0, N-2) (skip duplicate latest entry).
    • Else if p > 0 → decrement (p = p - 1).
    • Else (p = 0) → no-op (can’t rewind past placeholder).

  • Fast-forward

    • If p = ⊥ → no-op (already at cursor).
    • Else if p = N-1 → set p = ⊥ (snap to cursor).
    • Else → increment (p = p + 1).

  • Counter display

    • If p = ⊥ → show N / N.
    • Else → show (p+1) / N.

Rendering Policy

  • Image

    • p = ⊥ → preview latest output H[N-1].output.
    • p = 0 (placeholder) → use H[0].input.
    • Else → use H[p].output.

  • Prompt

    • p = ⊥ or p = N-1 → "Type to make edits".
    • p = 0 (placeholder) → "Initial design" or stored label.
    • Else → truncated H[p].prompt.

Data Shape

type HistoryItem =
  | { kind: 'placeholder'; input: Asset; label?: string }
  | { kind: 'completed'; input: Asset; output: Asset; prompt: string; meta?: any };

type HistoryState = {
  H: HistoryItem[]; // H[0] is always the placeholder
  p: number | null; // null = ⊥ = cursor
};

Transition Functions

function rewind(s: HistoryState): HistoryState {
  const N = s.H.length;
  if (s.p === null) return { ...s, p: Math.max(0, N - 2) };
  if (s.p > 0) return { ...s, p: s.p - 1 };
  return s;
}

function forward(s: HistoryState): HistoryState {
  const N = s.H.length;
  if (s.p === null) return s;
  if (s.p === N - 1) return { ...s, p: null };
  return { ...s, p: s.p + 1 };
}

function counter(s: HistoryState): string {
  const N = s.H.length;
  return s.p === null ? `${N}/${N}` : `${s.p + 1}/${N}`;
}

Edge Cases • Fresh session: H = [placeholder], p = ⊥. • Rewind guard: cannot go past p = 0. • Fast-forward guard: last edit skips directly to p = ⊥. • In-progress generation: don’t append until complete; cursor stays ⊥. • Failed/canceled generation: don’t append; show error instead. • Reloads: server always returns H[0] (placeholder); client can synthesize if missing. • Export/Cart/Share: skip placeholder rows when iterating H.

Optional Enhancements • Store a displayAssetId (e.g., before/after composite) per completed item for richer thumbnails. • Replace implicit ⊥ with an explicit "cursor" item if you want uniform list navigation. • Use parent_edit_id for branching histories (fork from any earlier item).