83 lines
2.5 KiB
TypeScript
83 lines
2.5 KiB
TypeScript
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export type Undoable<L, T> = {
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loc: L;
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value_initial: T;
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value_updated: T;
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};
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export class UndoStack<T extends Undoable<unknown, unknown>> {
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/**
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* All undo- and redo-able diffs, in order.
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*/
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private _diffs: T[][] = [];
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/**
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* Index of last delta currently represented in application state. Decrements
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* when undoing; increments when pushing or redoing. -1 when stack is empty.
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*/
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private _cursor: number = -1;
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private readonly _apply_diff: (diff: T[]) => unknown;
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/**
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* NOTE: `apply_diff()` is permitted to mutate the array it receives as a
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* parameter, but otherwise `UndoStack` expects that the deltas it receives
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* and provides will not be tampered with. Mutating arrays after providing
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* them to `.push()` or mutating the individual array items supplied to
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* `apply_diff()`, for example, is considered undefined behavior.
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*/
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constructor(
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{ apply_diff }: { apply_diff(deltas: T[]): unknown },
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) {
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// Always shallow copy array so that callback may perform some mutations.
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this._apply_diff = (diff: T[]) => apply_diff([...diff]);
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}
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/**
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* Pushes a batch of deltas to the end of the undo stack, and clears the redo
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* stack. `apply_diff()` will be immediately called with a copy of `deltas`.
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*/
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push(diff: T[]): undefined {
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// Clear redo stack. `Array.splice()` does nothing if start index > array
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// length, and `this._cursor` is always >= -1, so no conditional needed.
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this._diffs.splice(this._cursor + 1);
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this._diffs.push(diff);
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this._cursor += 1;
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// Call `_apply_diff()` after shifting cursor, in case it recursively
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// mutates this UndoStack.
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this._apply_diff(diff);
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}
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undo(): undefined {
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if (this._cursor > -1) {
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this._cursor -= 1;
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// Call `_apply_diff()` after shifting cursor, in case it recursively
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// mutates this UndoStack.
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this._apply_diff(this._diffs[this._cursor + 1].map(invert));
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}
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}
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redo(): undefined {
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if (this._diffs.length > this._cursor + 1) {
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this._cursor += 1;
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// Call `_apply_diff()` after shifting cursor, in case it recursively
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// mutates this UndoStack.
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this._apply_diff(this._diffs[this._cursor]);
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}
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}
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}
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/**
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* Returns a copy of the parameter with initial and updated values swapped.
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*/
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function invert<T extends Undoable<unknown, unknown>>(undoable: T): T {
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return {
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loc: undoable.loc,
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value_initial: undoable.value_updated,
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value_updated: undoable.value_initial,
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} as T;
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}
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