initiative/packages/domain/src/undo-redo.ts

import type { DomainError, Encounter } from "./types.js";

export interface UndoRedoState {
	readonly undoStack: readonly Encounter[];
	readonly redoStack: readonly Encounter[];
}

const MAX_UNDO_STACK = 50;

export const EMPTY_UNDO_REDO_STATE: UndoRedoState = {
	undoStack: [],
	redoStack: [],
};

export function pushUndo(
	state: UndoRedoState,
	snapshot: Encounter,
): UndoRedoState {
	const newStack = [...state.undoStack, snapshot];
	if (newStack.length > MAX_UNDO_STACK) {
		newStack.shift();
	}
	return { undoStack: newStack, redoStack: [] };
}

export function undo(
	state: UndoRedoState,
	currentEncounter: Encounter,
): { state: UndoRedoState; encounter: Encounter } | DomainError {
	if (state.undoStack.length === 0) {
		return {
			kind: "domain-error",
			code: "nothing-to-undo",
			message: "Nothing to undo",
		};
	}
	const restored = state.undoStack.at(-1) as Encounter;
	return {
		state: {
			undoStack: state.undoStack.slice(0, -1),
			redoStack: [...state.redoStack, currentEncounter],
		},
		encounter: restored,
	};
}

export function redo(
	state: UndoRedoState,
	currentEncounter: Encounter,
): { state: UndoRedoState; encounter: Encounter } | DomainError {
	if (state.redoStack.length === 0) {
		return {
			kind: "domain-error",
			code: "nothing-to-redo",
			message: "Nothing to redo",
		};
	}
	const restored = state.redoStack.at(-1) as Encounter;
	return {
		state: {
			undoStack: [...state.undoStack, currentEncounter],
			redoStack: state.redoStack.slice(0, -1),
		},
		encounter: restored,
	};
}

export function clearHistory(): UndoRedoState {
	return EMPTY_UNDO_REDO_STATE;
}