export const DELETE_PROPERTY = Symbol('DELETE_PROPERTY');


export class String_Keyed_Stack {
	constructor(target={}, stack=[]) {
		Object.assign(this, { target, stack });
	}

	push(updates={}) {
		const frame = {}
		this.stack.push(frame);
		for (const [key, value] of Object.entries(updates)) {

			if (key in this.target) {
				frame[key] = this.target[key];
			} else {
				frame[key] = DELETE_PROPERTY;
			}

			if (value === DELETE_PROPERTY) {
				delete this.target[key];
			} else {
				this.target[key] = value;
			}
		}
		return frame;
	}

	push_defined(updates={}) {
		this.push(Object.fromEntries(Object.entries(updates).filter(([k ,v]) => v !== undefined )));
	}

	pop(copy_previous_state=false) {
		const frame = this.stack.pop();
		const { target } = this;

		const return_value = copy_previous_state ? { ...target } : null;

		for (const [key, value] of Object.entries(frame)) {
			if (value === DELETE_PROPERTY) {
				delete target[key];
			} else {
				target[key] = value;
			}
		}

		return return_value;
	}

	get top_reverse_delta() {
		return this.stack.at(-1);
	}

}