spreadSheetI

/**
 * Tiny spreadsheet that supports numeric literals and formulas with + only.
 *
 * Cell value rules:
 * - number: treated as a literal.
 * - string starting with '=': formula, e.g. '=A1 + 5 + B2'.
 * - unset cell: behaves like 0.
 */
export default class Spreadsheet {
	constructor() {
		this.cells = new Map();
	}

	/**
	 * @param {string} cellId
	 * @param {number | string} value
	 * @returns {void}
	 */
	setCell(cellId, value) {
		this.cells.set(cellId, value);
	}

	/**
	 * @param {string} cellId
	 * @returns {number}
	 */
	getCell(cellId) {
		return this.#evaluateCell(cellId, new Map());
	}

	/**
	 * @param {string} cellId
	 * @param {Map<string, number>} memo
	 * @returns {number}
	 */
	#evaluateCell(cellId, memo) {
		if (memo.has(cellId)) {
			return memo.get(cellId);
		}

		const raw = this.cells.get(cellId);

		// Unset cells behave as zero.
		if (raw === undefined) {
			memo.set(cellId, 0);
			return 0;
		}

		// Number literal.
		if (typeof raw === 'number') {
			memo.set(cellId, raw);
			return raw;
		}

		// Non-formula string literal: parse as number fallback.
		if (!raw.startsWith('=')) {
			const parsed = Number(raw);
			const value = Number.isNaN(parsed) ? 0 : parsed;
			memo.set(cellId, value);
			return value;
		}

		// Formula supports + only.
		const expression = raw.slice(1);
		const parts = expression.split('+');
		let sum = 0;

		for (const part of parts) {
			const token = part.trim();

			// Numeric token in formula, e.g. '=A1 + 5'.
			// Formula left to right, supporting plus only
			if (/^-?\d+(?:\.\d+)?$/.test(token)) {
				sum += Number(token);
			} else {
				// Otherwise token is treated as a referenced cell id.
				sum += this.#evaluateCell(token, memo);
			}
		}

		memo.set(cellId, sum);
		return sum;
	}
}