mcalc/modules/compute.js

import { tonelliShanks, modinv, modpow, isprime } from "./math.js";

function binaryOpPop(stack) {
	const b = stack.pop();
	const a = stack.pop();
	if (a === undefined || b === undefined) {
		throw new Error("invalid expression");
	}

	return [a, b];
}

function compute(queue, modulus) {
	const stack = [];
	for (const token of queue) {
		if (typeof token === "bigint") {
			stack.push(token);
		} else if (token === "+") {
			let [a, b] = binaryOpPop(stack);
			a %= modulus;
			b %= modulus;
			const c = (a + b) % modulus;
			stack.push(c);
		} else if (token === "-") {
			let [a, b] = binaryOpPop(stack);
			a %= modulus;
			b %= modulus;
			const c = (a - b) % modulus;
			stack.push(c);
		} else if (token === "*") {
			let [a, b] = binaryOpPop(stack);
			a %= modulus;
			b %= modulus;
			const c = (a * b) % modulus;
			stack.push(c);
		} else if (token === "/") {
			let [a, b] = binaryOpPop(stack);
			a %= modulus;
			b %= modulus;
			const binv = modinv(b, modulus);
			const c = (a * binv) % modulus;
			stack.push(c);
		} else if (token === "u") {
			let a = stack.pop();
			if (a === undefined) {
				throw new Error("invalid expression");
			}

			a *= -1n;
			stack.push(a);
		} else if (token === "^") {
			const [a, b] = binaryOpPop(stack);
			const c = modpow(a, b, modulus);
			stack.push(c);
		} else if (token === "sqrt") {
			if (!isprime(modulus)) {
				throw new Error("modulus must be prime to compute square root");
			}

			const a = stack.pop();
			const s = tonelliShanks(a, modulus);
			stack.push(s);
		}
	}

	if (stack.length !== 1) {
		throw new Error("error evaluating expression");
	}

	let result = stack[0] % modulus;
	if (result < 0n) {
		result += modulus;
	}

	return result;
}

export { compute };