mcalc/main.js

import { tokenize } from "./modules/lexer.js";
import { shunt } from "./modules/parser.js";

function xgcd(a, b) {
	let [old_r, r] = [a, b];
	let [old_s, s] = [1n, 0n];
	let [old_t, t] = [0n, 1n];

	while (r !== 0n) {
		const quotient = old_r / r;
		[old_r, r] = [r, old_r - quotient * r];
		[old_s, s] = [s, old_s - quotient * s];
		[old_t, t] = [t, old_t - quotient * t];
	}

	return [old_r, old_s, old_t];
}

function modinv(x, modulus) {
	let [r, s, t] = xgcd(x, modulus);
	if (r !== 1n) {
		throw new Error(`no inverse exists - ${x} and ${modulus} are not coprime`);
	}

	if (s < 0n) {
		s += modulus;
	}

	return s;
}

function modpow(base, exponent, modulus) {
	if (exponent < 0n) {
		const p = modpow(base, -exponent, modulus);
		return modinv(p, modulus);
	}

	if (modulus === 1n) {
		return 0n;
	}

	let result = 1n;
	base %= modulus;

	while (exponent > 0) {
		if (exponent % 2n === 1n) {
			result *= base;
			result %= modulus;
		}

		exponent >>= 1n;
		base *= base;
		base %= modulus;
	}

	return result;
}

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 % modulus);
		} else if (token === "+") {
			const [a, b] = binaryOpPop(stack);
			const c = (a + b) % modulus;
			stack.push(c);
		} else if (token === "-") {
			const [a, b] = binaryOpPop(stack);
			const c = (a - b) % modulus;
			stack.push(c);
		} else if (token === "*") {
			const [a, b] = binaryOpPop(stack);
			const c = (a * b) % modulus;
			stack.push(c);
		} else if (token === "/") {
			const [a, b] = binaryOpPop(stack);
			const binv = modinv(b, modulus);
			const c = (a * binv) % modulus;
			stack.push(c);
		} else if (token === "^") {
			const [a, b] = binaryOpPop(stack);
			const c = modpow(a, b, modulus);
			stack.push(c);
		}
	}

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

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

	return result;
}

function calculate() {
	const expr = document.querySelector("#expr");
	const modulus = document.querySelector("#modulus");
	const m = BigInt(modulus.value);

	const tokens = tokenize(expr.value);
	console.log(tokens);

	const result = document.querySelector("#result");
	let queue = [];
	try {
		queue = shunt(tokens);
	} catch(e) {
		result.value = e;
		console.log(e);
		return;
	}

	console.log(queue);

	let r = 0;
	try {
		r = compute(queue, m);
	} catch(e) {
		result.value = e;
		console.log(e);
		return;
	}

	result.value = r;

}

document.querySelector("#enter").addEventListener("click", calculate);