handle unary minus

modules/compute.js

@@ -93,6 +93,14 @@ function compute(queue, 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);

modules/lexer.js

@@ -1,8 +1,7 @@
 function tokenize(expr) {
 	// NOTE: not handling whitespace
 	// NOTE: currently ends early if string doesn't match token
-	// FIXME: need to handle unary minus (e.g. on parentheses)
-	const regexp = /-?[0-9]+|[-+*/^]|\(|\)/gy;
+	const regexp = /[0-9]+|[-+*/^]|\(|\)/gy;
 	const matches = expr.matchAll(regexp);
 	
 	const tokens = [];

modules/parser.js

@@ -3,7 +3,7 @@ function isLeftAssociative(op) {
 }
 
 function popOps(opstack, queue, op, powInStack) {
-	const prec = {"+": 1, "-": 1, "*": 2, "/": 2, "^": 3}
+	const prec = {"+": 1, "-": 1, "*": 2, "/": 2, "u": 3, "^": 3}
 	while (opstack.length > 0) {
 		const op2 = opstack.at(-1);
 		if (op2 === "(") {
@@ -27,9 +27,11 @@ function popOps(opstack, queue, op, powInStack) {
 	// rather than complicate things by evaluating exponent expressions
 	// normally instead of modularly (and consequently needing to deal with
 	// fractions) we simply require exponents to be integers
-	if (powInStack) {
+	if (powInStack && op !== "u") {
 		throw new Error("exponent must be an integer, not an expression");
 	}
+
+	return powInStack;
 }
 
 function popBetweenParens(opstack, queue, powInStack) {
@@ -60,16 +62,24 @@ function empty(opstack, queue) {
 	}
 }
 
+function isUnaryMinus(token, lastToken) {
+	return token === "-" && (lastToken === null || /[(-+*/^]/.test(lastToken));
+}
+
 function shunt(tokens) {
 	const queue = [];
 	const opstack = [];
+	let lastToken = null;
 	let powInStack = false;
-	for (const token of tokens) {
+	for (let token of tokens) {
 		if (typeof token === "bigint") {
 			queue.push(token);
 		} else if (/[-+*/^]/.test(token)) {
-			popOps(opstack, queue, token, powInStack);
-			powInStack = false;
+			if (isUnaryMinus(token, lastToken)) {
+				token = "u";
+			}
+
+			powInStack = popOps(opstack, queue, token, powInStack);
 
 			opstack.push(token);
 			if (token === "^") {
@@ -80,6 +90,8 @@ function shunt(tokens) {
 		} else if (token === ")") {
 			powInStack = popBetweenParens(opstack, queue, powInStack);
 		}
+
+		lastToken = token;
 	}
 
 	empty(opstack, queue);