do not allow expressions in exponent

modules/compute.js

@@ -94,11 +94,6 @@ function compute(queue, modulus) {
 			const c = (a * binv) % modulus;
 			stack.push(c);
 		} else if (token === "^") {
-			// FIXME: not always valid - consider 2^3^4 mod 7
-			// also 2^(3+4)
-			// solution might be to change parser
-			// might also be better to just mod once at the end
-			// instead of with every op
 			const [a, b] = binaryOpPop(stack);
 			const c = modpow(a, b, modulus);
 			stack.push(c);

modules/parser.js

@@ -2,7 +2,7 @@ function isLeftAssociative(op) {
 	return op === "+" || op === "-" || op === "*" || op === "/";
 }
 
-function popOps(opstack, queue, op) {
+function popOps(opstack, queue, op, powInStack) {
 	const prec = {"+": 1, "-": 1, "*": 2, "/": 2, "^": 3}
 	while (opstack.length > 0) {
 		const op2 = opstack.at(-1);
@@ -13,25 +13,40 @@ function popOps(opstack, queue, op) {
 		if ((prec[op2] > prec[op]) || (prec[op2] === prec[op] && isLeftAssociative(op))) {
 			opstack.pop();
 			queue.push(op2);
+			if (op2 === "^") {
+				powInStack = false;
+			}
 		} else {
 			break;
 		}
 	}
 
-	opstack.push(op);
+	// practically, we want 2^(3+4) mod 7 to evaluate to 2^7 mod 7 = 2
+	// however, since our operators are all modular, this would instead
+	// evaluate as 2^0 mod 7 = 1
+	// 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) {
+		throw new Error("exponent must be an integer, not an expression");
+	}
 }
 
-function popBetweenParens(opstack, queue) {
+function popBetweenParens(opstack, queue, powInStack) {
 	while (opstack.at(-1) !== "(") {
 		if (opstack.length === 0) {
 			throw new Error("mismatched parentheses");
 		}
 
 		const op = opstack.pop();
+		if (op === "^") {
+			powInStack = false;
+		}
 		queue.push(op);
 	}
 
 	opstack.pop();
+	return powInStack;
 }
 
 function empty(opstack, queue) {
@@ -48,15 +63,22 @@ function empty(opstack, queue) {
 function shunt(tokens) {
 	const queue = [];
 	const opstack = [];
+	let powInStack = false;
 	for (const token of tokens) {
 		if (typeof token === "bigint") {
 			queue.push(token);
 		} else if (/[-+*/^]/.test(token)) {
-			popOps(opstack, queue, token);
+			popOps(opstack, queue, token, powInStack);
+			powInStack = false;
+
+			opstack.push(token);
+			if (token === "^") {
+				powInStack = true;
+			}
 		} else if (token === "(") {
 			opstack.push(token);
 		} else if (token === ")") {
-			popBetweenParens(opstack, queue);
+			powInStack = popBetweenParens(opstack, queue, powInStack);
 		}
 	}