calculator.html

<style>
body {
	font-family: Arial;
	font-size: 120%;
}
td, th {
	padding: 6px 16px;
}
table {
	margin: 2em auto;
}

</style>

<body>

<script>
/*

AddSub ::= MulDiv ('+' MulDiv | '-' MulDiv)*
MulDiv ::= Exp ('*' Exp | '/' Exp)*
Exp    ::= Factor ('**' Factor)*
Factor ::= ['-'] (Number | '(' AddSub ')')
Number ::= Digit+

*/
const {PI} = Math;
const blank = /\s/;
const num = /^NaN|^pi|^PI|^[0-9]+(\.[0-9]*)?(e[0-9]+)?|^[0-9]*\.[0-9]+(e[0-9]+)?/;

function parseAddSub(s, i=0){
	let [j, v] = parseMulDiv(s, i);
	while (j<s.length){
		if (blank.test(s[j])) j++;
		else if (s[j]=='+'){
			const [k, w] = parseMulDiv(s, j+1);
			[j, v] = [k, v+w];
		} else if (s[j]=='-'){
			const [k, w] = parseMulDiv(s, j+1);
			[j, v] = [k, v-w];
		} else break;
	}
	return [j,v];
}

function parseMulDiv(s, i){
	let [j, v] = parseExponent(s, i);
	while (j<s.length){
		if (blank.test(s[j])) j++;
		else if (s[j]=='*'){
			const [k, w] = parseExponent(s, j+1);
			[j, v] = [k, v*w];
		} else if (s[j]=='/'){
			const [k, w] = parseExponent(s, j+1);
			[j, v] = [k, v/w];
		} else break;
	}
	return [j,v];
}

function parseExponent(s, i){ // right associative
	let [j, v] = parseFactor(s, i);
	while (j<s.length){
		if (blank.test(s[j])) j++;
		else if (s[j]=='*'&&s[j+1]=='*') {
			const [k, w] = parseExponent(s, j+2);
			return [k, v**w];
		} else break;
	}
	return [j, v];
}

function parseFactor(s, i){
	let factor = 1;
	while (i<s.length){
		if (blank.test(s[i])) i++;
		else if (s[i]=='+') i++;
		else if (s[i]=='-') {factor *= -1; i++; }
		else if (s[i]=='(') {
			const [j, v] = parseAddSub(s, i+1);
			if (s[j]!==')') throw new Error('parentheses not matched '+s.slice(i,j));
			return [j+1, factor * v];
		} else {
			const [j, v] = parseNumber(s, i);
			return [j, factor * v];
		}
	}
}

function parseNumber(s, i){
	while (blank.test(s[i])) i++;
	let m = s.slice(i).match(num);
	if (!m) {
		debugger;
		throw new Error('expected number, received '+s.slice(i));
	}
	const v = m[0];
	return [i+v.length, v=='pi'||v=='PI'?PI:parseFloat(v)];
}

// todo make it work with a binary tree, instead of directly evaluating

var testCases =  [
	"2 * (4+2)",
	"2*  3 +4+3 *8*(7+1) + 6",
	" -2 ** 3",
	" (-.201e1) ** 3",
	" 2 ** 2 ** 3",
	" 2 ** (+2 - 3)",
	" 2 ** -3 ",
	" 2 ** -3.14 ",
	" 2.001 ** -PI ",
	"(1+2)*3+5*(4*6)",
	"(1+2)*3+6*5*(2*3)",
	"1+ (2*(4+3)+6)*8",
	"(4+5) + ((2*(1+3)+4)*4+5)*4 +5*6",
	"2*(4 - 2 -5) *6 +24 /5",
	"2+((9-4/7)*2-1)-18*(14/4/(2*(3+5)))"
]

var lines = testCases.map(s=>[s, parseAddSub(s)[1], evil(s)]);
document.body.innerHTML += `<table>
	<tr><th>input<th>calculated<th>eval
	${lines.map(([s, ev, evi])=>`<tr><td>${s}<td>${ev}<td>${evi}`).join('\n')}
</table>`;


// for (const s of testCases) {
// 	console.log(parseAddSub(s), evil(s));
// }


function evil(s){
	try {
		return eval(s);
	} catch(e) {return e}
}


</script>

</body>