fix(parse): Replace regex with hand-rolled parser (fb55#9)

Based on 9894c1d.

parse.js

@@ -1,40 +1,94 @@
 module.exports = parse;
 
-//following http://www.w3.org/TR/css3-selectors/#nth-child-pseudo
-
-//[ ['-'|'+']? INTEGER? {N} [ S* ['-'|'+'] S* INTEGER ]?
-var re_nthElement = /^([+\-]?\d*n)?\s*(?:([+\-]?)\s*(\d+))?$/;
-
-/*
-	parses a nth-check formula, returns an array of two numbers
-*/
-function parse(formula){
-	formula = formula.trim().toLowerCase();
-
-	if(formula === "even"){
-		return [2, 0];
-	} else if(formula === "odd"){
-		return [2, 1];
-	} else {
-		var parsed = formula.match(re_nthElement);
-
-		if(!parsed){
-			throw new SyntaxError("n-th rule couldn't be parsed ('" + formula + "')");
-		}
-
-		var a;
-
-		if(parsed[1]){
-			a = parseInt(parsed[1], 10);
-			if(isNaN(a)){
-				if(parsed[1].charAt(0) === "-") a = -1;
-				else a = 1;
-			}
-		} else a = 0;
-
-		return [
-			a,
-			parsed[3] ? parseInt((parsed[2] || "") + parsed[3], 10) : 0
-		];
-	}
+// Following http://www.w3.org/TR/css3-selectors/#nth-child-pseudo
+
+// Whitespace as per https://www.w3.org/TR/selectors-3/#lex is " \t\r\n\f"
+const whitespace = new Set([9, 10, 12, 13, 32]);
+const ZERO = "0".charCodeAt(0);
+const NINE = "9".charCodeAt(0);
+
+/**
+ * Parses an expression.
+ *
+ * @throws An `Error` if parsing fails.
+ * @returns An array containing the integer step size and the integer offset of the nth rule.
+ * @example nthCheck.parse("2n+3"); // returns [2, 3]
+ */
+function parse(formula) {
+    formula = formula.trim().toLowerCase();
+
+    if (formula === "even") {
+        return [2, 0];
+    } else if (formula === "odd") {
+        return [2, 1];
+    }
+
+    // Parse [ ['-'|'+']? INTEGER? {N} [ S* ['-'|'+'] S* INTEGER ]?
+
+    let idx = 0;
+
+    let a = 0;
+    let sign = readSign();
+    let number = readNumber();
+
+    if (idx < formula.length && formula.charAt(idx) === "n") {
+        idx++;
+        a = sign * (number ?? 1);
+
+        skipWhitespace();
+
+        if (idx < formula.length) {
+            sign = readSign();
+            skipWhitespace();
+            number = readNumber();
+        } else {
+            sign = number = 0;
+        }
+    }
+
+    // Throw if there is anything else
+    if (number === null || idx < formula.length) {
+        throw new Error(`n-th rule couldn't be parsed ('${formula}')`);
+    }
+
+    return [a, sign * number];
+
+    function readSign() {
+        if (formula.charAt(idx) === "-") {
+            idx++;
+            return -1;
+        }
+
+        if (formula.charAt(idx) === "+") {
+            idx++;
+        }
+
+        return 1;
+    }
+
+    function readNumber() {
+        const start = idx;
+        let value = 0;
+
+        while (
+            idx < formula.length &&
+            formula.charCodeAt(idx) >= ZERO &&
+            formula.charCodeAt(idx) <= NINE
+        ) {
+            value = value * 10 + (formula.charCodeAt(idx) - ZERO);
+            idx++;
+        }
+
+        // Return `null` if we didn't read anything.
+        return idx === start ? null : value;
+    }
+
+    function skipWhitespace() {
+        while (
+            idx < formula.length &&
+            whitespace.has(formula.charCodeAt(idx))
+        ) {
+            idx++;
+        }
+    }
 }