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fix(parse): Replace regex with hand-rolled parser (#9)
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fb55 committed Sep 14, 2021
1 parent 3ba66fc commit 9894c1d2010870c351f66c6f6efcf656e26bb726
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@@ -1,7 +1,9 @@
// Following http://www.w3.org/TR/css3-selectors/#nth-child-pseudo

// [ ['-'|'+']? INTEGER? {N} [ S* ['-'|'+'] S* INTEGER ]?
const RE_NTH_ELEMENT = /^([+-]?\d*n)?\s*(?:([+-]?)\s*(\d+))?$/;
// 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.
@@ -19,24 +21,72 @@ export function parse(formula: string): [a: number, b: number] {
return [2, 1];
}

const parsed = formula.match(RE_NTH_ELEMENT);
// Parse [ ['-'|'+']? INTEGER? {N} [ S* ['-'|'+'] S* INTEGER ]?

if (!parsed) {
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}')`);
}

let a;
return [a, sign * number];

if (parsed[1]) {
a = parseInt(parsed[1], 10);
if (isNaN(a)) {
a = parsed[1].startsWith("-") ? -1 : 1;
function readSign() {
if (formula.charAt(idx) === "-") {
idx++;
return -1;
}
} else a = 0;

const b =
(parsed[2] === "-" ? -1 : 1) *
(parsed[3] ? parseInt(parsed[3], 10) : 0);
if (formula.charAt(idx) === "+") {
idx++;
}

return [a, b];
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++;
}
}
}

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