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