parse.js
2.19 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
// 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]
*/
export 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 !== null && number !== void 0 ? 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++;
}
}
}
//# sourceMappingURL=parse.js.map