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1# nth-check [![Build Status](https://travis-ci.org/fb55/nth-check.svg)](https://travis-ci.org/fb55/nth-check)
2
3Parses and compiles CSS nth-checks to highly optimized functions.
4
5### About
6
7This module can be used to parse & compile nth-checks, as they are found in CSS 3's `nth-child()` and `nth-last-of-type()`. It can be used to check if a given index matches a given nth-rule, or to generate a sequence of indices matching a given nth-rule.
8
9`nth-check` focusses on speed, providing optimized functions for different kinds of nth-child formulas, while still following the [spec](http://www.w3.org/TR/css3-selectors/#nth-child-pseudo).
10
11### API
12
13```js
14import nthCheck, { parse, compile } from "nth-check";
15```
16
17##### `nthCheck(formula)`
18
19Parses and compiles a formula to a highly optimized function. Combination of `parse` and `compile`.
20
21If the formula doesn't match any elements, it returns [`boolbase`](https://github.com/fb55/boolbase)'s `falseFunc`. Otherwise, a function accepting an _index_ is returned, which returns whether or not the passed _index_ matches the formula.
22
23**Note**: The nth-rule starts counting at `1`, the returned function at `0`.
24
25**Example:**
26
27```js
28const check = nthCheck("2n+3");
29
30check(0); // `false`
31check(1); // `false`
32check(2); // `true`
33check(3); // `false`
34check(4); // `true`
35check(5); // `false`
36check(6); // `true`
37```
38
39##### `parse(formula)`
40
41Parses the expression, throws an `Error` if it fails. Otherwise, returns an array containing the integer step size and the integer offset of the nth rule.
42
43**Example:**
44
45```js
46parse("2n+3"); // [2, 3]
47```
48
49##### `compile([a, b])`
50
51Takes an array with two elements (as returned by `.parse`) and returns a highly optimized function.
52
53**Example:**
54
55```js
56const check = compile([2, 3]);
57
58check(0); // `false`
59check(1); // `false`
60check(2); // `true`
61check(3); // `false`
62check(4); // `true`
63check(5); // `false`
64check(6); // `true`
65```
66
67##### `generate([a, b])`
68
69Returns a function that produces a monotonously increasing sequence of indices.
70
71If the sequence has an end, the returned function will return `null` after the last index in the sequence.
72
73**Example:** An always increasing sequence
74
75```js
76const gen = nthCheck.generate([2, 3]);
77
78gen(); // `1`
79gen(); // `3`
80gen(); // `5`
81gen(); // `8`
82gen(); // `11`
83```
84
85**Example:** With an end value
86
87```js
88const gen = nthCheck.generate([-2, 5]);
89
90gen(); // 0
91gen(); // 2
92gen(); // 4
93gen(); // null
94```
95
96##### `sequence(formula)`
97
98Parses and compiles a formula to a generator that produces a sequence of indices. Combination of `parse` and `generate`.
99
100**Example:** An always increasing sequence
101
102```js
103const gen = nthCheck.sequence("2n+3");
104
105gen(); // `1`
106gen(); // `3`
107gen(); // `5`
108gen(); // `8`
109gen(); // `11`
110```
111
112**Example:** With an end value
113
114```js
115const gen = nthCheck.sequence("-2n+5");
116
117gen(); // 0
118gen(); // 2
119gen(); // 4
120gen(); // null
121```
122
123---
124
125License: BSD-2-Clause
126
127## Security contact information
128
129To report a security vulnerability, please use the [Tidelift security contact](https://tidelift.com/security).
130Tidelift will coordinate the fix and disclosure.
131
132## `nth-check` for enterprise
133
134Available as part of the Tidelift Subscription
135
136The maintainers of `nth-check` and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source dependencies you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact dependencies you use. [Learn more.](https://tidelift.com/subscription/pkg/npm-nth-check?utm_source=npm-nth-check&utm_medium=referral&utm_campaign=enterprise&utm_term=repo)

1	`# nth-check [![Build Status](https://travis-ci.org/fb55/nth-check.svg)](https://travis-ci.org/fb55/nth-check)`
2
3	`Parses and compiles CSS nth-checks to highly optimized functions.`
4
5	`### About`
6
7	This module can be used to parse & compile nth-checks, as they are found in CSS 3's `nth-child()` and `nth-last-of-type()`. It can be used to check if a given index matches a given nth-rule, or to generate a sequence of indices matching a given nth-rule.
8
9	`nth-check` focusses on speed, providing optimized functions for different kinds of nth-child formulas, while still following the [spec](http://www.w3.org/TR/css3-selectors/#nth-child-pseudo).
10
11	`### API`
12
13	```js
14	`import nthCheck, { parse, compile } from "nth-check";`
15	```
16
17	##### `nthCheck(formula)`
18
19	Parses and compiles a formula to a highly optimized function. Combination of `parse` and `compile`.
20
21	If the formula doesn't match any elements, it returns [`boolbase`](https://github.com/fb55/boolbase)'s `falseFunc`. Otherwise, a function accepting an _index_ is returned, which returns whether or not the passed _index_ matches the formula.
22
23	Note: The nth-rule starts counting at `1`, the returned function at `0`.
24
25	`Example:`
26
27	```js
28	`const check = nthCheck("2n+3");`
29
30	check(0); // `false`
31	check(1); // `false`
32	check(2); // `true`
33	check(3); // `false`
34	check(4); // `true`
35	check(5); // `false`
36	check(6); // `true`
37	```
38
39	##### `parse(formula)`
40
41	Parses the expression, throws an `Error` if it fails. Otherwise, returns an array containing the integer step size and the integer offset of the nth rule.
42
43	`Example:`
44
45	```js
46	`parse("2n+3"); // [2, 3]`
47	```
48
49	##### `compile([a, b])`
50
51	Takes an array with two elements (as returned by `.parse`) and returns a highly optimized function.
52
53	`Example:`
54
55	```js
56	`const check = compile([2, 3]);`
57
58	check(0); // `false`
59	check(1); // `false`
60	check(2); // `true`
61	check(3); // `false`
62	check(4); // `true`
63	check(5); // `false`
64	check(6); // `true`
65	```
66
67	##### `generate([a, b])`
68
69	`Returns a function that produces a monotonously increasing sequence of indices.`
70
71	If the sequence has an end, the returned function will return `null` after the last index in the sequence.
72
73	`Example: An always increasing sequence`
74
75	```js
76	`const gen = nthCheck.generate([2, 3]);`
77
78	gen(); // `1`
79	gen(); // `3`
80	gen(); // `5`
81	gen(); // `8`
82	gen(); // `11`
83	```
84
85	`Example: With an end value`
86
87	```js
88	`const gen = nthCheck.generate([-2, 5]);`
89
90	`gen(); // 0`
91	`gen(); // 2`
92	`gen(); // 4`
93	`gen(); // null`
94	```
95
96	##### `sequence(formula)`
97
98	Parses and compiles a formula to a generator that produces a sequence of indices. Combination of `parse` and `generate`.
99
100	`Example: An always increasing sequence`
101
102	```js
103	`const gen = nthCheck.sequence("2n+3");`
104
105	gen(); // `1`
106	gen(); // `3`
107	gen(); // `5`
108	gen(); // `8`
109	gen(); // `11`
110	```
111
112	`Example: With an end value`
113
114	```js
115	`const gen = nthCheck.sequence("-2n+5");`
116
117	`gen(); // 0`
118	`gen(); // 2`
119	`gen(); // 4`
120	`gen(); // null`
121	```
122
123	`---`
124
125	`License: BSD-2-Clause`
126
127	`## Security contact information`
128
129	`To report a security vulnerability, please use the [Tidelift security contact](https://tidelift.com/security).`
130	`Tidelift will coordinate the fix and disclosure.`
131
132	## `nth-check` for enterprise
133
134	`Available as part of the Tidelift Subscription`
135
136	The maintainers of `nth-check` and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source dependencies you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact dependencies you use. [Learn more.](https://tidelift.com/subscription/pkg/npm-nth-check?utm_source=npm-nth-check&utm_medium=referral&utm_campaign=enterprise&utm_term=repo)