# nth-check [![Build Status](https://travis-ci.org/fb55/nth-check.svg)](https://travis-ci.org/fb55/nth-check)

Parses and compiles CSS nth-checks to highly optimized functions.

### About

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.

`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).

### API

```js
import nthCheck, { parse, compile } from "nth-check";
```

##### `nthCheck(formula)`

Parses and compiles a formula to a highly optimized function. Combination of `parse` and `compile`.

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.

**Note**: The nth-rule starts counting at `1`, the returned function at `0`.

**Example:**

```js
const check = nthCheck("2n+3");

check(0); // `false`
check(1); // `false`
check(2); // `true`
check(3); // `false`
check(4); // `true`
check(5); // `false`
check(6); // `true`
```

##### `parse(formula)`

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.

**Example:**

```js
parse("2n+3"); // [2, 3]
```

##### `compile([a, b])`

Takes an array with two elements (as returned by `.parse`) and returns a highly optimized function.

**Example:**

```js
const check = compile([2, 3]);

check(0); // `false`
check(1); // `false`
check(2); // `true`
check(3); // `false`
check(4); // `true`
check(5); // `false`
check(6); // `true`
```

##### `generate([a, b])`

Returns a function that produces a monotonously increasing sequence of indices.

If the sequence has an end, the returned function will return `null` after the last index in the sequence.

**Example:** An always increasing sequence

```js
const gen = nthCheck.generate([2, 3]);

gen(); // `1`
gen(); // `3`
gen(); // `5`
gen(); // `8`
gen(); // `11`
```

**Example:** With an end value

```js
const gen = nthCheck.generate([-2, 5]);

gen(); // 0
gen(); // 2
gen(); // 4
gen(); // null
```

##### `sequence(formula)`

Parses and compiles a formula to a generator that produces a sequence of indices. Combination of `parse` and `generate`.

**Example:** An always increasing sequence

```js
const gen = nthCheck.sequence("2n+3");

gen(); // `1`
gen(); // `3`
gen(); // `5`
gen(); // `8`
gen(); // `11`
```

**Example:** With an end value

```js
const gen = nthCheck.sequence("-2n+5");

gen(); // 0
gen(); // 2
gen(); // 4
gen(); // null
```

---

License: BSD-2-Clause

## Security contact information

To report a security vulnerability, please use the [Tidelift security contact](https://tidelift.com/security).
Tidelift will coordinate the fix and disclosure.