nth-check 

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

$3

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.

$3

``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'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

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