feat: import nested categories

[curbengh]

Fixes #36 cc @jashsayani @adnan360

Example categories:

1. lorem => ipsum
2. dolor

lorem and dolor are parent (top-level) categories, ipsum is child of lorem.

After import, the front-matter should have:

categories:
  - - lorem
    - ipsum
  - - dolor

Equivalent to:

categories:
  - [lorem, ipsum]
  - [dolor]

---

Next PR can add an option to skip Uncategorized category.

[coveralls]

Coverage increased (+0.5%) to 96.273% when pulling b17721c on curbengh:post-category into b5b2565 on hexojs:master.

[curbengh]

Currently it's only limited to two-level categories. b17721c

Input:

1. lorem => ipsum => dolor (dolor is child of ipsum, ipsum is child of lorem)
2. foo => bar
3. baz

I have limited skill. If anyone wants to help, the objective is to have following output for the above input.

Output: [ ['lorem', 'ipsum', 'dolor'], ['foo', 'bar'], ['baz'] ]

WP describes the relationship like this:

[
  { name: 'lorem', parent: '' },
  { name: 'ipsum', parent: 'lorem' },
  { name: 'dolor', parent: 'ipsum' },
  { name: 'foo', parent: '' },
  { name: 'bar', parent: 'foo' },
  { name: 'baz', parent: '' }
]

Then converted to object for easier access,

{
  lorem: '',
  ipsum: 'lorem',
  dolor: 'ipsum',
  foo: '',
  bar: 'foo',
  baz: '',
}

The goal is to formats the object into that output. Each category can only have one parent (one-to-many).

[adnan360]

@curbengh Here is a code. It should support infinite nesting of categories:

var cats = [
	{ name: 'lorem', parent: '' },
	{ name: 'ipsum', parent: 'lorem' },
	{ name: 'dolor', parent: 'ipsum' },
	{ name: 'foo', parent: '' },
	{ name: 'bar', parent: 'foo' },
	{ name: 'baz', parent: '' }
];

console.log(cats);
console.log('---------------------------');

// To enable deep level flatten. Uses recursion with reduce and concat.
// @source https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat#Alternatives
function flatDeep(arr, d = 1) {
	return d > 0 ? arr.reduce((acc, val) => acc.concat(Array.isArray(val) ? flatDeep(val, d - 1) : val), [])
				: arr.slice();
};

// Nests array based on their parent data.
// @source https://stackoverflow.com/a/55241491
const nest = (items, name = '', link = 'parent') =>
	items
		.filter(item => item[link] === name)
		.map(item => ( flatDeep( [ item.name, nest(items, item.name) ], Infinity ) ));
console.log( nest(cats) );

Outputs:

[ [ 'lorem', 'ipsum', 'dolor' ], [ 'foo', 'bar' ], [ 'baz' ] ]

[curbengh]

@adnan360 Fantastic.

Since this plugin requires Node 12+, Array.flat() can be utilised

const nest = (items, name = '', link = 'parent') => {
  return items
    .filter(item => item[link] === name)
    .map(item => [item.name, nest(items, item.name)].flat(Infinity));
}

[curbengh]

@jashsayani @adnan360

This PR is ready for test, it now supports non-nested categories and >2 levels categories.

[adnan360]

Working fine on my end