Usage

`javascript
var tabulateByAsync = require( '@stdlib/utils-async-tabulate-by' );
`

#### tabulateByAsync( collection, \[options,] indicator, done )

Generates a frequency table according to an indicator function (i.e., a function which specifies how to categorize an element in the input collection).

`javascript
function indicator( value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
next( null, (value > 2000) );
}
}

function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
}

var arr = [ 3000, 2500, 1000, 750 ];

tabulateByAsync( arr, indicator, done );
/* =>
750
1000
2500
3000
[ [ false, 2, 0.5 ], [ true, 2, 0.5 ] ]
*/
`

The returned frequency table is an array of arrays. Each sub-array corresponds to a unique value in the input collection and is structured as follows:

- 0: unique value
- 1: value count
- 2: frequency percentage

The function accepts the following options:

- limit: the maximum number of pending invocations at any one time. Default: infinity.
- series: boolean indicating whether to sequentially invoke the indicator function for each collection element. If true, the function sets options.limit=1. Default: false.
- thisArg: the execution context for indicator.

By default, all elements are processed concurrently, which means that the function does not guarantee completion order. To process each collection element sequentially, set the series option to true.

`javascript
function indicator( value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
next( null, (value > 2000) );
}
}

function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
}

var arr = [ 3000, 2500, 1000, 750 ];

var opts = {
'series': true
};

tabulateByAsync( arr, opts, indicator, done );
/* =>
3000
2500
1000
750
[ [ true, 2, 0.5 ], [ false, 2, 0.5 ] ]
*/
`

To limit the maximum number of pending function invocations, set the limit option.

`javascript
function indicator( value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
next( null, (value > 2000) );
}
}

function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
}

var arr = [ 3000, 2500, 1000, 750 ];

var opts = {
'limit': 2
};

tabulateByAsync( arr, opts, indicator, done );
/* =>
2500
3000
1000
750
[ [ true, 2, 0.5 ], [ false, 2, 0.5 ] ]
*/
`

To set the execution context of the indicator function, set the thisArg option.

`javascript
function indicator( value, next ) {
this.count += 1;
setTimeout( onTimeout, value );
function onTimeout() {
next( null, (value > 2000) );
}
}

var arr = [ 3000, 2500, 1000, 750 ];

var context = {
'count': 0
};

var opts = {
'thisArg': context
};

tabulateByAsync( arr, opts, indicator, done );

function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
// => [ [ false, 2, 0.5 ], [ true, 2, 0.5 ] ]

console.log( context.count );
// => 4
}
`

When invoked, the indicator function is provided a maximum of four arguments:

- value: collection value.
- index: collection index.
- collection: the input collection.
- next: a callback which should be called once the indicator function has finished processing a collection value.

The actual number of provided arguments depends on function length. If the indicator function accepts two arguments, the indicator function is provided value and next. If the indicator function accepts three arguments, the indicator function is provided value, index, and next. For every other indicator function signature, the indicator function is provided all four arguments.

`javascript
function indicator( value, i, collection, next ) {
console.log( 'collection: %s. %d: %d', collection.join( ',' ), i, value );
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
next( null, (value > 2000) );
}
}

function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
}

var arr = [ 3000, 2500, 1000, 750 ];

tabulateByAsync( arr, indicator, done );
/* =>
collection: 3000,2500,1000,750. 0: 3000
collection: 3000,2500,1000,750. 1: 2500
collection: 3000,2500,1000,750. 2: 1000
collection: 3000,2500,1000,750. 3: 750
750
1000
2500
3000
[ [ false, 2, 0.5 ], [ true, 2, 0.5 ] ]
*/
`

#### tabulateByAsync.factory( \[options,] indicator )

Returns a function which invokes an indicator function once for each element in a collection and generates a frequency table.

`javascript
function indicator( value, next ) {
setTimeout( onTimeout, value );
function onTimeout() {
console.log( value );
next( null, (value > 2000) );
}
}

function done( error, result ) {
if ( error ) {
throw error;
}
console.log( result );
}

var f = tabulateByAsync.factory( indicator );

var arr1 = [ 3000, 2500, 1000, 750 ];

f( arr1, done );
/* =>
750
1000
2500
3000
[ [ false, 2, 0.5 ], [ true, 2, 0.5 ] ]
*/

var arr2 = [ 300, 250, 100 ];

f( arr2, done );
/* =>
100
250
300
[ [ false, 3, 1.0 ] ]
*/
`

The function accepts the same options as tabulateByAsync().