bifurcateByAsync

Split values into two groups according to a predicate function.

Usage

var bifurcateByAsync = require( '@stdlib/utils/async/bifurcate-by' );

bifurcateByAsync( collection, [options,] predicate, done )

Splits values into two groups according to a predicate function, which specifies which group an element in the input collection belongs to. If a predicate function calls a provided callback with a truthy group value, a collection element belongs to the first group; otherwise, a collection element belongs to the second group.

function predicate( 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 ];

bifurcateByAsync( arr, predicate, done );
/* =>
    1000
    2500
    3000
    [ [ 2500, 3000 ], [ 1000 ] ]
*/

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 predicate function for each collection element. If true, the function sets options.limit=1. Default: false.
returns: specifies the output format. If the option equals 'values', the function returns group results as element values. If the option equals 'indices', the function returns group results as element indices. If the option equals '*', the function returns group results as both element indices and values. Default: 'values'.
thisArg: the execution context for predicate.

By default, the function returns group results as element values. To return element indices, set the returns option to 'indices'.

function predicate( 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 ];

var opts = {
    'returns': 'indices'
};
bifurcateByAsync( arr, opts, predicate, done );
/* =>
    1000
    2500
    3000
    [ [ 1, 0 ], [ 2 ] ]
*/

To return index-value pairs, set the returns option to '*'.

function predicate( 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 ];

var opts = {
    'returns': '*'
};
bifurcateByAsync( arr, opts, predicate, done );
/* =>
    1000
    2500
    3000
    [ [ [ 1, 2500 ], [ 0, 3000 ] ], [ [ 2, 1000 ] ] ]
*/

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.

function predicate( 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 ];

var opts = {
    'series': true
};

bifurcateByAsync( arr, opts, predicate, done );
/* =>
    3000
    2500
    1000
    [ [ 3000, 2500 ], [ 1000 ] ]
*/

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

function predicate( 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 ];

var opts = {
    'limit': 2
};

bifurcateByAsync( arr, opts, predicate, done );
/* =>
    2500
    3000
    1000
    [ [ 2500, 3000 ], [ 1000 ] ]
*/

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

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

var arr = [ 3000, 2500, 1000 ];

var context = {
    'count': 0
};

var opts = {
    'thisArg': context
};

bifurcateByAsync( arr, opts, predicate, done );

function done( error, result ) {
    if ( error ) {
        throw error;
    }
    console.log( result );
    // => [ [ 2500, 3000 ], [ 1000 ] ]

    console.log( context.count );
    // => 3
}

When invoked, the predicate 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 predicate function has finished processing a collection value.

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

function predicate( 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 ];

bifurcateByAsync( arr, predicate, done );
/* =>
    collection: 3000,2500,1000. 0: 3000
    collection: 3000,2500,1000. 1: 2500
    collection: 3000,2500,1000. 2: 1000
    1000
    2500
    3000
    [ [ 2500, 3000 ], [ 1000 ] ]
*/

bifurcateByAsync.factory( [options,] predicate )

Returns a function which invokes a predicate function once for each element in a collection.

function predicate( 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 = bifurcateByAsync.factory( predicate );

var arr1 = [ 3000, 2500, 1000 ];

f( arr1, done );
/* =>
    1000
    2500
    3000
    [ [ 2500, 3000 ], [ 1000 ] ]
*/

var arr2 = [ 300, 250, 100 ];

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

The function accepts the same options as bifurcateByAsync().

Notes

A collection may be either an Array, Typed Array, or an array-like Object (excluding strings and functions).
If a provided function calls the next callback with a truthy error argument, the function suspends execution and immediately calls the done callback for subsequent error handling.
The function does not support dynamic collection resizing.
The function does not skip undefined elements.
If provided an empty collection, the function calls the done callback with an empty array for the group results.
Neither bifurcateByAsync nor the function returned by the factory method guarantee asynchronous execution. To guarantee asynchrony, wrap the done callback in a function which either executes at the end of the current stack (e.g., nextTick) or during a subsequent turn of the event loop (e.g., setImmediate, setTimeout).

Examples

var resolve = require( 'path' ).resolve;
var readFile = require( '@stdlib/fs/read-file' );
var bifurcateByAsync = require( '@stdlib/utils/async/bifurcate-by' );

var files = [
    resolve( __dirname, 'package.json' ),
    resolve( __dirname, 'README.md' ),
    resolve( __dirname, 'beep.boop.md' )
];

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

function predicate( file, next ) {
    var opts = {
        'encoding': 'utf8'
    };
    readFile( file, opts, onFile );

    function onFile( error ) {
        if ( error ) {
            return next( null, false );
        }
        next( null, true );
    }
}

bifurcateByAsync( files, predicate, done );