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* @license Apache-2.0
*
* Copyright (c) 2018 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
'use strict';
// MODULES //
var logger = require( 'debug' );
var hasOwnProp = require( '@stdlib/assert/has-own-property' );
// VARIABLES //
var debug = logger( 'group-by-async:limit' );
// MAIN //
/**
* Invokes an indicator function once for each element in a collection, limiting the number of concurrently pending functions.
*
* ## Notes
*
* - We need to cache the collection value to prevent the edge case where, during the invocation of the indicator function, the element at index `i` is swapped for some other value. For some, that might be a feature; here, we take the stance that one should be less clever.
* - Checking for an "own" property is necessary to guard against the edge case where an indicator function returns a group identifier which matches a method or property on the `Object` prototype.
*
* @private
* @param {Collection} collection - input collection
* @param {Options} opts - function options
* @param {*} [opts.thisArg] - execution context
* @param {PositiveInteger} [opts.limit] - maximum number of pending function invocations
* @param {string} [options.returns] - output format
* @param {Function} indicator - indicator function
* @param {Callback} done - function to invoke upon completion or upon encountering an error
* @returns {void}
*/
function limit( collection, opts, indicator, done ) {
var maxIndex;
var count;
var flg;
var lim;
var len;
var idx;
var out;
var i;
len = collection.length;
debug( 'Collection length: %d', len );
out = {};
if ( len === 0 ) {
debug( 'Finished processing a collection.' );
return done( null, out );
}
if ( len < opts.limit ) {
lim = len;
} else {
lim = opts.limit;
}
debug( 'Concurrency limit: %d', lim );
debug( 'Number of arguments: %d', indicator.length );
maxIndex = len - 1;
count = 0;
idx = -1;
for ( i = 0; i < lim; i++ ) {
// This guard is necessary to protect against synchronous functions which exhaust all collection elements...
if ( idx < maxIndex ) {
next(); // eslint-disable-line node/callback-return
}
}
/**
* Callback to invoke a provided function for the next element in a collection.
*
* @private
*/
function next() {
var v;
var j;
idx += 1;
j = idx;
v = collection[ j ];
debug( 'Collection element %d: %s.', j, JSON.stringify( v ) );
if ( indicator.length === 2 ) {
indicator.call( opts.thisArg, v, cb );
} else if ( indicator.length === 3 ) {
indicator.call( opts.thisArg, v, j, cb );
} else {
indicator.call( opts.thisArg, v, j, collection, cb );
}
/**
* Callback invoked once a provided function finishes processing a collection element.
*
* @private
* @param {*} [error] - error
* @param {*} [group] - group
* @returns {void}
*/
function cb( error, group ) {
if ( flg ) {
// Prevent further processing of collection elements:
return;
}
if ( error ) {
flg = true;
return clbk( error );
}
debug( 'Collection element %d group: %s.', j, group );
// Default is to return values...
if ( opts.returns === 'indices' ) {
if ( hasOwnProp( out, group ) ) {
out[ group ].push( j );
} else {
out[ group ] = [ j ];
}
} else if ( opts.returns === '*' ) {
if ( hasOwnProp( out, group ) ) {
out[ group ].push( [ j, v ] );
} else {
out[ group ] = [ [ j, v ] ];
}
} else if ( hasOwnProp( out, group ) ) {
out[ group ].push( v );
} else {
out[ group ] = [ v ];
}
clbk();
}
}
/**
* Callback invoked once ready to process the next collection element.
*
* @private
* @param {*} [error] - error
* @returns {void}
*/
function clbk( error ) {
if ( error ) {
debug( 'Encountered an error: %s', error.message );
return done( error );
}
count += 1;
debug( 'Processed %d of %d collection elements.', count, len );
if ( idx < maxIndex ) {
return next();
}
if ( count === len ) {
debug( 'Finished processing a collection.' );
return done( null, out );
}
}
}
// EXPORTS //
module.exports = limit;
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