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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 isCollection = require( '@stdlib/assert/is-collection' );
var isFunction = require( '@stdlib/assert/is-function' );
var hasOwnProp = require( '@stdlib/assert/has-own-property' );
var format = require( '@stdlib/string/format' );
var validate = require( './validate.js' );
// MAIN //
/**
* Groups values according to an indicator function and returns group counts.
*
* @param {Collection} collection - input collection
* @param {Options} [options] - function options
* @param {*} [options.thisArg] - execution context
* @param {Function} indicator - indicator function specifying which group an element in the input collection belongs to
* @throws {TypeError} first argument must be a collection
* @throws {TypeError} options argument must be an object
* @throws {TypeError} last argument must be a function
* @throws {TypeError} must provide valid options
* @returns {Object} counts
*
* @example
* function indicator( v ) {
* return v[ 0 ];
* }
* var arr = [ 'beep', 'boop', 'foo', 'bar' ];
*
* var out = countBy( arr, indicator );
* // returns { 'b': 3, 'f': 1 }
*/
function countBy( collection, options, indicator ) {
var thisArg;
var opts;
var err;
var out;
var len;
var cb;
var g;
var i;
if ( !isCollection( collection ) ) {
throw new TypeError( format( 'invalid argument. First argument must be a collection. Value: `%s`.', collection ) );
}
opts = {};
if ( arguments.length === 2 ) {
cb = options;
} else {
err = validate( opts, options );
if ( err ) {
throw err;
}
cb = indicator;
}
if ( !isFunction( cb ) ) {
throw new TypeError( format( 'invalid argument. Last argument must be a function. Value: `%s`.', cb ) );
}
thisArg = opts.thisArg;
len = collection.length;
out = {};
for ( i = 0; i < len; i++ ) {
g = cb.call( thisArg, collection[ i ], i );
if ( hasOwnProp( out, g ) ) {
out[ g ] += 1;
} else {
out[ g ] = 1;
}
}
return out;
}
// EXPORTS //
module.exports = countBy;
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