@stdlib/iter/strided/lib/main.js

Standard iterator utilities.

/**
* @license Apache-2.0
*
* Copyright (c) 2019 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 setReadOnly = require( '@stdlib/utils/define-nonenumerable-read-only-property' );
var isNonNegativeInteger = require( '@stdlib/assert/is-nonnegative-integer' ).isPrimitive;
var isPositiveInteger = require( '@stdlib/assert/is-positive-integer' );
var isBoolean = require( '@stdlib/assert/is-boolean' ).isPrimitive;
var isFunction = require( '@stdlib/assert/is-function' );
var isIteratorLike = require( '@stdlib/assert/is-iterator-like' );
var iteratorSymbol = require( '@stdlib/symbol/iterator' );
var format = require( '@stdlib/string/format' );


// VARIABLES //

var DEFAULT_OFFSET = 0;
var DEFAULT_EAGER = false;


// MAIN //

/**
* Returns an iterator which steps by a specified amount.
*
* @param {Iterator} iterator - input iterator
* @param {PositiveInteger} stride - stride
* @param {NonNegativeInteger} [offset=0] - offset
* @param {boolean} [eager=false] - boolean indicating whether to eagerly advance the input iterator when provided a non-zero offset
* @throws {TypeError} first argument must be an iterator protocol-compliant object
* @throws {TypeError} second argument must be a positive integer
* @throws {TypeError} third argument must be a nonnegative integer
* @throws {TypeError} fourth argument must be a boolean
* @returns {Iterator} iterator
*
* @example
* var array2iterator = require( '@stdlib/array/to-iterator' );
*
* var arr = array2iterator( [ 0, 1, 2, 3, 4, 5, 6, 7 ] );
*
* var iter = iterStrided( arr, 2, 1 );
*
* var r = iter.next().value;
* // returns 1
*
* r = iter.next().value;
* // returns 3
*
* r = iter.next().value;
* // returns 5
*
* // ...
*/
function iterStrided( iterator, stride, offset, eager ) {
	var iter;
	var bool;
	var FLG;
	var idx;
	var ret;
	var o;
	var v;
	var i;
	if ( !isIteratorLike( iterator ) ) {
		throw new TypeError( format( 'invalid argument. First argument must be an iterator protocol-compliant object. Value: `%s`.', iterator ) );
	}
	if ( !isPositiveInteger( stride ) ) {
		throw new TypeError( format( 'invalid argument. Second argument must be a positive integer. Value: `%s`.', stride ) );
	}
	if ( arguments.length === 3 ) {
		if ( !isNonNegativeInteger( offset ) ) {
			throw new TypeError( format( 'invalid argument. Third argument must be a nonnegative integer. Value: `%s`.', offset ) );
		}
		idx = offset;
		bool = DEFAULT_EAGER;
	} else if ( arguments.length > 3 ) {
		if ( !isNonNegativeInteger( offset ) ) {
			throw new TypeError( format( 'invalid argument. Third argument must be a nonnegative integer. Value: `%s`.', offset ) );
		}
		idx = offset;
		if ( !isBoolean( eager ) ) {
			throw new TypeError( format( 'invalid argument. Fourth argument must be a boolean. Value: `%s`.', eager ) );
		}
		bool = eager;
	} else {
		idx = DEFAULT_OFFSET;
		bool = DEFAULT_EAGER;
	}
	if ( bool ) {
		for ( i = 0; i < idx; i++ ) {
			v = iterator.next();
			if ( v.done ) {
				FLG = true;
				ret = v;
				break;
			}
		}
		// NOTE: if we've successfully advanced the input iterator, `i` is currently equal to `idx` (i.e., the offset)
		i -= 1;
	} else {
		i = -1;
	}
	// Create an iterator protocol-compliant object:
	iter = {};
	setReadOnly( iter, 'next', next );
	setReadOnly( iter, 'return', end );

	// If an environment supports `Symbol.iterator` and the provided iterator is iterable, make the iterator iterable:
	if ( iteratorSymbol && isFunction( iterator[ iteratorSymbol ] ) ) {
		o = idx;
		setReadOnly( iter, iteratorSymbol, factory );
	}
	return iter;

	/**
	* Returns an iterator protocol-compliant object containing the next iterated value.
	*
	* @private
	* @returns {Object} iterator protocol-compliant object
	*/
	function next() {
		var v;
		if ( FLG ) {
			if ( ret ) {
				v = ret;
				ret = null;
				return v;
			}
			return {
				'done': true
			};
		}
		i += 1;
		while ( i < idx ) {
			v = iterator.next();
			if ( v.done ) {
				FLG = true;
				return v;
			}
			i += 1;
		}
		idx += stride;
		v = iterator.next();
		if ( v.done ) {
			FLG = true;
		}
		return v;
	}

	/**
	* Finishes an iterator.
	*
	* @private
	* @param {*} [value] - value to return
	* @returns {Object} iterator protocol-compliant object
	*/
	function end( value ) {
		FLG = true;
		if ( arguments.length ) {
			return {
				'value': value,
				'done': true
			};
		}
		return {
			'done': true
		};
	}

	/**
	* Returns a new iterator.
	*
	* @private
	* @returns {Iterator} iterator
	*/
	function factory() {
		return iterStrided( iterator[ iteratorSymbol ](), stride, o, bool );
	}
}


// EXPORTS //

module.exports = iterStrided;