A Functional Guide to Cat Herding with PHP Generators

A Functional Guide to
Cat Herding with PHP
Generators
The Filter/Map/Reduce Pattern for PHP
Generators

A Functional Guide to Cat Herding with PHP Generators
• Blog Post
http://markbakeruk.net/2016/01/19/a-functional-guide-to-cat-herding-with-
php-generators/
• Code Examples
https://github.com/MarkBaker/GeneratorFunctionExamples

<?xml version="1.0" encoding="ISO-8859-1"?>
<gpx version="1.1"
creator="Memory-Map 5.4.2.1089 http://www.memory-map.com"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns="http://www.topografix.com/GPX/1/1"
xsi:schemaLocation="http://www.topografix.com/GPX/1/1 http://www.topografix.com/GPX/1/1/gpx.xsd">
<trk>
<name>Wythburn</name>
<type>Track</type>
<trkseg>
<trkpt lat="54.5131924947" lon="-3.0448236664"><time>2015-03-02T07:59:35Z</time></trkpt>
<trkpt lat="54.5131921768" lon="-3.0450893323"><time>2015-03-02T08:00:31Z</time></trkpt>
<trkpt lat="54.5131534894" lon="-3.0448548317"><ele>192</ele><time>2015-03-
02T08:00:51Z</time></trkpt>
...
<trkpt lat="54.4399968465" lon="-2.9721705119"><ele>52</ele><time>2015-03-
02T14:50:49Z</time></trkpt>
</trkseg>
</trk>
</gpx>

namespace GpxReader;
class GpxHandler {
protected $gpxReader;
public function __construct($gpxFilename) {
$this->gpxReader = new XMLReader();
$this->gpxReader->open($gpxFilename);
}
public function getElements($elementType) {
while ($this->gpxReader->read()) {
if ($this->gpxReader->nodeType == XMLREADER::ELEMENT &&
$this->gpxReader->name == $elementType) {
$doc = new DOMDocument('1.0', 'UTF-8');
$xml = simplexml_import_dom($doc->importNode($this->gpxReader->expand(), true));
$gpxAttributes = $this->readAttributes($this->gpxReader);
$gpxElement = $this->readChildren($xml);
$gpxElement->position = $gpxAttributes;
yield $gpxElement->timestamp => $gpxElement;
}
}
}
}

// Create our initial Generator to read the gpx file
$gpxReader = new GpxReaderGpxHandler($gpxFilename);
// Iterate over the trackpoint set from the gpx file,
// displaying each point detail in turn
foreach ($gpxReader->getElements('trkpt') as $time => $element) {
printf(
'%s' . PHP_EOL .
' latitude: %7.4f longitude: %7.4f elevation: %d' . PHP_EOL,
$time->format('Y-m-d H:i:s'),
$element->position->latitude,
$element->position->longitude,
$element->elevation
);
}

2015-03-02 07:59:35
latitude: 54.5132, longitude: -3.0448, elevation: 0
2015-03-02 08:00:31
2015-03-02 08:00:51
...
2015-03-02 14:50:39
2015-03-02 14:50:49

Cat Herding with PHP Generators – Filter
• A filter selects only a subset of values from the Traversable.
• The rules for filtering are defined in a callback function.
• If no callback is provided, then only non-empty values are returned.

/**
* The `$flag` option (and the constants ARRAY_FILTER_USE_KEY and ARRAY_FILTER_USE_BOTH)
* were introduced in PHP 5.6.0
*
**/
function filter(Traversable $filter, Callable $callback = null, $flag = 0) {
if ($callback === null) {
$callback = 'notEmpty';
}
foreach ($filter as $key => $value) {
switch($flag) {
case ARRAY_FILTER_USE_KEY:
if ($callback($key)) {
}
break;
case ARRAY_FILTER_USE_BOTH:
if ($callback($value, $key)) {
}
break;
default:
if ($callback($value)) {
}
break;
}
}
}

// Define the date/time filter parameters
$startTime = new DateTime('2015-03-02 13:20:00Z');
$endTime = new DateTime('2015-03-02 13:30:00Z');
// Create the filter callback with the date/time parameters we've just defined
$timeFilter = function($timestamp) use ($startTime, $endTime) {
return $timestamp >= $startTime && $timestamp <= $endTime;
};

// Iterate over the trackpoint set from the gpx file,
foreach (filter($gpxReader->getElements('trkpt'), $timeFilter, ARRAY_FILTER_USE_KEY)
as $time => $element) {
printf(
'%s' . PHP_EOL . ' latitude: %7.4f longitude: %7.4f elevation: %d' . PHP_EOL,
$element->elevation
);
}

2015-03-02 13:20:21
latitude: 54.4692 longitude: -2.9677 elevation: 634
2015-03-02 13:21:13
2015-03-02 13:21:58
...
2015-03-02 13:29:47
2015-03-02 13:29:58

Cat Herding with PHP Generators – Map
• A map is like a foreach loop that transforms each value in the
Traversable.
• Each input value is transformed into a new output value.
• The rules for the transformation are defined in a callback function.

function map(Callable $callback, Traversable $iterator) {
foreach ($iterator as $key => $value) {
yield $key => $callback($value);
}
}

namespace GpxReaderHelpers;
class DistanceCalculator {
public function setDistance(GpxReaderGpxElement $point) {
$point->distance = $this->calculateDistance($point);
return $point;
}
}

// Set the mapper to calculate the distance between a trackpoint
// and the previous trackpoint
$distanceCalculator = new GpxReaderHelpersDistanceCalculator();

// Iterate over the trackpoint set from the gpx file, mapping the distances as we go,
foreach (map([$distanceCalculator, 'setDistance'], $gpxReader->getElements('trkpt'))
as $time => $element) {
printf(
'%s' . PHP_EOL . ' latitude: %7.4f longitude: %7.4f elevation: %d' . PHP_EOL .
' distance from previous point: %5.2f m' . PHP_EOL,
$element->elevation,
$element->distance
);
}

2015-03-02 07:59:35
distance from previous point: 0.00 m
2015-03-02 08:00:31
2015-03-02 08:00:51
...
2015-03-02 14:50:39
2015-03-02 14:50:49

Cat Herding with PHP Generators – Reduce
• A reduce aggregates all the values in the Traversable to a single value.
• A callback function determines the process for the aggregation.

function reduce(Traversable $iterator, Callable $callback, $initial = null) {
$result = $initial;
foreach($iterator as $value) {
$result = $callback($result, $value);
}
return $result;
}

// Reduce our trackpoint set from the gpx file (mapping the distance as we go)
// and summing the results to calculate the total distance travelled
$totalDistance = reduce(
map([$distanceCalculator, 'setDistance'], $gpxReader->getElements('trkpt')),
function($runningTotal, $value) {
$runningTotal += $value->distance;
return $runningTotal;
},
0.0
);
// Display the results of our reduce
printf(
'Total distance travelled is %5.2f km' . PHP_EOL,
$totalDistance / 1000
);

Total distance travelled is 19.27 km

https://github.com/lstrojny/functional-php

No cats were forced to walk anywhere that they didn't want to go
during the writing of this presentation.

?
Questions

Who am I?
Mark Baker
Design and Development Manager
InnovEd (Innovative Solutions for Education) Ltd
Coordinator and Developer of:
Open Source PHPOffice library
PHPExcel, PHPWord,PHPPowerPoint, PHPProject, PHPVisio
Minor contributor to PHP core
@Mark_Baker
https://github.com/MarkBaker
http://uk.linkedin.com/pub/mark-baker/b/572/171

A Functional Guide to Cat Herding with PHP Generators

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