A real world application for Cat Tracking, demonstrating the filter/map/reduce pattern with PHP Generators

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

2. 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

3. A Functional Guide to Cat Herding with PHP Generators

4. A Functional Guide to Cat Herding with PHP Generators

5. A Functional Guide to Cat Herding with PHP Generators
<?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>

6. A Functional Guide to Cat Herding with PHP Generators
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;
}
}
}
}

7. A Functional Guide to Cat Herding with PHP Generators
// 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
);
}

8. A Functional Guide to Cat Herding with PHP Generators
2015-03-02 07:59:35
latitude: 54.5132, longitude: -3.0448, elevation: 0
2015-03-02 08:00:31
latitude: 54.5132, longitude: -3.0451, elevation: 0
2015-03-02 08:00:51
latitude: 54.5132, longitude: -3.0449, elevation: 192
...
2015-03-02 14:50:39
latitude: 54.4392, longitude: -2.9714, elevation: 52
2015-03-02 14:50:49
latitude: 54.4400, longitude: -2.9722, elevation: 52

9. A Functional Guide to Cat Herding with PHP Generators

10. 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.

11. Cat Herding with PHP Generators – Filter
function notEmpty($value) {
return !empty($value);
}
/**
* Version of filter to use with versions of PHP prior to 5.6.0,
* without the `$flag` option
*
**/
function filter(Traversable $filter, Callable $callback = null) {
if ($callback === null) {
$callback = 'notEmpty';
}
foreach ($filter as $key => $value) {
if ($callback($value)) {
yield $key => $value;
}
}
}

12. Cat Herding with PHP Generators – Filter
/**
* 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)) {
yield $key => $value;
}
break;
case ARRAY_FILTER_USE_BOTH:
if ($callback($value, $key)) {
yield $key => $value;
}
break;
default:
if ($callback($value)) {
yield $key => $value;
}
break;
}
}
}

13. Cat Herding with PHP Generators – Filter
// Create our initial Generator to read the gpx file
$gpxReader = new GpxReaderGpxHandler($gpxFilename);
// 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;
};

14. Cat Herding with PHP Generators – Filter
// Iterate over the trackpoint set from the gpx file,
// displaying each point detail in turn
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,
$time->format('Y-m-d H:i:s'),
$element->position->latitude,
$element->position->longitude,
$element->elevation
);
}

15. A Functional Guide to Cat Herding with PHP Generators
2015-03-02 13:20:21
latitude: 54.4692 longitude: -2.9677 elevation: 634
2015-03-02 13:21:13
latitude: 54.4691 longitude: -2.9677 elevation: 628
2015-03-02 13:21:58
latitude: 54.4690 longitude: -2.9676 elevation: 621
...
2015-03-02 13:29:47
latitude: 54.4658 longitude: -2.9673 elevation: 533
2015-03-02 13:29:58
latitude: 54.4657 longitude: -2.9674 elevation: 531

16. 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.

17. Cat Herding with PHP Generators – Map
function map(Callable $callback, Traversable $iterator) {
foreach ($iterator as $key => $value) {
yield $key => $callback($value);
}
}

18. Cat Herding with PHP Generators – Map
namespace GpxReaderHelpers;
class DistanceCalculator {
public function setDistance(GpxReaderGpxElement $point) {
$point->distance = $this->calculateDistance($point);
return $point;
}
}

19. Cat Herding with PHP Generators – Map
// Create our initial Generator to read the gpx file
$gpxReader = new GpxReaderGpxHandler($gpxFilename);
// Set the mapper to calculate the distance between a trackpoint
// and the previous trackpoint
$distanceCalculator = new GpxReaderHelpersDistanceCalculator();

20. Cat Herding with PHP Generators – Map
// Iterate over the trackpoint set from the gpx file, mapping the distances as we go,
// displaying each point detail in turn
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,
$time->format('Y-m-d H:i:s'),
$element->position->latitude,
$element->position->longitude,
$element->elevation,
$element->distance
);
}

21. Cat Herding with PHP Generators – Map
2015-03-02 07:59:35
latitude: 54.5132 longitude: -3.0448 elevation: 0
distance from previous point: 0.00 m
2015-03-02 08:00:31
latitude: 54.5132 longitude: -3.0451 elevation: 0
distance from previous point: 17.15 m
2015-03-02 08:00:51
latitude: 54.5132 longitude: -3.0449 elevation: 192
distance from previous point: 15.74 m
...
2015-03-02 14:50:39
latitude: 54.4392 longitude: -2.9714 elevation: 52
distance from previous point: 98.87 m
2015-03-02 14:50:49
latitude: 54.4400 longitude: -2.9722 elevation: 52
distance from previous point: 106.70 m

22. 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.

23. Cat Herding with PHP Generators – Reduce
function reduce(Traversable $iterator, Callable $callback, $initial = null) {
$result = $initial;
foreach($iterator as $value) {
$result = $callback($result, $value);
}
return $result;
}

24. Cat Herding with PHP Generators – Reduce
// 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
);

25. Cat Herding with PHP Generators – Map
Total distance travelled is 19.27 km

26. A Functional Guide to Cat Herding with PHP Generators

27. A Functional Guide to Cat Herding with PHP Generators

28. A Functional Guide to Cat Herding with PHP Generators
https://github.com/lstrojny/functional-php

29. A Functional Guide to Cat Herding with PHP Generators
No cats were forced to walk anywhere that they didn't want to go
during the writing of this presentation.

30. A Functional Guide to Cat Herding with PHP Generators
?
Questions

31. 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