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="UTF-8" standalone="no" ?>
<gpx 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>Roman 2015-11-23</name>
<trkseg>
<trkpt lat="53.54398800" lon="-2.7403680000">
<ele>146.100</ele><time>2015-11-23T11:05:07Z</time>
</trkpt>
<trkpt lat="53.54401600" lon="-2.7402620000">
</trkpt>
...
<trkpt lat="53.54384500" lon="-2.7426130000">
</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
);
}

2015-11-23 11:05:07
latitude: 53.5440 longitude: -2.7404 elevation: 146
2015-11-23 11:05:15
2015-11-23 11:05:25
...
2015-11-23 14:58:47
2015-11-23 14:58:57

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.

array_filter()
Filters elements of an array using a callback function.
array array_filter ( array $array [, callable $callback] )
Iterates over each value in the array passing them to the callback function. If the
callback function returns true, the current value from array is returned into the
result array, otherwise it is discarded. Array keys are preserved.

array_filter()
Filters elements of an array using a callback function.
array array_filter ( array $array [, callable $callback] )
array array_filter ( array $array [, callable $callback [, int $flag = 0 ]] )
Iterates over each value in the array passing them to the callback function. If the
callback function returns true, the current value from array is returned into the
result array, otherwise it is discarded. Array keys are preserved.
[PHP < 5.6.0]
[PHP >= 5.6.0]

/**
* 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:
...
case ARRAY_FILTER_USE_BOTH:
...
default:
...
}
}
}

• Time Range
Where was Roman between 11:30 and 12:00?
• Geo-Fencing (“Bounding Box”)
• Inside
Did Osiris go anywhere near the main road?
• Outside
Has Lexie left the house at all?

// 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;
};

foreach (filter($gpxReader->getElements('trkpt'), $timeFilter, ARRAY_FILTER_USE_KEY)
as $time => $element) {
printf(
$element->elevation
);
}

namespace GpxReaderHelpers;
class BoundingBox {
/**
* Identify whether a trackpoint falls inside the defined bounding box
*
* @param GpxReaderGpxElement The trackpoint
* @return boolean If the trackpoint falls outside (false)
* or inside (true) the bounding box
**/
public function inside(GpxReaderGpxElement $point) {
return (($point->position->longitude >= $this->left) &&
($point->position->longitude <= $this->right) &&
($point->position->latitude >= $this->bottom) &&
($point->position->latitude <= $this->top));
}
}

// Create a bounding box defining the coordinates we want to test each point against
// This bounding box is for inside the house/garden
$boundaries = new GpxReaderHelpersBoundingBox();
$boundaries->setLatitudes(53.54382, 53.54340);
$boundaries->setLongitudes(-2.74059, -2.74005);
// We want to set the filter to include only points inside the bounding box
$boundingBoxFilter = [$boundaries, 'inside'];

foreach (filter($gpxReader->getElements('trkpt'), $boundingBoxFilter) as
$time => $element) {
printf(
$element->elevation
);
}

// applying both a time filter (12:00:00-12:20:00 on 2015-11-23)
// and a bounding box filter for inside the house/garden
foreach (filter(
filter(
$gpxReader->getElements('trkpt'), $timeFilter, ARRAY_FILTER_USE_KEY
),
$boundingBoxFilter
) as $time => $element) {
printf(
...
);
}

Cat Herding with PHP Generators
Osiris
Located
Achievement
Unlocked

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.

array_map()
Applies the callback to the elements of the given arrays.
array array_map ( callable $callback , array $array1 [, array $... ] )
array_map() returns an array containing all the elements of array1 after applying
the callback function to each one. The number of parameters that the callback
function accepts should match the number of arrays passed to the array_map().

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

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
);
}

Lexie
Located
Achievement
Unlocked

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.

array_reduce()
Iteratively reduces the array to a single value using a callback function.
mixed array_reduce ( array $array , callable $callback [, mixed $initial = NULL ] )
array_reduce() applies the callback function iteratively to the elements of the array,
so as to reduce the array to a single value.

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

• Bounding Box
What Coordinates should I use for the bounding box when displaying the track
on a map?
• Distance
How far has Roman walked while he’s been out?
What is the furthest distance that Osiris has travelled from home?

// Iterate over our trackpoint set from the gpx file (mapping the distance as we go)
// and reducing 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 4.33 km

class BoundingBox {
function calculate($discard, GpxReaderGpxElement $point) {
$this->top = max($point->position->latitude, $this->top);
$this->bottom = min($point->position->latitude, $this->bottom);
$this->left = min($point->position->longitude, $this->left);
$this->right = max($point->position->longitude, $this->right);
return $this;
}
}

// Set our bounding box callback
$boundaries = new GpxReaderHelpersBoundingBox();
// Reduce our trackpoint set from the gpx file against the bounding box callback
$boundingBox = reduce( $gpxReader->getElements('trkpt'), [$boundaries, 'calculate'] );
// Display the results of our reduce
printf(
'Top: %7.4f Bottom: %7.4f' . PHP_EOL . 'Left: %7.4f Right: %7.4f' . PHP_EOL,
$boundingBox->top,
$boundingBox->bottom,
$boundingBox->left,
$boundingBox->right
);

Top: 53.5445 Bottom: 53.5426
Left: -2.7433 Right: -2.7393

Roman
Located
Achievement
Unlocked

iterator_apply()
Call a function for every element in an iterator.
int iterator_apply ( Traversable $iterator , callable $function [, array $args ] )

iterator_count()
Count the elements in an iterator.
int iterator_count ( Traversable $iterator )

iterator_to_array()
Copy all values from the iterator into an array.
array iterator_to_array ( Traversable $iterator [, bool $use_keys = true ] )

// Create an instance of the fluent Generator Helper and set our filters,
// mapper offset and limits
// and the display callback to "do" (or execute)
// In this case, the "do" is a display callback,
// but it could also be a "reduce" callback
withGenerator($trackPoints)
->filteredBy($boundingBoxFilter)
->filteredBy($timeFilter, ARRAY_FILTER_USE_KEY)
->map([$distanceCalculator, 'setDistance'])
->offset(1)
->limit(1)
->do($display);

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, PHPPresentation (formerly PHPPowerPoint), PHPProject, PHPVisio
Minor contributor to PHP core
@Mark_Baker
https://github.com/MarkBaker
http://uk.linkedin.com/pub/mark-baker/b/572/171
http://markbakeruk.net

A Functional Guide to Cat Herding with PHP Generators

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A Functional Guide to Cat Herding with PHP Generators