Slides from presentation given to the Brighton PHP group on 15th December 2014

1. SPL
The Undiscovered Library
Exploring DataStructures

2. 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

3. SPL – Standard PHP Library
• SPL provides a standard set of interfaces for PHP5
• The aim of SPL is to implement some efficient data access interfaces
and classes for PHP
• Introduced with PHP 5.0.0
• Included as standard with PHP since version 5.3.0
• SPL DataStructures were added for version 5.3.0

4. SPL DataStructures
Dictionary DataStructures (Maps)
• Fixed Arrays
Linear DataStructures
• Doubly-Linked Lists
• Stacks
• Queues
Tree DataStructures
• Heaps

5. SPL DataStructures – Why use them?
• Can improve performance
• When the right structures are used in the right place
• Can reduce memory usage
• When the right structures are used in the right place
• Already implemented and tested in PHP core
• Saves work!
• Can be type-hinted in function/method definitions
• Adds semantics to your code

6. SPL DataStructures
Dictionary DataStructures (Maps)
• Fixed Arrays
Linear DataStructures
Tree DataStructures

7. Fixed Arrays
• Predefined Size
• Enumerated indexes only, not Associative
• Indexed from 0
• Is an object
• No hashing required for keys
• Implements
• Iterator
• ArrayAccess
• Countable

8. Fixed Arrays – Uses
• Returned Database resultsets, Record collections
• Hours of Day
• Days of Month/Year
• Hotel Rooms, Airline seats
As a 2-d fixed array

9. Fixed Arrays – Big-O Complexity
• Insert an element O(1)
• Delete an element O(1)
• Lookup an element O(1)
• Resize a Fixed Array O(n)

10. Fixed Arrays
Standard Arrays SPLFixedArray
Data Record 1
Key 12345
Data Record 2
Key 23456
Data Record 4
Key 34567
Data Record 3
Key 45678
[0]
[1]
[2]
[…]
[…]
[12]
[n-1]
Hash
Function
Key 12345
Key 23456
Key 45678
Key 34567
Data Record 1
Key 0
Data Record 2
Key 1
Data Record 3
Key 2
Data Record 4
Key 3
[0]
[1]
[2]
[…]
[…]
[12]
[n-1]
Key 0
Key 1
Key 2
Key 3

11. Fixed Arrays
$a = array();
for ($i = 0; $i < $size; ++$i) {
$a[$i] = $i;
}
// Random/Indexed access
for ($i = 0; $i < $size; ++$i) {
$r = $a[$i];
}
// Sequential access
foreach($a as $v) {
}
// Sequential access with keys
foreach($a as $k => $v) {
}
Initialise: 0.0000 s
Set 1,000,000 Entries: 0.4671 s
Read 1,000,000 Entries: 0.3326 s
Iterate values for 1,000,000 Entries: 0.0436 s
Iterate keys and values for 1,000,000 Entries: 0.0839 s
Total Time: 0.9272 s
Memory: 82,352.55 k

12. Fixed Arrays
$a = new SPLFixedArray($size);
for ($i = 0; $i < $size; ++$i) {
$a[$i] = $i;
}
// Random/Indexed access
for ($i = 0; $i < $size; ++$i) {
$r = $a[$i];
}
// Sequential access
foreach($a as $v) {
}
// Sequential access with keys
foreach($a as $k => $v) {
}
Initialise: 0.0013 s
Set 1,000,000 Entries: 0.3919 s
Read 1,000,000 Entries: 0.3277 s
Iterate values for 1,000,000 Entries: 0.1129 s
Iterate keys and values for 1,000,000 Entries: 0.1531 s
Total Time: 0.9869 s
Memory: 35,288.41 k

13. 0.0000
0.0100
0.0200
0.0300
0.0400
0.0500
0.0600
0.0700
Initialise (s) Set Values (s) Sequential Read (s) Random Read (s) Pop (s)
Speed
SPL Fixed Array Standard PHP Array
Fixed Arrays
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
Current Memory (k) Peak Memory (k)
Memory Usage
SPL Fixed Array Standard PHP Array

14. Fixed Arrays
• Faster direct access
• Lower memory usage
• Faster for random/indexed access than for sequential access

15. Fixed Arrays – Gotchas
• Can be extended, but at a cost in speed
• Standard array functions won’t work with SPLFixedArray
e.g. array_walk(), sort(), array_pop(), implode()
• Avoid unsetting elements if possible
• Unlike standard PHP enumerated arrays, this leaves empty nodes that trigger
an Exception if accessed

16. SPL DataStructures
Dictionary DataStructures (Maps)
Linear DataStructures
• Doubly-Linked Lists
• Stacks
• Queues
Tree DataStructures

17. Doubly Linked Lists

18. Doubly Linked Lists
• Iterable Lists
• Top to Bottom
• Bottom to Top
• Unindexed
• Good for sequential access
• Not good for random/indexed access
• Implements
• Iterator
• ArrayAccess
• Countable

19. Doubly Linked Lists – Uses
• Stacks
• Queues
• Most-recently used lists
• Undo functionality
• Trees
• Memory Allocators
• Fast dynamic, iterable arrays (not PHP’s hashed arrays)
• iNode maps
• Video frame queues

20. Doubly Linked Lists – Big-O Complexity
• Insert an element by index O(1)
• Delete an element by index O(1)
• Lookup by index O(n)
• I have seen people saying that SPLDoublyLinkedList behaves like a hash table
for lookups, which would make it O(1); but timing tests prove otherwise
• Access a node at the beginning of the list O(1)
• Access a node at the end of the list O(1)

21. Doubly Linked Lists
Head Tail
A B C D E

22. Doubly Linked Lists
$a = array();
for ($i = 0; $i < $size; ++$i) {
$a[$i] = $i;
}
// Random/Indexed access
for ($i = 0; $i < $size; ++$i) {
$r = $a[$i];
}
// Sequential access
for ($i = 0; $i < $size; ++$i) {
$r = array_pop($a);
}
Initialise: 0.0000 s
Set 100,000 Entries: 0.0585 s
Read 100,000 Entries: 0.0378 s
Pop 100,000 Entries: 0.1383 s
Total Time: 0.2346 s
Memory: 644.55 k
Peak Memory: 8457.91 k

23. Doubly Linked Lists
$a = new SplDoublyLinkedList();
for ($i = 0; $i < $size; ++$i) {
$a->push($i);
}
// Random/Indexed access
for ($i = 0; $i < $size; ++$i) {
$r = $a->offsetGet($i);
}
// Sequential access
for ($i = $size-1; $i >= 0; --$i) {
$a->pop();
}
Initialise: 0.0000 s
Set 100,000 Entries: 0.1514 s
Read 100,000 Entries: 22.7068 s
Pop 100,000 Entries: 0.1465 s
Total Time: 23.0047 s
Memory: 133.67 k
Peak Memory: 5603.09 k

24. Doubly Linked Lists
• Fast for sequential access
• Lower memory usage
• Traversable in both directions
• Size limited only by memory
• Slow for random/indexed access
• Insert into middle of list only available from PHP 5.5.0

25. SPL DataStructures
Dictionary DataStructures (Maps)
Linear DataStructures
• Doubly-Linked Lists
• Stacks
• Queues
Tree DataStructures

26. Stacks

27. Stacks
• Implemented as a Doubly-Linked List
• LIFO
• Last-In
• First-Out
• Essential Operations
• push()
• pop()
• Optional Operations
• count()
• isEmpty()
• peek()

28. Stack – Uses
• Undo mechanism (e.g. In text editors)
• Backtracking (e.g. Finding a route through a maze)
• Call Handler (e.g. Defining return location for nested calls)
• Shunting Yard Algorithm (e.g. Converting Infix to Postfix notation)
• Evaluating a Postfix Expression
• Depth-First Search

29. Stacks – Big-O Complexity
• Push an element O(1)
• Pop an element O(1)

30. Stacks
class StandardArrayStack {
private $_stack = array();
public function count() {
return count($this->_stack);
}
public function push($data) {
$this->_stack[] = $data;
}
public function pop() {
if (count($this->_stack) > 0) {
return array_pop($this->_stack);
}
return NULL;
}
function isEmpty() {
return count($this->_stack) == 0;
}
}

31. Stacks
$a = new StandardArrayStack();
for ($i = 1; $i <= $size; ++$i) {
$a->push($i);
}
while (!$a->isEmpty()) {
$i = $a->pop();
}
PUSH 100,000 ENTRIES
Push Time: 0.5818 s
Current Memory: 8.75
POP 100,000 ENTRIES
Pop Time: 1.6657 s
Current Memory: 2.25
Total Time: 2.2488 s
Current Memory: 2.25
Peak Memory: 8.75

32. Stacks
class StandardArrayStack {
private $_stack = array();
private $_count = 0;
public function count() {
return $this->_count;
}
public function push($data) {
++$this->_count;
$this->_stack[] = $data;
}
public function pop() {
if ($this->_count > 0) {
--$this->_count;
return array_pop($this->_stack);
}
return NULL;
}
function isEmpty() {
return $this->_count == 0;
}
}

33. Stacks
$a = new StandardArrayStack();
for ($i = 1; $i <= $size; ++$i) {
$a->push($i);
}
while (!$a->isEmpty()) {
$i = $a->pop();
}
PUSH 100,000 ENTRIES
Push Time: 0.5699 s
Current Memory: 8.75
POP 100,000 ENTRIES
Pop Time: 1.1005 s
Current Memory: 1.75
Total Time: 1.6713 s
Current Memory: 1.75
Peak Memory: 8.75

34. Stacks
$a = new SPLStack();
for ($i = 1; $i <= $size; ++$i) {
$a->push($i);
}
while (!$a->isEmpty()) {
$i = $a->pop();
}
PUSH 100,000 ENTRIES
Push Time: 0.4301 s
Current Memory: 5.50
POP 100,000 ENTRIES
Pop Time: 0.6413 s
Current Memory: 0.75
Total Time: 1.0723 s
Current Memory: 0.75
Peak Memory: 5.50

35. Stacks
0.0796 0.0782
0.0644
0.1244
0.0998
0.0693
8.75 8.75
5.50
0
1
2
3
4
5
6
7
8
9
10
0.0000
0.0200
0.0400
0.0600
0.0800
0.1000
0.1200
0.1400
StandardArrayStack StandardArrayStack2 SPLStack
Memory(MB)
Time(seconds)
Stack Timings
Push Time (s)
Pop Time (s)
Memory after Push (MB)

36. Stacks – Gotchas
• Peek (view an entry from the middle of the stack)
• StandardArrayStack
public function peek($n = 0) {
if ((count($this->_stack) - $n) < 0) {
return NULL;
}
return $this->_stack[count($this->_stack) - $n - 1];
}
• StandardArrayStack2
public function peek($n = 0) {
if (($this->_count - $n) < 0) {
return NULL;
}
return $this->_stack[$this->_count - $n - 1];
}
• SPLStack
$r = $a->offsetGet($n);

37. Stacks – Gotchas
0.0075 0.0077 0.0064
0.0111
0.0078
0.1627
0.0124
0.0098
0.0066
1.00 1.00
0.75
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0.0000
0.0200
0.0400
0.0600
0.0800
0.1000
0.1200
0.1400
0.1600
0.1800
StandardArrayStack StandardArrayStack2 SPLStack
Memory(MB)
Time(seconds)
Stack Timings
Push Time (s)
Peek Time (s)
Pop Time (s)
Memory after Push (MB)

38. Stacks – Gotchas
• Peek
When looking through the stack, SPLStack has to follow each link in the “chain”
until it finds the nth entry

39. SPL DataStructures
Dictionary DataStructures (Maps)
Linear DataStructures
• Doubly-Linked Lists
• Stacks
• Queues
Tree DataStructures

40. Queues

41. Queues
• Implemented as a Doubly-Linked List
• FIFO
• First-In
• First-Out
• Essential Operations
• enqueue()
• dequeue()
• Optional Operations
• count()
• isEmpty()
• peek()

42. Queues – Uses
• Job/print/message submissions
• Breadth-First Search
• Request handling (e.g. a Web server)

43. Queues – Big-O Complexity
• Enqueue an element O(1)
• Dequeue an element O(1)

44. Queues
class StandardArrayQueue {
private $_queue = array();
private $_count = 0;
public function count() {
return $this->_count;
}
public function enqueue($data) {
++$this->_count;
$this->_queue[] = $data;
}
public function dequeue() {
if ($this->_count > 0) {
--$this->_count;
return array_shift($this->_queue);
}
return NULL;
}
function isEmpty() {
return $this->_count == 0;
}
}

45. Queues
$a = new StandardArrayQueue();
for ($i = 1; $i <= $size; ++$i) {
$a->enqueue($i);
}
while (!$a->isEmpty()) {
$i = $a->dequeue();
}
ENQUEUE 100,000 ENTRIES
Enqueue Time: 0.6884
Current Memory: 8.75
DEQUEUE 100,000 ENTRIES
Dequeue Time: 335.8434
Current Memory: 1.75
Total Time: 336.5330
Current Memory: 1.75
Peak Memory: 8.75

46. Queues
$a = new SPLQueue();
for ($i = 1; $i <= $size; ++$i) {
$a->enqueue($i);
}
while (!$a->isEmpty()) {
$i = $a->dequeue();
}
ENQUEUE 100,000 ENTRIES
Enqueue Time: 0.4087
Current Memory: 5.50
DEQUEUE 100,000 ENTRIES
Dequeue Time: 0.6148
Current Memory: 0.75
Total Time: 1.0249
Current Memory: 0.75
Peak Memory: 5.50

47. Queues
0.0075 0.0080 0.00640.0087 0.0070
0.1582
0.6284 0.6277
0.0066
1.00 1.00
0.75
0.00
0.20
0.40
0.60
0.80
1.00
1.20
0.0000
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
StandardArrayQueue StandardArrayQueue2 SPLQueue
Memory(MB)
Time(seconds)
Queue Timings
Enqueue Time (s)
Peek Time (s)
Dequeue Time (s)
Memory after Enqueue (MB)

48. Queues – Gotchas
• Dequeue
In standard PHP enumerated arrays, shift() and unshift() are expensive
operations because they re-index the entire array
This problem does not apply to SPLQueue
• Peek
When looking through the queue, SPLQueue has to follow each link in the
“chain” until it finds the nth entry

49. SPL DataStructures
Dictionary DataStructures (Maps)
Linear DataStructures
Tree DataStructures
• Heaps

50. Heaps
 

51. Heaps
• Ordered Lists
• Random Input
• Ordered Output
• Implemented as a binary tree structure
• Essential Operations
• Insert
• Extract
• Ordering Rule
• Abstract that requires extending with the implementation of a compare()
algorithm
• compare() is reversed in comparison with usort compare callbacks
• Partially sorted on data entry

52. Heaps – Uses
• Heap sort
• Selection algorithms (e.g. Max, Min, Median)
• Graph algorithms
• Prim’s Minimal Spanning Tree (connected weighted undirected graph)
• Dijkstra’s Shortest Path (network or traffic routing)
• Priority Queues

53. Heaps – Big-O Complexity
• Insert an element O(log n)
• Delete an element O(log n)
• Access root element O(1)

54. Heaps

55. Heaps
class ExtendedSPLHeap extends SPLHeap {
protected function compare($a, $b) {
if ($a->latitude == $b->latitude) {
return 0;
}
return ($a->latitude < $b->latitude) ? -1 : 1;
}
}
$citiesHeap = new ExtendedSPLHeap();
$file = new SplFileObject("cities.csv");
$file->setFlags(
SplFileObject::DROP_NEW_LINE |
SplFileObject::SKIP_EMPTY
);
while (!$file->eof()) {
$cityData = $file->fgetcsv();
if ($cityData !== NULL) {
$city = new StdClass;
$city->name = $cityData[0];
$city->latitude = $cityData[1];
$city->longitude = $cityData[2];
$citiesHeap->insert($city);
}
}

56. Heaps
echo 'There are ', $citiesHeap->count(),
' cities in the heap', PHP_EOL;
echo 'FROM NORTH TO SOUTH', PHP_EOL;
foreach($citiesHeap as $city) {
echo sprintf(
"%-20s %+3.4f %+3.4f" . PHP_EOL,
$city->name,
$city->latitude,
$city->longitude
);
}
echo 'There are ', $citiesHeap->count(),
' cities in the heap', PHP_EOL;

57. Heaps
echo 'There are ', $citiesHeap->count(),
' cities in the heap', PHP_EOL;
echo 'FROM NORTH TO SOUTH', PHP_EOL;
foreach($citiesHeap as $city) {
echo sprintf(
"%-20s %+3.4f %+3.4f" . PHP_EOL,
$city->name,
$city->latitude,
$city->longitude
);
}
echo 'There are ', $citiesHeap->count(),
' cities in the heap', PHP_EOL;
There are 69 cities in the heap
FROM NORTH TO SOUTH
Inverness +57.4717 -4.2254
Aberdeen +57.1500 -2.1000
Dundee +56.4500 -2.9833
Perth +56.3954 -3.4353
Stirling +56.1172 -3.9397
Edinburgh +55.9500 -3.2200
Glasgow +55.8700 -4.2700
Derry +54.9966 -7.3086
Newcastle upon Tyne +54.9833 -1.5833
Carlisle +54.8962 -2.9316
Sunderland +54.8717 -1.4581
Durham +54.7771 -1.5607
Belfast +54.6000 -5.9167
Lisburn +54.5097 -6.0374
Armagh +54.2940 -6.6659
Newry +54.1781 -6.3357
Ripon +54.1381 -1.5223

58. Heaps
class ExtendedSPLHeap extends SPLHeap {
const NORTH_TO_SOUTH = 'north_to_south';
const SOUTH_TO_NORTH = 'south_to_north';
const EAST_TO_WEST = 'east_to_west';
const WEST_TO_EAST = 'west_to_east';
protected $_sortSequence = self::NORTH_TO_SOUTH;
protected function compare($a, $b) {
switch($this->_sortSequence) {
case self::NORTH_TO_SOUTH :
if ($a->latitude == $b->latitude)
return 0;
return ($a->latitude < $b->latitude) ? -1 : 1;
case self::SOUTH_TO_NORTH :
if ($a->latitude == $b->latitude)
return 0;
return ($b->latitude < $a->latitude) ? -1 : 1;
case self::EAST_TO_WEST :
if ($a->longitude == $b->longitude)
return 0;
return ($a->longitude < $b->longitude) ? -1 : 1;
case self::WEST_TO_EAST :
if ($a->longitude == $b->longitude)
return 0;
return ($b->longitude < $a->longitude) ? -1 : 1;
}
}
public function setSortSequence(
$sequence = self::NORTH_TO_SOUTH
) {
$this->_sortSequence = $sequence;
}
}
$sortSequence = ExtendedSPLHeap::WEST_TO_EAST;
$citiesHeap = new ExtendedSPLHeap();
$citiesHeap->setSortSequence($sortSequence);
$file = new SplFileObject("cities.csv");
$file->setFlags(
SplFileObject::DROP_NEW_LINE |
SplFileObject::SKIP_EMPTY
);
while (!$file->eof()) {
$cityData = $file->fgetcsv();
if ($cityData !== NULL) {
$city = new StdClass;
$city->name = $cityData[0];
$city->latitude = $cityData[1];
$city->longitude = $cityData[2];
$citiesHeap->insert($city);
}
}

59. Heaps
class ExtendedSPLHeap extends SPLHeap {
protected $_longitude = 0;
protected $_latitude = 0;
protected function compare($a, $b) {
if ($a->distance == $b->distance)
return 0;
return ($a->distance > $b->distance) ? -1 : 1;
}
public function setLongitude($longitude) {
$this->_longitude = $longitude;
}
public function setLatitude($latitude) {
$this->_latitude = $latitude;
}
…..
public function insert($value) {
$value->distance =
$this->_calculateDistance($value);
parent::insert($value);
}
}
$citiesHeap = new ExtendedSPLHeap();
// Latitude and Longitude for Brighton
$citiesHeap->setLatitude(50.8300);
$citiesHeap->setLongitude(-0.1556);
$file = new SplFileObject("cities.csv");
$file->setFlags(
SplFileObject::DROP_NEW_LINE |
SplFileObject::SKIP_EMPTY
);
while (!$file->eof()) {
$cityData = $file->fgetcsv();
if ($cityData !== NULL) {
$city = new StdClass;
$city->name = $cityData[0];
$city->latitude = $cityData[1];
$city->longitude = $cityData[2];
$citiesHeap->insert($city);
}
}

60. Heaps – Gotchas
• Compare method is reversed logic from a usort() callback
• Traversing the heap removes elements from the heap

61. SPL – Standard PHP Library
E-Book
Mastering the SPL Library
Joshua Thijssen
Available in PDF, ePub, Mobi
http://www.phparch.com/books/mastering-the-spl-library/

62. SPL DataStructures
?
Questions