Slides from the talk on SPL Datastructures that I gave at PHPBarcelona 2015

1. Exploring DataStructures

2. 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

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

4. 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

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

6. Fixed Arrays – SplFixedArray
• Predefined Size
• Enumerated indexes only, not Associative
• Indexed from 0, Increments by 1
• Is an object
• No hashing required for keys
• Implements
• Iterator
• ArrayAccess
• Countable

7. 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 of fixed array

8. 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)

9. 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

10. 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: 0.1323 s
Random Read 1,000,000 Entries: 0.3311 s
Iterate values for 1,000,000 Entries: 0.0146 s
Iterate keys and values for 1,000,000 Entries: 0.0198 s
Total Time: 0.4979 s
Memory: 41,668.32 k

11. 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: 0.0011 s
Set 1,000,000 Entries: 0.1061 s
Random Read 1,000,000 Entries: 0.3144 s
Iterate values for 1,000,000 Entries: 0.0394 s
Iterate keys and values for 1,000,000 Entries: 0.0476 s
Total Time: 0.5086 s
Memory: 17,889.43 k

12. Fixed Arrays
0.0000
0.0500
0.1000
0.1500
0.2000
0.2500
0.3000
0.3500
Initialise (s) Set Values (s) Sequential Read (s) Sequential Read
with Key (s)
Random Read (s) Pop (s)
Speed
SPL Fixed Array Standard PHP Array
0.00
5,000.00
10,000.00
15,000.00
20,000.00
25,000.00
30,000.00
35,000.00
40,000.00
45,000.00
Current Memory (k) Peak Memory (k)
Memory Usage
SPL Fixed Array Standard PHP Array

13. Fixed Arrays
• Faster to populate
• Lower memory usage
• Faster for random/indexed access than standard PHP arrays
• Slower for sequential access than standard PHP arrays

14. 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(), array_intersect(), etc
• Avoid unsetting elements if possible
• Unlike standard PHP enumerated arrays, this leaves empty nodes that trigger
an Exception if accessed

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

16. Doubly Linked Lists – SplDoublyLinkedList

17. 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

18. Doubly Linked Lists – Uses
• Stacks
• Queues
• Most-recently used lists
• Undo functionality
• Trees
• Memory Allocators
• Fast dynamic, iterable arrays
• iNode maps
• Video frame queues

19. 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)

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

21. 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 = $size-1; $i >= 0; --$i) {
$r = array_pop($a);
}
Initialise: 0.0000 s
Set 100,000 Entries: 0.0585 s
Random 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

22. 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
Random 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

23. Doubly Linked Lists
• Fast for sequential access
• Lower memory usage
• Traversable in both directions
Use setIteratorMode() to determine direction
• Size limited only by memory
• Slow for random/indexed access
• Insert into middle of list only available from PHP 5.5.0

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

25. Stacks – SplStack

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

27. Stack – Uses
• Undo mechanism (e.g. In text editors)
• Backtracking (e.g. Finding a route through a maze or network)
• 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

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

29. 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;
}
}

30. 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

31. Stacks
class StandardArrayStack2 {
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;
}
}

32. Stacks
$a = new StandardArrayStack2();
for ($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

33. 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

34. 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)

35. 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);

36. 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)

37. Stacks – Gotchas
• Peek
When looking through the stack, SplStack has to follow each link in the “chain”
until it finds the nth entry
O(n)

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

39. Queues – SplQueue

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

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

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

43. Queues
class StandardArrayQueue2 {
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;
}
}

44. Queues
$a = new StandardArrayQueue2();
for ($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

45. 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

46. 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)

47. 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

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

49. Heaps – SplHeap
 

50. 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

51. Heaps
• Implements
• Iterator
• Countable

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
class DistanceSplHeap extends SplHeap {
protected $longitude = 0.0;
protected $latitude = 0.0;
private $comparator;
public function __construct(Callable $comparator,
$longitude, $latitude) {
$this->longitude = $longitude;
$this->latitude = $latitude;
$this->comparator = $comparator;
}
protected function compare($a, $b) {
return call_user_func(
$this->comparator, $a, $b
);
}
public function insert($value) {
$value->distance =
$this->calculateDistance($value);
parent::insert($value);
}
}
$comparator = function($a, $b) {
if ($a->distance == $b->distance)
return 0;
return ($a->distance > $b->distance)
? -1
: 1;
};
// Latitude and Longitude for Barcelona
$speakersHeap = new DistanceSplHeap(
$comparator,
2.1833,
41.3833
);

55. Heaps
$file = new SplFileObject("speakers.csv");
$file->setFlags(
SplFileObject::DROP_NEW_LINE |
SplFileObject::SKIP_EMPTY
);
while (!$file->eof()) {
$speakerData = $file->fgetcsv();
$speaker = new StdClass;
$speaker->name = $speakerData[0];
$speaker->from = $speakerData[1];
$speaker->latitude = $speakerData[2];
$speaker->longitude = $speakerData[3];
$speakersHeap->insert($speaker);
}
"Anthony Ferrara","New Jersey, USA",40.0000,-74.5000
"Bastian Hofmann","Berlin, Germany",52.5167,13.3833
"Damien Seguy","The Hague, Netherlands",52.0833,4.3167
"Derick Rethans","London, UK",51.5072,-0.1275
"Juozas Kaziukėnas","New York, USA",40.7127,-74.0059
"Marcello Duarte","London, UK",51.5072,-0.1275
"Mark Baker","Wigan, UK",53.5448,-2.6318
"Mathias Verraes","Belgium",50.8500,4.3500
"Matthias Noback","Utrecht, Netherlands",52.0833,5.1167
"Nikita Popov","Berlin, Germany",52.5167,13.3833
"Paweł Jędrzejewski","Łódź, Poland",51.7833,19.4667
"Steve Maraspin","Udine, Italy",46.0667,13.2333
"Tudor Barbu","Barcelona, Catalonia",41.3833,2.1833
"Zeev Suraski","Israel",31.0000,35.0000

56. Heaps
echo 'There are ', $speakersHeap->count(),
' speakers at PHPBarcelona 2015',
PHP_EOL, PHP_EOL;
echo 'Distance that they have travelled to
reach Barcelona', PHP_EOL, PHP_EOL;
foreach($speakersHeap as $speaker) {
echo sprintf(
"%-22s from %-
24s has travelled %6.1f miles".PHP_EOL,
$speaker->name,
$speaker->from,
$speaker->distance
);
}
echo PHP_EOL;
There are 14 speakers at PHPBarcelona 2015
Distance that they have travelled to reach Barcelona
Tudor Barbu from Barcelona, Catalonia has travelled 0.0 miles
Steve Maraspin from Udine, Italy has travelled 638.8 miles
Mathias Verraes from Belgium has travelled 662.2 miles
Derick Rethans from London, UK has travelled 708.0 miles
Marcello Duarte from London, UK has travelled 708.0 miles
Damien Seguy from The Hague, Netherlands has travelled 746.0 miles
Matthias Noback from Utrecht, Netherlands has travelled 752.0 miles
Mark Baker from Wigan, UK has travelled 869.3 miles
Nikita Popov from Berlin, Germany has travelled 930.8 miles
Bastian Hofmann from Berlin, Germany has travelled 930.8 miles
Paweł Jędrzejewski from Łódź, Poland has travelled 1085.9 miles
Zeev Suraski from Israel has travelled 1951.0 miles
Juozas Kaziukėnas from New York, USA has travelled 3831.8 miles
Anthony Ferrara from New Jersey, USA has travelled 3877.9 miles

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

58. SPL – Standard PHP Library
Other SPL Datastructures
• SplMaxHeap
• SplMinHeap
• SplPriorityQueue
• SplObjectStorage

59. 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/

60. SPL – Standard PHP Library
E-Book
Iterating PHP Iterators
Cal Evans
Available in PDF, ePub, Mobi
https://leanpub.com/iteratingphpiterators

61. SPL DataStructures
?
Questions

62. 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

63. SPL: The Undiscovered Library
– Exploring Datastructures
http://joind.in/talk/view/15874