PHP data structures (and the impact of php 7 on them), phpDay Verona 2015, Italy

PHP Data Structures
(and the impact of PHP 7 on them)
Patrick Allaert
phpDay Verona 2015, Italy

About me
● Patrick Allaert
● Founder of Libereco and co-founder of catchy.io
● Playing with PHP/Linux for +15 years
● eZ Publish core developer
● Author of the APM PHP extension
● @patrick_allaert
● patrickallaert@php.net
● http://github.com/patrickallaert/
● http://patrickallaert.blogspot.com/

PHP native datatypes
● NULL
● Booleans
● Integers
● Floating point numbers
● Strings
● Arrays
● Objects
● Resources

Datatypes on Wikipedia
● 2-3-4 tree
● 2-3 heap
● 2-3 tree
● AA tree
● Abstract syntax
tree
● (a,b)-tree
● Adaptive k-d tree
● Adjacency list
● Adjacency matrix
● AF-heap
● Alternating
decision tree
● And-inverter
graph
● And–or tree
● Array
● AVL tree
● Beap
● Bidirectional map
● Bin
● Binary decision
diagram
● Binary heap
● Binary search tree
● Binary tree
● Binomial heap
● Bit array
● Bitboard
● Bit field
● Bitmap
● BK-tree
● Bloom filter
● Boolean
● Bounding
interval
hierarchy
● B sharp tree
● BSP tree
● B-tree
● B*-tree
● B+ tree
● B-trie
● Bx-tree
● Cartesian tree
● Char
● Circular buffer
● Compressed
suffix array
● Container
● Control table
● Cover tree
● Ctrie
● Dancing tree
● D-ary heap
● Decision tree
● Deque
● Directed acyclic
graph
● Directed graph
● Disjoint-set
● Distributed hash
table
● Double
● Doubly connected
edge list
● Doubly linked list
● Dynamic array
● Enfilade
● Enumerated type
● Expectiminimax
tree
● Exponential tree
● Fenwick tree
● Fibonacci heap
● Finger tree
● Float
● FM-index
● Fusion tree
● Gap buffer
● Generalised suffix
tree
● Graph
● Graph-structured
stack
● Hash
● Hash array
mapped trie
● Hashed array
tree
● Hash list
● Hash table
● Hash tree
● Hash trie
● Heap
● Heightmap
● Hilbert R-tree
● Hypergraph
● Iliffe vector
● Image
● Implicit kd-tree
● Interval tree
● Int
● Judy array
● Kdb tree
● Kd-tree
● Koorde
● Leftist heap
● Lightmap
● Linear octree
● Link/cut tree
● Linked list
● Lookup table
● Map/Associative
array/Dictionary
● Matrix
● Metric tree
● Minimax tree
● Min/max kd-tree
● M-tree
● Multigraph
● Multimap
● Multiset
● Octree
● Pagoda
● Pairing heap
● Parallel array
● Parse tree
● Plain old data
structure
● Prefix hash tree
● Priority queue
● Propositional
directed acyclic
graph
● Quad-edge
● Quadtree
● Queap
● Queue
● Radix tree
● Randomized
binary search tree
● Range tree
● Rapidly-exploring
random tree
● Record (also called
tuple or struct)
● Red-black tree
● Rope
● Routing table
● R-tree
● R* tree
● R+ tree
● Scapegoat tree
● Scene graph
● Segment tree
● Self-balancing
binary search tree
● Self-organizing list
● Set
● Skew heap
● Skip list
● Soft heap
● Sorted array
● Spaghetti stack
● Sparse array
● Sparse matrix
● Splay tree
● SPQR-tree
● Stack
● String
● Suffix array
● Suffix tree
● Symbol table
● Syntax tree
● Tagged union
(variant record,
discriminated union,
disjoint union)
● Tango tree
● Ternary heap
● Ternary search tree
● Threaded binary tree
● Top tree
● Treap
● Tree
● Trees
● Trie
● T-tree
● UB-tree
● Union
● Unrolled linked list
● Van Emde Boas tree
● Variable-length array
● VList
● VP-tree
● Weight-balanced tree
● Winged edge
● X-fast trie
● Xor linked list
● X-tree
● Y-fast trie
● Zero suppressed
decision diagram
● Zipper
● Z-order

Game:
Can you recognize some structures?

PHP data structures (and the impact of php 7 on them), phpDay Verona 2015, Italy

Array: PHP's untruthfulness
PHP“Arrays”are not true Arrays!

PHP“Arrays”are not true Arrays!
An array typically looks like this:
Data DataDataData Data Data
0 1 2 3 4 5

PHP“Arrays”can dynamically grow and be iterated
both directions (reset(), next(), prev(), end()),
exclusively with O(1) operations.

PHP“Arrays”can dynamically grow and be iterated
both directions (reset(), next(), prev(), end()),
exclusively with O(1) operations.
Let's have a Doubly Linked List (DLL):
Data Data Data Data Data
Head Tail
Enables Queue, Stack and Deque implementations

PHP“Arrays”elements are always accessible using a
key (index).

PHP“Arrays”elements are always accessible using a
key (index).
Let's have an Hash Table:
Data Data Data Data Data
Head Tail
Bucket Bucket Bucket Bucket Bucket
Bucket pointers array
Bucket *
0
Bucket *
1
Bucket *
2
Bucket *
3
Bucket *
4
Bucket *
5 ...
Bucket *
nTableSize -1

http://php.net/manual/en/language.types.array.php:
“This type is optimized for several
different uses; it can be treated as an
array, list (vector), hash table (an
implementation of a map),
dictionary, collection, stack, queue,
and probably more.”

Optimized for anything ≈ Optimized for nothing!

● In C: 100 000 integers (using long on 64bits => 8
bytes) can be stored in 0.76 MiB.
● In PHP 5:
● it will take 13.97 MiB!≅
● A variable (containing an integer) takes 48 bytes.
● The overhead for every“array”entries is about 96 bytes.

● In C: 100 000 integers (using long on 64bits => 8
bytes) can be stored in 0.76 MiB.
● In PHP 5 7:
● it will take ≅ 13.97 4 MiB!
● A variable (containing an integer) takes 48 16 bytes.
● The overhead for every“array”entries is about 96 20
bytes.

Structs (or records, tuples,...)

Structs (or records, tuples,...)
● A struct is a value containing other values which
are typically accessed using a name.
● Example:
Person => firstName / lastName
ComplexNumber => realPart / imaginaryPart

Structs – Using array
$person = [
"firstName" => "Patrick",
"lastName" => "Allaert",
];

Structs – Using a class
$person = new PersonStruct(
"Patrick", "Allaert"
);

(Implementation)
class PersonStruct
{
public $firstName;
public $lastName;
public function __construct($firstName, $lastName)
{
$this->firstName = $firstName;
$this->lastName = $lastName;
}
}

(Implementation)
class PersonStruct
{
public $firstName;
public $lastName;
public function __construct($firstName, $lastName)
{
$this->firstName = $firstName;
$this->lastName = $lastName;
}
public function __set($key, $value)
{
// a. Do nothing
// b. trigger_error()
// c. Throws an exception
}
}

Structs – Pros and Cons
Creating 107
“person”structs
array object
0
1000
2000
3000
4000
5000
6000 5621
41274098
1403
PHP 5.6
PHP 7
Memory(MiB)
array object
0
0,5
1
1,5
2
2,5
1,52
2,26
0,5
0,9
PHP 5.6
PHP 7
Time(s)

Structs – Pros and Cons
Using a class implementation
+ Type hinting possible
+ Rigid structure
+ More OO
+ Uses ~ 26% less memory
- Slower to create by ~ 50%
Starting PHP 7:
+ Uses ~ 66% less memory
- Slower to create by a factor 2!

(true) Arrays
● An array is a fixed size collection where elements
are each identified by a numeric index.

(true) Arrays
● An array is a fixed size collection where elements
are each identified by a numeric index.
0 1 2 3 4 5

(true) Arrays – Using SplFixedArray
$array = new SplFixedArray(3);
$array[0] = 1; // or $array->offsetSet()
$array[0]; // gives 1

(true) Arrays – Pros and Cons
Creating/iterating 104
arrays of 1000 elements
array SplFixedArray
0
200
400
600
800
1000
1200
1400
1600
1378
539
353
159
PHP 5.6
PHP 7
Memory(MiB)
array (create) array (iterate) SplFixedArray (create) SplFixedArray (iterate)
0
0,5
1
1,5
2
2,5
3
2,49
0,2
0,92
0,360,33
0,09
0,24 0,19
PHP 5.6
PHP 7
Time(s)

(true) Arrays – Pros and Cons
Using SplFixedArray
+ Uses much less memory
+ Takes less time at creation
- Takes a bit more time to iterate

Queues
● A queue is an ordered collection respecting First
In, First Out (FIFO) order.
● Elements are inserted at one end and removed at
the other.

Queues
● A queue is an ordered collection respecting First
In, First Out (FIFO) order.
● Elements are inserted at one end and removed at
the other.
Data
Data
Enqueue
Dequeue

Queues – Using array
$queue = [];
$queue[] = 1; // or array_push()
array_shift($queue); // gives 1

Queues – Using SplQueue
$queue = new SplQueue();
$queue[] = 1; // or $queue->enqueue()
$queue->dequeue(); // gives 1

Stacks
● A stack is an ordered collection respecting Last In,
First Out (LIFO) order.
● Elements are inserted and removed on the same
end.

Stacks
● A stack is an ordered collection respecting Last In,
First Out (LIFO) order.
● Elements are inserted and removed on the same
end.
Data
Data
Push
Pop

Stacks – Using array
$stack = [];
$stack[] = 1; // or array_push()
array_pop($stack); // gives 3

Stacks – Using SplStack
$stack = new SplStack();
$stack[] = 1; // or $stack->push()
$stack->pop(); // gives 3

Stack/Queue – Pros and Cons
Creating 104
stacks/queues of 103
elements
array Spl(Queue|Stack)
0
200
400
600
800
1000
1200
1400
1600
1378
920
353
541
PHP 5.6
PHP 7
Memory(MiB)
array (create) array (iterate) Spl(Stack|Queue) (create) Spl(Stack|Queue) (iterate)
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
0,57
0,2
1,62
0,27
0,17
0,09
1,22
0,18
PHP 5.6
PHP 7
Time(s)

Queues/Stacks – Pros and Cons
SplQueue / SplStack
+ Uses less memory
+ Type hinting
+ More OO
- A bit more cpu intensive
Starting PHP 7 (comparatively to arrays):
- Uses more memory
- Much more cpu intensive
=> They haven't received as much attention as arrays did (yet?).

Sets
People with
strong views on
the distinction
between geeks
and nerds
Geeks Nerds

Sets
● A set is a collection with no particular ordering
especially suited for testing the membership of a
value against a collection or to perform
union/intersection/complement operations
between them.

Sets
● A set is a collection with no particular ordering
especially suited for testing the membership of a
value against a collection or to perform
union/intersection/complement operations
between them.
Data
Data
Data
Data
Data

Sets – Using array
$set = [];
// Adding elements to a set
$set[] = 1;
$set[] = 2;
$set[] = 3;
// Checking presence in a set
in_array(2, $set); // true
in_array(5, $set); // false
array_merge($set1, $set2); // union
array_intersect($set1, $set2); // intersection
array_diff($set1, $set2); // complement

Sets – Using array
$set = [];
$set[] = 1;
$set[] = 2;
$set[] = 3;
in_array(2, $set); // true
in_array(5, $set); // false
array_merge($set1, $set2); // union
array_intersect($set1, $set2); // intersection
array_diff($set1, $set2); // complement
True
performance
killers!

Sets – Mis-usage
if (in_array($value, ["val1", "val2", "val3"]))
{
// ...
}

Sets – Mis-usage
if ($value === "val1" || $value === "val2" || $value ===
"val3")))
{
// ...
}

Sets – Mis-usage
switch ($value)
{
case "val1":
case "val2":
case "val3":
// ...
}

Sets – Mis-usage
Testing 5 * 107
membership against set of 3 elements
in_array compare switch optimized way ;)
0
5
10
15
20
25
19,59
3,15
5,2
1,97
3,43
2,34
1,53
0,75
PHP 5.6
PHP 7
Time(s)

Sets – Using array (simple types)
$set = [];
$set[1] = true; // Any dummy value
$set[2] = true; // is good but NULL!
$set[3] = true;
isset($set[2]); // true
isset($set[5]); // false
$set1 + $set2; // union
array_intersect_key($set1, $set2); // intersection
array_diff_key($set1, $set2); // complement

Sets – Using array (simple types)
$set = [];
$set[1] = true; // Any dummy value
$set[2] = true; // is good but NULL!
$set[3] = true;
isset($set[2]); // true
isset($set[5]); // false
● Remember that PHP Array keys can be integers or
strings only!

Sets – Using array (objects)
$set = [];
$set[spl_object_hash($object1)] = $object1;
isset($set[spl_object_hash($object2)]); // true
isset($set[spl_object_hash($object5)]); // false

Sets – Using array (objects)
$set = [];
isset($set[spl_object_hash($object2)]); // true
isset($set[spl_object_hash($object5)]); // false
Store a
reference of
the object!

Sets – Using SplObjectStorage
(objects)
$set = new SplObjectStorage();
$set->attach($object1); // or $set[$object1] = null;
isset($set[$object2]); // true
isset($set[$object5]); // false
$set1->$addAll($set2); // union
$set1->removeAllExcept($set2); // intersection
$set1->removeAll($set2); // complement

Sets – Using QuickHash (int)
● No union/intersection/complement operations
(yet?)
● Yummy features like (loadFrom|saveTo)(String|File)
$set = new QuickHashIntSet(64,QuickHashIntSet::CHECK_FOR_DUPES);
$set->add(1);
$set->add(2);
$set->add(3);
$set->exists(2); // true
$set->exists(5); // false
isset($set[2]);

Sets – Using bitsets
function remove(
$path, $files = true, $dir = true,
$links = true, $exec = true
)
{
if (!$files && is_file($path))
return false;
if (!$dir && is_dir($path))
return false;
if (!$links && is_link($path))
return false;
if (!$exec && is_executable($path))
return false;
// ...
}

Sets – Using bitsets (example)
remove("/tmp/removeMe", true, false, true, false);

remove("/tmp/removeMe", true, false, true, false);
// WTF ?!

E_ERROR
E_WARNING
E_PARSE
E_NOTICE

define("E_ERROR", 1);
define("E_WARNING", 2);
define("E_PARSE", 4);
define("E_NOTICE", 8);

E_ERROR
E_PARSE
E_NOTICE
10000000
00100000
00010000

E_ERROR
E_PARSE
E_NOTICE
E_ERROR | E_PARSE | E_NOTICE
10000000
00100000
00010000
10110000

define("E_ERROR", 1 << 0);
define("E_WARNING", 1 << 1);
define("E_PARSE", 1 << 2);
define("E_NOTICE", 1 << 3);
E_ERROR
E_PARSE
E_NOTICE
E_ERROR | E_PARSE | E_NOTICE
10000000
00100000
00010000
10110000

define("E_ERROR", 1 << 0);
define("E_WARNING", 1 << 1);
define("E_PARSE", 1 << 2);
define("E_NOTICE", 1 << 3);
$set = 0;
$set |= E_ERROR;
$set |= E_WARNING;
$set |= E_PARSE;
$set & E_ERROR; // true
$set & E_NOTICE; // false
$set1 | $set2; // union
$set1 & $set2; // intersection
$set1 ^ $set2; // complement

define("REMOVE_FILES", 1 << 0);
define("REMOVE_DIRS", 1 << 1);
define("REMOVE_LINKS", 1 << 2);
define("REMOVE_EXEC", 1 << 3);
define("REMOVE_ALL", ~0); // Setting all bits
function remove($path, $options = REMOVE_ALL)
{
if (~$options & REMOVE_FILES && is_file($path))
return false;
if (~$options & REMOVE_DIRS && is_dir($path))
return false;
if (~$options & REMOVE_LINKS && is_link($path))
return false;
if (~$options & REMOVE_EXEC && is_executable($path))
return false;
// ...
}

remove("/tmp/removeMe", REMOVE_FILES | REMOVE_LINKS);

remove("/tmp/removeMe", REMOVE_FILES | REMOVE_LINKS);
// Much better :)

Sets: Conclusions
● Use the key and not the value when using PHP
Arrays.
● Use QuickHash for set of integers if possible.
● Use SplObjectStorage as soon as you are playing
with objects.
● Use bitsets when playing with finite number of
elements (and known in advance).
● Avoid array_unique() / in_array() at all price!

Maps
● A map is a collection of key/value pairs where all
keys are unique.

Maps – Using array
● Don't use array_merge() on maps.
$map = [];
$map["ONE"] = 1;
$map["TWO"] = 2;
$map["THREE"] = 3;
// Merging maps:
array_merge($map1, $map2); // SLOW!
$map2 + $map1; // Fast :)

Maps – Using array
Testing 107
merges against 2 maps of 5 elements
array_merge +
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
5
4,74
2,77
1,42
1,09
PHP 5.6
PHP 7
Time(s)

Multikey Maps – Using array
$map = [];
$map["ONE"] = 1;
$map["UN"] =& $map["ONE"];
$map["UNO"] =& $map["ONE"];
$map["TWO"] = 2;
$map["DEUX"] =& $map["TWO"];
$map["DUE"] =& $map["TWO"];
$map["UNO"] = "once";
$map["DEUX"] = "twice";
var_dump($map);
/*
array(6) {
["ONE"] => &string(4) "once"
["UN"] => &string(4) "once"
["UNO"] => &string(4) "once"
["TWO"] => &string(5) "twice"
["DEUX"] => &string(5) "twice"
["DUE"] => &string(5) "twice"
}
*/

Heap
● A heap is a tree-based structure in which all
elements are ordered with largest key at the top,
and the smallest one as leafs.

Heap – Using Spl(Min|Max)Heap
$heap = new SplMinHeap;
$heap->insert(30);
$heap->insert(20);
$heap->insert(25);
var_dump($heap->top());
/* int(20) */

Heaps: Conclusions
● MUCH faster than having to re-sort() an array at
every insertion.
● If you don't require a collection to be sorted at
every single step and can insert all data at once
and then sort(). Array is a much better/faster
approach.
● SplPriorityQueue is very similar, consider it is the
same as SplHeap but where the sorting is made on
the key rather than the value.

Bloom filters
● A bloom filter is a space-efficient probabilistic data
structure used to test whether an element is
member of a set.
● False positives are possible, but false negatives are
not!

Bloom filters – Using bloomy
$bloom = new BloomFilter(
10000, // capacity
0,001 // (optional) error rate
// (optional) random seed
);
$bloom->add("An element");
$bloom->has("An element"); // true for sure
$bloom->has("Foo"); // false, most probably

Other related projects
● SPL Types: Various types implemented as object:
SplInt, SplFloat, SplEnum, SplBool and SplString
http://pecl.php.net/package/SPL_Types

● Judy: Sparse dynamic arrays implementation
http://pecl.php.net/package/Judy

● Judy: Sparse dynamic arrays implementation
http://pecl.php.net/package/Judy
● Weakref: Weak references implementation.
Provides a gateway to an object without
preventing that object from being collected by the
garbage collector.

Conclusions
● Use appropriate data structure. It will keep your
code clean and fast.

Conclusions
● Think about the time and space complexity
involved by your algorithms.

Conclusions
● Think about the time and space complexity
involved by your algorithms.
● Name your variables accordingly: use“Map”,“Set”,
“List”,“Queue”,... to describe them instead of using
something like: $ordersArray.

Thanks
Don't forget to rate this talk on https://joind.in/14535
Stay in touch!
@patrick_allaert
patrickallaert@php.net

Photo Credits
● Tuned car:
http://www.flickr.com/photos/gioxxswall/5783867752
● London Eye Structure:
http://www.flickr.com/photos/photographygal123/4883546484
● Heap structure:
http://en.wikipedia.org/wiki/File:Max-Heap.svg
● Drawers:
http://www.flickr.com/photos/jamesclay/2312912612
● Stones stack:
http://www.flickr.com/photos/silent_e/2282729987
● Tree:
http://www.flickr.com/photos/drewbandy/6002204996

PHP data structures (and the impact of php 7 on them), phpDay Verona 2015, Italy

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