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Package bitset
Short Description A simple Bit Set implementation
License MIT
Homepage https://github.com/pwm/bitset
Informations about the package bitset
BitSet
A simple bit set implementation for compact storage of sets of values. The library itself is stateless providing only pure mapping functions.
Table of Contents
- Requirements
- Installation
- Usage
- How it works
- Tests
- Changelog
- Licence
Requirements
PHP 7.1+
Installation
$ composer require pwm/bitset
Usage
In this example we are going to use a bit set to put users in groups.
Let's create a Group
class. It has a list of groups, named G1 to G5 for this example, and a map between groups and binary values of powers of 2 provided by BitSet. Finally it has 2 functions that map between group names and bit values.
Let's also create a User
class. A user can belong to groups. The $groups
property starts from 0 that represents the empty set. It has 5 methods to manipulate its groups, corresponding to BitSet's 5 functions: get()
, set()
, add()
, remove()
and has()
.
Now we can use the above the following way:
How it works
As an example let's take the numbers 1, 4 and 8. Their sum is 13. Pretty uninteresting so far. But let's look at their binary representations: 0b1, 0b100 and 0b1000 respectively. Their sum 13 is 0b1101. See how the base-2 representations of 1, 4 and 8 "fill unique slots" in 13's, leaving only one slot unset? This is because 1, 4 and 8 are all powers of 2 so their binary representations starts with a 1 followed by zero or more 0s. 0b1 = 2^0 = 1, 0b100 = 2^2 = 4, 0b1000 = 2^3 = 8. Their sum: 2^0 + 2^2 + 2^3 = 0b1 + 0b100 + 0b1000 = 0b1101 = 13. Now, say in a game, we could call 1 "north", 2 "south", 4 "east" and 8 "west" and then 13 could mean that our character can move all directions from its current position except south as 2 (0b10) is not in 13 (0b1101).
The idea behind a bit set is using a base-2 representation of a number to singal the presence or absence of values by having its bits set to 1 or 0. The values themselves are also numbers with the restriction that they must be powers of 2 starting from 2^0 and in general, depending on the underlying system, going up to 2^32 or 2^64. For example on a 64 bit system one can store up to 64 unique values in a bit set.
The BitSet
library itself is a collection of 5 pure functions that map between values and their bit set:
set(array $values): int
Maps a list of unique positive integers to a single integer, the bit set, that is their sum. All integers in the input list must be powers of 2.
get(int $bitSet): array
Maps an integer, the bit set, to a list of unique positive integers. All integers in the output list must be powers of 2.
add(int $bitSet, array $values): int
Adds a list of unique positive integers to a bit set. All integers in the list must be powers of 2.
remove(int $bitSet, array $values): int
Subtracts a list of unique positive integers from a bit set. All integers in the list must be powers of 2.
has(int $bitSet, int $value): bool
Predicate that tells whether an element is in a bit set or not.
Tests
$ vendor/bin/phpunit
$ composer phpcs
$ composer phpstan
Changelog
Click here
Licence
MIT