Codec for VLQ (variable-length quantity) Base64 algorithm
Codec for VLQ (variable-length quantity) Base64 algorithm (PHP).
- The library does not require any dependencies (except composer packages).
- Tested on PHP 5.4+, PHP 7, HHVM (on Linux), PHP 5.5 (on Windows).
composer require axy/codecs-base64vlq.
- License: MIT.
Base64 allows us to represent a sequence of numbers in a text string that can be stored and transmit in text formats (JSON, XML, etc).
VLQ allows us to represent an integer in a sequence numbers with little digit capacity. For example 6 bit is the limit for Base64. The resulting numbers are called "VLQ digits". Small input number is represented by fewer VLQ-digits than big number. Thus VLQ is most effective if the input sequence is contains mainly the small numbers.
VLQ+Base64 allows us effectively represented a sequence of integers (dominated by a small number of) in the text format.
For example, we have a block of numbers:
[12345, -12345, 0].
(1). VLQ only works with unsigned integers. Transfer the sign bit to the end of the integer.
12345 in binary is
0 (positive) to the end:
-12345 take a positive form and add
1 (negative) to the end:
[110000001110010, 110000001110011, 0].
(2). Transform to the VLQ-sequence. For Base64 we need a block of 6-bit numbers. Most significant bit is reserved - it is "continuation".
Split numbers to groups of 5 bits:
[11000 00011 10010, 11000 00011 10011, 00000].
Output starting from the least significant bits.
If the group is not the last in the current number then set the continuation bit.
[110010 100011 011000 110011 100011 011000 000000].
[50, 35, 24, 51, 35, 24, 0].
These are VLQ digits.
(3). Replace the numbers on the letters of the Base64-alphabet.
The standard alphabet is
use axy\codecs\base64vlq\Encoder; $encoder = new Encoder(); $encoder->encode([12345, -12345, 0]); // yjYzjYA $encoder->decode('yjYzjYA'); // [12345, 012345, 9] $encoder->decode('Variable+Length+QuantitY'); // [-10, 13, -13349, -13 ... -12797139]
Or default encoder can be obtained as follow:
$encoder = Encoder::getStandardInstance();
In this case, the object creation and preliminary calculations are performed only once.
The standard encoder uses standard options:
- Transfer the sign bit.
- 6-bit VLQ digits.
- Standard alphabet:
You can apply your settings:
Encoder::__construct([array|string $alphabet, int $bits, bool $signed = true])
/* Custom alphabet, 4 bits, no sign transfer */ $encoder = new Encoder('My Alphabet', 3, false); $encoder->encode([12345, 6789]); // phalllApplhhhy
Custom alphabet can be specified as a string (
My Alphabet) or as an array (
[1 => 'A', 10 => 'B', 15 => 'C', 20 => 'D']).
The error classes are located in the namespace
$encoder->decode('Az'); // VLQ sequence is invalid: [0,51]
z is 51 (the continuation bit = 1).
The last digit must have 0 in continuation bit.
$encoder->decode('A*A'); // Base-64 string is invalid: "A*A"
* are not in the standard Base64 alphabet.
For the standard encoder this exception should not occur.
Can occur for incorrect custom options:
$encoder = new Encoder('qwe', 10); $encoder->encode([10, 20, 30]); // Number 20 is not found in Base64 alphabet
10 bits is 1024 variants, but the alphabet contains only 3 letter.