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Package codecs-base64vlq
Short Description Codec for VLQ (variable-length quantity) Base64 algorithm
License MIT
Homepage https://github.com/axypro/codecs-base64vlq
Informations about the package codecs-base64vlq
axy\codecs\base64vlq
Codec for VLQ (variable-length quantity) Base64 algorithm (PHP).
Documentation
VQL + Base64
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, it used in JavaScript/CSS source map.
The Algorithm
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 11000000111001
.
Added 0
(positive) to the end: 110000001110010
.
For -12345
take a positive form and add 1
(negative) to the end: 110000001110011
.
Result is [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.
Result: [110010 100011 011000 110011 100011 011000 000000]
.
Or decimal [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 A..Za..z0..9+/
.
Result is yjYzjYA
.
How to use the library
Or default encoder can be obtained as follow:
In this case, the object creation and preliminary calculations are performed only once.
Custom options
The standard encoder uses standard options:
- Transfer the sign bit.
- 6-bit VLQ digits.
- Standard alphabet:
A..Za..z0..9+/
.
You can apply your settings:
Custom alphabet can be specified as a string (My Alphabet
) or as an array ([1 => 'A', 10 => 'B', 15 => 'C', 20 => 'D']
).
Exceptions
The error classes are located in the namespace axy\codecs\base64vlq\Encoder\Errors
.
- Error
- VLQ
- InvalidVLQSequence
- Base64
- InvalidBase64
- InvalidBase64Input
- VLQ
InvalidVLQSequence
z
is 51 (the continuation bit = 1).
The last digit must have 0 in continuation bit.
InvalidBase64
*
are not in the standard Base64 alphabet.
InvalidBase64Input
For the standard encoder this exception should not occur.
Can occur for incorrect custom options:
10 bits is 1024 variants, but the alphabet contains only 3 letter.