A one-to-one mapping function between integers and fixed length strings, such that sequential integers are mapped to non-sequential strings. In other words you can obfuscate user ids for use in urls.
Converts an integer value to a string of fixed length (default is 8). As of 1.2 the maximum encodable value is the same as the alphabet maximum. For example the default 36 character alphabet and 8 character fixed length can encoded numbers between 0 and 2,821,109,907,455 = 36**8 - 1.
Given a valid string returns the decoded number. Note that the two operations are reversible and adjacent values are unlikely to return adjacent strings (See Stats section below). For example, using the default configuration:
FixedLengthEncoder.decode(FixedLengthEncoder.encode(100)) == 100
FixedLengthEncoder.encode(100) == 'ycxk2ntw'
FixedLengthEncoder.encode(101) == 'd8gxk24x'
npm install fixed_length_encoder
require 'fixed_length_encoder'
FixedLengthEncoder.encode(100)
FixedLengthEncoder.decode('ycxk2ntw')
FixedLengthEncoder.encode(42, 3)
FixedLengthEncoder.decode('5c4')
FixedLengthEncoder::MESSAGE_LENGTH = 10
The ALPHABET, ENCODE_MAP and DECODE_MAP must all work together. The two maps must also be
reversible. For example, for an alphabet of 62 characters you will need to build two maps of
length 62**2 - 1 such that DECODE_MAP[ENCODE_MAP[x]] == x. One such way to do this would be:
max = 62*62 - 1
ENCODE_MAP = (0..max).to_a.shuffle
DECODE_MAP = []
(0..max).each { |i| DECODE_MAP[ENCODE_MAP[i]] = i }
Then, hard code these results into your application. You will have three lines much like the lines
that define the default ALPHABET, ENCODE_MAP and DECODE_MAP in the FixedLengthEncoded:
FixedLengthEncoder::ALPHABET = 'abcdefg'
FixedLengthEncoder::ENCODE_MAP = [19, 22, 25, 44, 17, 21, 33, 48, 39, 0, 16, 20, 29, 40, 43, 23, 3, 41, 12, 35, 7, 14, 10, 32, 46, 38, 9, 11, 27, 31, 26, 18, 34, 24, 4, 42, 47, 5, 1, 36, 13, 37, 30, 15, 45, 2, 8, 28, 6]
FixedLengthEncoder::DECODE_MAP = [9, 38, 45, 16, 34, 37, 48, 20, 46, 26, 22, 27, 18, 40, 21, 43, 10, 4, 31, 0, 11, 5, 1, 15, 33, 2, 30, 28, 47, 12, 42, 29, 23, 6, 32, 19, 39, 41, 25, 8, 13, 17, 35, 14, 3, 44, 24, 36, 7]
Consider a random value using the FixedLengthEncoder default LENGTH of 8 and ALPHABET of 36
characters. If we encode value and value + 1 and compute the difference between them in base 36
we get a delta. The table below compares the distribution of 10M pairs of two adjacent encoded values with 10M pairs of two random numbers.
Both sets are taken from the range 0 to 36**8 = 2,821,109,907,456. As expected the number of
negative deltas is near 50% with the encoded values 49.9860% negative and random 49.9989%.
It's interesting to note that there are no random occurances of two adjecent values, but in the
encoded values there are 674.
| : Encoded : | : Random : | |
|---|---|---|
| Negative deltas | 4,998,610 | 4,999,894 |
| Delta equals one | 674 | 0 |
| Maximum Delta | 2,820,278,456,877 | 2,820,579,691,973 |
| Average Delta | 935,745,508,922 | 940,183,477,180 |
| Std Dev | 1,148,460,034,903 | 1,151,442,250,985 |
- Author :: Brett Pontarelli brett@paperyfrog.com
- Website :: http://brett.pontarelli.com