How to unpack 7-bits at a time in ruby?

Question

I'm trying to format a UUIDv4 into a url friendly string. The typical format in base16 is pretty long and has dashes:

xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx

To avoid dashes and underscores I was going to use base58 (like bitcoin does) so each character fully encode sqrt(58).floor = 7 bits.

I can pack the uuid into binary with:

[ uuid.delete('-') ].pack('H*')

To get 8-bit unsigned integers its:

binary.unpack('C*')

How can i unpack every 7-bits into 8-bit unsigned integers? Is there a pattern to scan 7-bits at a time and set the high bit to 0?

This gem may help: https://github.com/dougal/base58 – John La Rooy Nov 24 '15 at 21:58 — John La Rooy, Nov 24 '15 at 21:58

John La Rooy · Accepted Answer · 2015-11-24T22:47:57.183

1

require 'base58'
uuid ="123e4567-e89b-12d3-a456-426655440000"
Base58.encode(uuid.delete('-').to_i(16))
=> "3fEgj34VWmVufdDD1fE1Su"

and back again

Base58.decode("3fEgj34VWmVufdDD1fE1Su").to_s(16)
 => "123e4567e89b12d3a456426655440000"

A handy pattern to reconstruct the uuid format from a template

template = 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'
src = "123e4567e89b12d3a456426655440000".each_char
template.each_char.reduce(''){|acc, e| acc += e=='-' ? e : src.next}  
 => "123e4567-e89b-12d3-a456-426655440000"

edited Nov 24 '15 at 22:47

answered Nov 24 '15 at 22:08

John La Rooy

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Yes had a feeling was over-thinking it. – AJcodez Nov 24 '15 at 23:24

score 1 · Answer 2 · answered Nov 25 '15 at 06:59

John La Rooy's answer is great, but I just wanted to point out how simple the Base58 algorithm is because I think it's neat. (Loosely based on the base58 gem, plus bonus original int_to_uuid function):

ALPHABET = "123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ".chars
BASE = ALPHABET.size

def base58_to_int(base58_val)
  base58_val.chars
    .reverse_each.with_index
    .reduce(0) do |int_val, (char, index)|
      int_val + ALPHABET.index(char) * BASE ** index
    end
end

def int_to_base58(int_val)
  ''.tap do |base58_val|
    while int_val > 0
      int_val, mod = int_val.divmod(BASE)
      base58_val.prepend ALPHABET[mod]
    end
  end
end

def int_to_uuid(int_val)
  base16_val = int_val.to_s(16)
  [ 8, 4, 4, 4, 12 ].map do |n|
    base16_val.slice!(0...n)
  end.join('-')
end

uuid = "123e4567-e89b-12d3-a456-426655440000"
int_val = uuid.delete('-').to_i(16)
base58_val = int_to_base58(int_val)
int_val2 = base58_to_int(base58_val)
uuid2 = int_to_uuid(int_val2)

printf <<END, uuid, int_val, base_58_val, int_val2, uuid2
Input UUID: %s
Input UUID as integer: %d
Integer encoded as base 58: %s
Integer decoded from base 58: %d
Decoded integer as UUID: %s
END

Output:

Input UUID: 123e4567-e89b-12d3-a456-426655440000
Input UUID as integer: 24249434048109030647017182302883282944
Integer encoded as base 58: 3fEgj34VWmVufdDD1fE1Su
Integer decoded from base 58: 24249434048109030647017182302883282944
Decoded integer as UUID: 123e4567-e89b-12d3-a456-426655440000

How to unpack 7-bits at a time in ruby?

2 Answers2