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

Answer 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"

Answer 2

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