Ruby: Why does unpack('Q') give a different result than manual conversion?

Question

I'm trying to write a function that will .unpack('Q') (unpack to uint64_t) without access to the unpack method.

When I manually convert from string to binary to uint64, I get a different result than .unpack('Q'):

Integer('abcdefgh'.unpack('B*').first, 2) # => 7017280452245743464

'abcdefgh'.unpack('Q').first # => 7523094288207667809

I don't understand what's happening here.

I also don't understand why the output of .unpack('Q') is fixed regardless of the size of the input. If I add a thousand characters after 'abcdefgh' and then unpack('Q') it, I still just get [7523094288207667809]?

Answer 1

Byte order matters:

 Integer('abcdefgh'.
           each_char.
           flat_map { |c| c.unpack('B*') }.
           reverse.
           join, 2)
 #⇒ 7523094288207667809
 'abcdefgh'.unpack('Q*').first
 #⇒ 7523094288207667809

Your code produces the wrong result because after converting to binary, bytes should be reversed.

---

For the last part of your question, the reason the output of .unpack('Q') doesn't change with a longer input string is because the format is specifying a single 64-bit value so any characters after the first 8 are ignored. If you specified a format of Q2 and a 16 character string you'd decode 2 values:

> 'abcdefghihjklmno'.unpack('Q2')
=> [7523094288207667809, 8029475498074204265]

and again you'd find adding additional characters wouldn't change the result:

> 'abcdefghihjklmnofoofoo'.unpack('Q2')
=> [7523094288207667809, 8029475498074204265]

A format of Q* would return as many values as multiples of 64-bits were in the input:

> 'abcdefghihjklmnopqrstuvw'.unpack('Q*')
=> [7523094288207667809, 8029475498074204265, 8608196880778817904]
> 'abcdefghihjklmnopqrstuvwxyz'.unpack('Q*')
=> [7523094288207667809, 8029475498074204265, 8608196880778817904]