Ruby's pack and unpack explained

Question

Even after reading the standard documentation, I still can\'t understand how Ruby\'s Array#pack and String#unpack exactly work. Here is the example that\'s causing me the most t

Answer 1

I expected both these operations to return the same output: "abc".

The easiest way to understand why your approach didn't work, is to simply start with what you are expecting:

"abc".unpack("H*")
# => ["616263"]

["616263"].pack("H*")
# => "abc"

So, it seems that Ruby expects your hex bytes in one long string instead of separate elements of an array. So the simplest answer to your original question would be this:

chars = ["61", "62", "63"]
[chars.join].pack("H*")
# => "abc"

This approach also seems to perform comparably well for large input:

require 'benchmark'

chars = ["61", "62", "63"] * 100000

Benchmark.bmbm do |bm|
  bm.report("join pack") do [chars.join].pack("H*") end
  bm.report("big pack") do chars.pack("H2" * chars.size) end
  bm.report("map pack") do chars.map{ |s| [s].pack("H2") }.join end
end

#                 user     system      total        real
# join pack   0.030000   0.000000   0.030000 (  0.025558)
# big pack    0.030000   0.000000   0.030000 (  0.027773)
# map pack    0.230000   0.010000   0.240000 (  0.241117)