File with random data but specific size

I am trying to generate a file in ruby that has a specific size. The content doesn't matter.

Here is what I got so far (and it works!):

File.open("done/#{NAME}.txt", 'w') do |f|
  contents = "x" * (1024*1024)
  SIZE.to_i.times { f.write(contents) }
end

The problem is: Once I zip or rar this file the created archive is only a few kb small. I guess thats because the random data in the file got compressed.

How do I create data that is more random as if it were just a normal file (for example a movie file)? To be specific: How to create a file with random data that keeps its size when archived?

Solution

You cannot guarantee an exact file size when compressing. However, as you suggest in the question, completely random data does not compress.

You can generate a random String using most random number generators. Even simple ones are capable of making hard-to-compress data, but you would have to write your own string-creation code. Luckily for you, Ruby comes with a built-in library that already has a convenient byte-generating method, and you can use it in a variation of your code:

require 'securerandom'
one_megabyte = 2 ** 20 # or 1024 * 1024, if you prefer

# Note use 'wb' mode to prevent problems with character encoding
File.open("done/#{NAME}.txt", 'wb') do |f|
  SIZE.to_i.times { f.write( SecureRandom.random_bytes( one_megabyte ) ) }
end

This file is not going to compress much, if at all. Many compressors will detect that and just store the file as-is (making a .zip or .rar file slightly larger than the original).