~/.bin/entropy: Calculate entropy per input line

Inspired by [this comment][comment] and very shortly described [on my
blog][blog].

[comment]: https://news.ycombinator.com/item?id=12819675
[blog]: https://papill0n.org/blog.html#git-ls-files-xargs-cat-entropy-rb-sort-tail-n20

.bin/entropy.rb

#!/usr/bin/env ruby
def shannon_entropy(s)
  d = {}
  s.each_char do |c|
    d[c] ||= 0.0
    d[c] += 1
  end
  res = 0.0
  d.each_value do |v|
    freq = v / s.length
    res -= freq * (Math.log(freq) / Math.log(2))
  end
  res
end
if __FILE__ == $0
  $stdin.each_line do |line|
    e = shannon_entropy(line)
    puts format("%.4f\t%s", e, line)
  end
end