Benchmarking a few solutions from the #ruby IRC channel

mark.rb

require 'benchmark/ips'

Benchmark.ips do |bm|
  bm.report('String chars') { 64.to_s.chars.map(&:to_i).map(&:abs2).join.to_i }
  bm.report('String digits') { 64.digits.map(&:abs2).reverse.join.to_i }
  bm.report('Integer digits') { 64.digits.map.with_index { |n, i| n * n * 10 ** (i * 2) }.sum }

  bm.compare!
end

result.txt

ruby mark.rb
Warming up --------------------------------------
        String chars    46.038k i/100ms
       String digits    64.010k i/100ms
      Integer digits    97.347k i/100ms
Calculating -------------------------------------
        String chars    644.869k (± 3.4%) i/s -      3.223M in   5.003541s
       String digits    869.585k (± 7.3%) i/s -      4.353M in   5.033445s
      Integer digits      1.251M (± 4.2%) i/s -      6.328M in   5.067135s

Comparison:
      Integer digits:  1250994.2 i/s
       String digits:   869585.4 i/s - 1.44x  slower
        String chars:   644869.1 i/s - 1.94x  slower