Ranking an array of integers in Ruby - http://stackoverflow.com/q/25042260/179125

01_rank_array_benchmark.rb

require "benchmark/ips"

def falsetru(a)
  rank = 1
  a.group_by {|x| x }
    .map { |k,v|
      ret = [rank] * v.size
      rank += v.size
      ret
    }.flatten
end

def falsetru_2(a)
  rank, i = 1, 0
  a.map { |x|
    i += 1
    x != a[i-2] ? rank = i : rank
  }
end

def sawa(a)
  a.group_by {|e| e}
    .each_with_object([]) {|(_, v), arr| arr.concat( [arr.length + 1] * v.length ) }
end

def jordan(a)
  run = rank = 0
  last_n = nil

  a.map do |n|
    run += 1
    next rank if n == last_n
    last_n = n
    rank += run
    run = 0
    rank
  end
end

def iceman(a)
  a.map {|e| a.index(e) + 1 }
end

def simong(a)
  rank = 1
  a.each_with_index.map {|value, i| a[i-1] == value ? rank : rank = i + 1 }
end

def cary_s(a)
  a.each_with_index
    .chunk { |e,_| e }
    .flat_map { |_,arr| [arr.first.last + 1] * arr.size }
end

test_arr = Array.new(10_000) { rand 100 }.sort.reverse

Benchmark.ips do |b|
  b.report("falsetru")         { falsetru test_arr }
  b.report("falsetru (2)")     { falsetru_2 test_arr }
  b.report("sawa")             { sawa test_arr }
  b.report("Jordan")           { jordan test_arr }
  b.report("Iceman")           { iceman test_arr }
  b.report("simongarnier")     { simong test_arr }
  b.report("Cary Swoveland")   { cary_s test_arr }
end

02_output.txt

Calculating -------------------------------------
            falsetru        72 i/100ms
        falsetru (2)       130 i/100ms
                sawa       100 i/100ms
              Jordan       165 i/100ms
              Iceman         1 i/100ms
        simongarnier       104 i/100ms
      Cary Swoveland        52 i/100ms
-------------------------------------------------
            falsetru      730.9 (±2.1%) i/s -       3672 in   5.025755s
        falsetru (2)     1289.9 (±2.7%) i/s -       6500 in   5.042749s
                sawa      986.9 (±2.1%) i/s -       5000 in   5.068450s
              Jordan     1644.9 (±1.9%) i/s -       8250 in   5.017334s
              Iceman        6.4 (±0.0%) i/s -         32 in   5.035015s
        simongarnier     1053.9 (±1.9%) i/s -       5304 in   5.034452s
      Cary Swoveland      511.4 (±3.5%) i/s -       2600 in   5.090605s