https://stackoverflow.com/questions/54180056/ruby-index-of-array-elements/

ruby-index-of-array-elements.rb

require 'benchmark/ips'

my_array = [100, 101, 100, 102, 100, 100, 101, 100, 250, 251, 253, 260, 250, 200, 100, 100, 100, 100, 100, 100, 100, 100, 100, 120]

index_and_repetitions = lambda { |my_array|
  stk = {}
  previous = my_array[0]
  last_index = 0
  stk[last_index] = 1
  my_array.drop(0).each_with_index do |item, index|
    if item == previous
      stk[last_index] += 1
    else
      last_index = index
      stk[last_index] = 1
      previous = item
    end
  end
  stk
}

Benchmark.ips do |x|
  x.time = 5
  x.warmup = 2

  x.report('oshanz') do |_t|
    stk = index_and_repetitions.call(my_array)
    stk.key(stk.values.max)
  end

  x.report('cary') do |_t|
    my_array.each_index.chunk { |i| my_array[i] }
            .max_by { |_, a| a.size }
            .last
            .first
  end

  x.report('zhou') do |_t|
    my_array.index.with_index { |_value, index| my_array[index, 6].uniq.size == 1 }
  end

  x.report('ray') do |_t|
    groups = my_array[1..-1].each_with_object([[my_array[0]]]) { |n, m| m.last[0] == n ? m.last << n : m << [n]; }
    groups[0, groups.index(groups.max_by(&:count))].flatten.count
  end

  x.compare!
end

Warming up --------------------------------------

           oshanz    10.471k i/100ms
            cary     4.771k i/100ms
            zhou     5.868k i/100ms
             ray     5.741k i/100ms

Calculating -------------------------------------

           oshanz      1.147B (±17.8%) i/s -      5.022B in   4.721988s
            cary    245.846M (±27.1%) i/s -      1.057B in   4.860784s
            zhou    373.943M (±22.9%) i/s -      1.652B in   4.856765s
             ray    330.414M (±21.7%) i/s -      1.471B in   4.836672s

Comparison:

             oshanz: 1147085295.2 i/s
            zhou: 373943146.9 i/s - 3.07x  slower
             ray: 330413853.0 i/s - 3.47x  slower
            cary: 245845639.8 i/s - 4.67x  slower