count k number of occurences

require 'pry'
require 'benchmark'
require 'benchmark-memory'

class Array

  def top_occurrence_0(k)
    counter = Hash.new{ |k, v| k[ v ] = 0 }
    self.each { |id| counter[ id ] += 1 }
    counter.sort_by{ |id, counts| -counts }[0, k].map{ |id_count| id_count[0] }
  end

  def top_occurrence_1(k)
    self.each_with_object( Hash.new( 0 ) ) do |id, counter|
      counter[ id ] += 1
    end.sort_by do |id, counts|
      -counts
    end[0, k].map do |id_count|
      id_count[0]
    end
  end

  def top_occurrence_2(k)
    self.group_by do |id|
      id
    end.sort_by do |id, counts|
      -counts.count
    end.first( k ).map do |id_count|
      id_count[0]
    end
  end

  def top_occurrence_3(k)
    counter = Hash.new{ |k, v| k[ v ] = 0 }
    self.each do |id|
      counter[ id ] += 1
    end
    counter.sort_by do |id, counts|
      -counts
    end[0, k].map do |id_count|
      id_count[0]
    end
  end

  def top_occurrence_4(k)
    self.each_with_object( Hash.new( 0 ) ) do |id, counter|
      counter[ id ] += 1
    end.sort_by do |id, counts|
      -counts
    end.to_h.keys[0, k]
  end

  def self.test_a_method( args = {} )
    puts args[:array].send( args[:method], args[:k] )
  end

  def self.benchmark( args = {} )
    Benchmark.bm do |x|
      [ 0, 1, 2, 3, 4 ].each do |n|
        x.report("top_occurrence_#{ n }") { args[:array].send("top_occurrence_#{ n }", args[:k]) }
      end
    end
  end

  def self.benchmark_mem( args = {} )
    Benchmark.memory do |x|
      [ 0, 1, 2, 3, 4 ].each do |n|
        x.report("top_occurrence_#{ n }") { args[:array].send("top_occurrence_#{ n }", args[:k]) }
      end
      x.compare!
    end
  end

end

k     = 3
array =  [ 1,1, 3, 4, 6, 7,3, 1, 6, 8, 8, 8, 8, 8, 2, 1 ] * 1000000
Array.benchmark(k: k, array: array)
Array.benchmark_mem(k: k, array: array)
# Array.test_a_method(k: k, array: array, method: :top_occurrence_1)
# array.top_occurrence_1(3)

#        user     system      total        real
# top_occurrence_0  2.330000   0.010000   2.340000 (  2.343289)
# top_occurrence_1  2.940000   0.010000   2.950000 (  2.949879)
# top_occurrence_2  2.370000   0.080000   2.450000 (  2.458058)
# top_occurrence_3  2.710000   0.000000   2.710000 (  2.719345)
# top_occurrence_4  3.340000   0.020000   3.360000 (  3.364670)
# Calculating -------------------------------------
#     top_occurrence_0     1.488k memsize (     0.000  retained)
#                         15.000  objects (     0.000  retained)
#                          2.000  strings (     0.000  retained)
#     top_occurrence_1     1.232k memsize (     0.000  retained)
#                         13.000  objects (     0.000  retained)
#                          2.000  strings (     0.000  retained)
#     top_occurrence_2   168.000M memsize (     0.000  retained)
#                         20.000  objects (     0.000  retained)
#                          2.000  strings (     0.000  retained)
#     top_occurrence_3     1.488k memsize (     0.000  retained)
#                         15.000  objects (     0.000  retained)
#                          2.000  strings (     0.000  retained)
#     top_occurrence_4     1.800k memsize (     0.000  retained)
#                         14.000  objects (     0.000  retained)
#                          2.000  strings (     0.000  retained)
# 
# Comparison:
#     top_occurrence_1:       1232 allocated
#     top_occurrence_0:       1488 allocated - 1.21x more
#     top_occurrence_3:       1488 allocated - 1.21x more
#     top_occurrence_4:       1800 allocated - 1.46x more
#     top_occurrence_2:  168000304 allocated - 136363.88x more