pingpongninja_expected_results.rb

#!/usr/bin/env ruby
# pingpongninja_expected_results -- simulate a series of games on pingpongninja
#
# algorithm extracted from
# https://github.com/jdennes/pingpongapp/blob/master/pingpong/rankings.py
def new_ranks(rank1, rank2, points1, points2)
  decay_factor = 10
  game_ranking_points = (rank1 + rank2) / 2
  ranking_change1 = game_ranking_points + (points1 - points2) * 100 / [points1, points2].max
  ranking_change2 = game_ranking_points + (points2 - points1) * 100 / [points1, points2].max

  new_rank1 = rank1 / decay_factor * (decay_factor - 1) + ranking_change1 / decay_factor
  new_rank2 = rank2 / decay_factor * (decay_factor - 1) + ranking_change2 / decay_factor

  [new_rank1, new_rank2]
end

def run_game(player1, rank1, player2, rank2, score1, score2)
  new_rank1, new_rank2 = new_ranks(rank1, rank2, score1, score2)
  delta = new_rank1 - rank1
  puts "#{score1} - #{score2}: #{delta.round(2)}"
end

def simulate(player1, rank1, player2, rank2)
  rank1 = rank1.to_f
  rank2 = rank2.to_f

  puts "#{player1} beats #{player2} and gains points"
  0.upto(9).each do |points|
    run_game(player1, rank1, player2, rank2, 11, points)
  end
  10.upto(14) do |point1|
    run_game(player1, rank1, player2, rank2, point1+2, point1)
  end

  puts "#{player1} loses to #{player2} and gains points"
  0.upto(9).each do |points|
    run_game(player1, rank1, player2, rank2, points, 11)
  end
  10.upto(14) do |point1|
    run_game(player1, rank1, player2, rank2, point1, point1+2)
  end
end

if ARGV.size != 4
  puts "Usage: #{File.basename($0)} <p1_name> <p1_rank> <p2_name> <p2_rank>"
  exit 1
end

simulate(*ARGV)