#!/usr/bin/env ruby

#
#
# Copyright (c) 2004 Stephen Waits
# 
# This software is provided 'as-is', without any express or implied warranty. In
# no event will the authors be held liable for any damages arising from the use
# of this software.
# 
# Permission is granted to anyone to use this software for any purpose,
# including commercial applications, and to alter it and redistribute it freely,
# subject to the following restrictions:
# 
# 1. The origin of this software must not be misrepresented; you must not claim
#    that you wrote the original software. If you use this software in a
#    product, an acknowledgment in the product documentation would be
#    appreciated but is not required.
# 
# 2. Altered source versions must be plainly marked as such, and must not be
#    misrepresented as being the original software.
# 
# 3. This notice may not be removed or altered from any source distribution.
#
#


# Glicko-2 Rating calculator class.
#
# This class implements the Glicko-2 rating system algorithm written by 
# Professor Mark E. Glickman.  All rating inputs and outputs are Glicko ratings,
# but internally everything is converted to and considered Glicko-2 ratings.
#
# Glicko-2 is an improvement on the original Glicko, which was, in turn, an
# improvement on the ELO system.
#
# The Glicko-2 system is specified on http://www.glicko.com/
# 
class Glicko2

	protected

		# Glicko2 reprensentation (Glicko2 uses different scales from Glicko)
		attr_accessor :g2rating, :g2deviation, :g2volatility
	
		# a nested Class to hold a single result
		Result = Struct.new(:opponent, :result)

	public

		WIN  = 1.0
		LOSS = 0.0
		DRAW = 0.5
		
		# Constructor with rating, rating deviation, and volatility optionally
		# specified.  If nothing specified, nitializes to a rating of 1500, a 
		# rating deviation of 350, and a volatility of 0.06.
		def initialize(rating = 1500.0, deviation = 350.0, volatility = 0.06)
			@dvolatility    = 0.3
			self.rating     = rating
			self.deviation  = deviation
			self.volatility = volatility
			clear_results
		end

		include Comparable

		# Comparison operator
		def <=>(other)
			self.rating <=> other.rating
		end

		# Get the current rating.  This rating is a Glicko rating, not a Glicko2 rating.
		# For details, please see http://www.glicko.com/
		def rating
			(@g2rating * 173.7178) + 1500.0
		end

		# Set Glicko rating.  Internally this is converted to a Glicko2 rating.
		# For details, please see http://www.glicko.com/
		def rating=(r)
			@g2rating = (r - 1500.0) / 173.7178
		end

		# Get the current rating deviation.  This is a Glicko rating deviation, not a 
		# Glicko-2 RD.  For details, please see http://www.glicko.com/
		def deviation
			@g2deviation * 173.7178
		end

		# Set Glicko rating deviation.  Internally this is converted to a Glicko-2 RD.
		# For details, please see http://www.glicko.com/
		def deviation=(d)
			@g2deviation = d / 173.7178
		end

		# Get the current rating volatility.
		def volatility
			@g2volatility
		end

		# Set rating volatility.
		def volatility=(v)
			@g2volatility = v
		end

		# Clear all results previously added via add_result(), add_win(), add_loss(),
		# and/or add_draw().  This method is called automatically whenever update()
		# is called.
		def clear_results
			@opponents = []
			@results = []
		end

		# Add a result to this rating.  Note that no calculation is performed until
		# update() is called.
		def add_result(opponent,result)
			@results.push( Result.new(opponent.clone, result) )
		end

		# Add a win result to this rating.  Note that no calculation is performed until
		# update() is called.
		def add_win(opponent)
			add_result(opponent,Glicko2::WIN)
		end

		# Add a loss result to this rating.  Note that no calculation is performed until
		# update() is called.
		def add_loss(opponent)
			add_result(opponent,Glicko2::LOSS)
		end

		# Add a draw result to this rating.  Note that no calculation is performed until
		# update() is called.
		def add_draw(opponent)
			add_result(opponent,Glicko2::DRAW)
		end
	
		# Update rating based on current results list, and clear results.
		def update
		
			# util func
			def Glicko2.g(deviation)
				1.0 / (Math.sqrt(1.0 + 3.0 * deviation ** 2.0 / (Math::PI ** 2.0)))
			end
		
			# util func
			def Glicko2.E(rating, rating_opponent, deviation_opponent)
				1.0 / (1.0 + Math.exp(-Glicko2.g(deviation_opponent)*(rating - rating_opponent)));
			end
		
			# bail if no opponents set
			if @results.empty?
				return nil
			end

			# compute variance

			#variance = 0.0
			#@results.inject(0.0) do |variance,r|
			#	g_i = Glicko2.g(r.opponent.g2deviation)
			#	e_i = Glicko2.E(@g2rating,r.opponent.g2rating,r.opponent.g2deviation)
			#	g_i ** 2.0 * e_i * (1.0 - e_i)
			#end

			variance = 0.0
			@results.each do |r|
				g_i = Glicko2.g(r.opponent.g2deviation)
				e_i = Glicko2.E(@g2rating,r.opponent.g2rating,r.opponent.g2deviation)
				variance += g_i ** 2.0 * e_i * (1.0 - e_i)
			end
			variance = 1.0 / variance
		
			# compute delta
			delta = 0.0
			@results.each do |r|
				delta += Glicko2.g(r.opponent.g2deviation) * (r.result - Glicko2.E(@g2rating, r.opponent.g2rating, r.opponent.g2deviation))
			end
			delta *= variance
		
			# determine new volatility
			new_volatility = 0.0
			a              = Math.log( @g2volatility**2.0 )
			x              = 0.0
			x_new          = a
			while ( (x - x_new).abs > 0.0000001 )
				x     = x_new
				d     = @g2deviation**2.0 + variance + Math.exp(x)
				h1    = -(x - a)/(@dvolatility**2.0) - 0.5*Math.exp(x)/d + 0.5*Math.exp(x)*(delta/d)*(delta/d)
				h2    = -1.0/(@dvolatility**2.0) - 0.5*Math.exp(x)*(@g2deviation**2.0+variance)/(d**2.0) + 0.5*(delta**2.0)*Math.exp(x)*((@g2deviation**2.0) + variance - Math.exp(x))/(d**3.0)
				x_new = x - h1/h2
			end
			new_volatility = Math.exp(x_new / 2.0)
		
			# update the rating deviation to the new pre-rating period value
			pre_deviation = Math.sqrt( @g2deviation**2.0 + new_volatility**2.0 )

			# update the rating and deviation
			new_deviation = 1.0 / (Math.sqrt( 1.0/(pre_deviation**2.0) + 1.0 / variance))
			new_rating    = 0.0
			@results.each do |r|
			    new_rating += Glicko2.g(r.opponent.g2deviation) * (r.result - Glicko2.E(@g2rating, r.opponent.g2rating, r.opponent.g2deviation))
			end
			new_rating  = new_rating * new_deviation**2.0
			new_rating += @g2rating

			# wipe our result lists
			clear_results

			# copy new values
			@g2deviation  = new_deviation
			@g2volatility = new_volatility
			@g2rating     = new_rating
		end
end