Ch6, Ex16

We can divide M users in K groups of equal size and in UserStep, user I can ask for performance information from the 3 users belonging to his group only.

These are the changes:

%We add another variable K that
%denotes the number of groups. K should be chosen
%so that (M mod K) is 0 i.e. It should be able
%to divide M users in K groups of equal size
declare
N=10000 M=500000 T=200 K=100 %K is the number of groups

%Sets A and B to the extremes of the group in
%which I fall into
proc {GroupEdges I ?A ?B}
   X1 = (K*I) div M
   X2 = (K*I) mod M
in
   if X2==0 then
      A = (X1-1)*(M div K)+1
      B = X1*(M div K)
   else
      A = X1*(M div K) + 1
      B = (X1+1)*(M div K)
   end
end

%Chose users from the same group only
proc {UserStep I}
   U = Users.I
   local A B in
      {GroupEdges I A B}
      % Ask three users(belonging to I's group only) for
      % their performance information
      L = {List.map [{UniformI A B} {UniformI A B}
                     {UniformI A B}]
           fun{$ X}
              (Users.X.currentSite) #
              Sites.(Users.X.currentSite).performance
              + {Gauss}*{IntToFloat N}
           end}
   end
   % Calculate the best site
   MS#MP = {List.foldL L
            fun {$ X1 X2} if X2.2>X1.2 then X2 else X1 end end
            U.currentSite #
            Sites.(U.currentSite).performance
            + {Abs {Gauss}*{IntToFloat N}}
           }
in
   if MS\=U.currentSite then
      Sites.(U.currentSite).hits :=
      Sites.(U.currentSite).hits - 1
      U.currentSite := MS
      Sites.MS.hits := Sites.MS.hits + 1
   end
end 

==================================================================
Complete Code to run word-of-mouth simulation:

%Download File.oz from book's suppliment site then
%create ozc file by running following command
%$ozc -c File.oz

%CHANGE THESE PATHS
declare FileOzrc SimResults
FileOzrc=
'/path/File.ozf'
SimResults=
'/path/wordofmouth.txt'
declare
[File]= {Module.link [FileOzrc]}

declare NewRand
local
   A=333667
   B=213453321
   M=1000000000
in
   proc {NewRand ?Rand ?Init ?Max}
      X={NewCell 0}
   in
      proc {Init Seed} X:=Seed end
      fun {Rand} X:=(A*@X+B) mod M in @X end
      Max=M
   end
end

declare Rand Init Max Uniform UniformI Gauss
{NewRand Rand Init Max}
local
   FMax={IntToFloat Max}
in
   fun {Uniform}
      {IntToFloat {Rand}}/FMax
   end
   fun {UniformI A B}
      A+{FloatToInt {Floor {Uniform}*{IntToFloat B-A+1}}}
   end
end
local
   TwoPi=4.0*{Float.acos 0.0}
   GaussCell={NewCell nil} in
   fun {Gauss}
      Prev={Exchange GaussCell $ nil}
   in
      if Prev\=nil then Prev
      else R Phi in
         R={Sqrt ~2.0*{Log {Uniform}}}
         Phi=TwoPi*{Uniform}
         GaussCell:=R*{Cos Phi}
         R*{Sin Phi}
      end
   end
end

declare
N=10000 M=500000 T=200
{Init 0}
{File.writeOpen SimResults}
proc {Out S}
   {File.write {Value.toVirtualString S 10 10}#"\n"}
end


declare
Sites={MakeTuple sites N}
for I in 1..N do
   Sites.I={Record.toDictionary
            o(hits:0 performance:{IntToFloat
                                   {UniformI 1 80000}})}
end

declare
Users={MakeTuple users M}
for I in 1..M do
   S={UniformI 1 N}
in
   Users.I={Record.toDictionary o(currentSite:S)}
   Sites.S.hits := Sites.S.hits + 1
end

proc {UserStep I}
   U = Users.I
   % Ask three users for their performance information
   L = {List.map [{UniformI 1 M} {UniformI 1 M} {UniformI 1 M}]
        fun{$ X}
           (Users.X.currentSite) #
           Sites.(Users.X.currentSite).performance
           + {Gauss}*{IntToFloat N}
        end}
   % Calculate the best site
   MS#MP = {List.foldL L
            fun {$ X1 X2} if X2.2>X1.2 then X2 else X1 end end
            U.currentSite #
            Sites.(U.currentSite).performance
            + {Abs {Gauss}*{IntToFloat N}}
           }
in
   if MS\=U.currentSite then
      Sites.(U.currentSite).hits :=
      Sites.(U.currentSite).hits - 1
      U.currentSite := MS
      Sites.MS.hits := Sites.MS.hits + 1
   end
end

for J in 1..N do
   {Out {Record.adjoinAt {Dictionary.toRecord site Sites.J}
         name J}}
end
{Out endOfRound(time:0 nonZeroSites:N)}
for I in 1..T do
   X = {NewCell 0}
in
   for U in 1..M do {UserStep U} end
   for J in 1..N do
      H=Sites.J.hits in
      if H\=0 then
         {Out {Record.adjoinAt
               {Dictionary.toRecord site Sites.J} name J}}
         X:=1+@X
      end
   end
   {Out endOfRound(time:I nonZeroSites:@X)}
end
{Out finished}
{File.writeClose}