Ch6, Ex17

Following are the changes:

%Add  a variable for threhold number of users on one site
declare N=10000 M=500000 T=200 Threshold=100

%Add methods for incrementing/decrementing hits
%Procedures for incrementing/decrementing by 1 user on site
proc {IncrementHits S}
   C = 1.0
in
   Sites.S.hits := Sites.S.hits + 1
   if Sites.S.hits>Threshold then
      Sites.S.performance := Sites.S.performance - C
   end
end
proc {DecrementHits S}
   C = 1.0
in
   if Sites.S.hits>Threshold then
      Sites.S.performance := Sites.S.performance + C
   end
   Sites.S.hits := Sites.S.hits - 1
end

%Never increment/decrement hits manualy but use above methods
declare
Users={MakeTuple users M}
for I in 1..M do
   S={UniformI 1 N}
in
   Users.I={Record.toDictionary o(currentSite:S)}
   {IncrementHits S}
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
      {DecrementHits U.currentSite}
      U.currentSite := MS
      {IncrementHits MS}
   end
end