function C=adaptive(H,N,R)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% 功能:   自適應的比特分配:按照信道響應降序排列,子載波間爭奪比特和功率
%         步長為2
% 輸入:   H為信道的參數
%         N為子載波數
% 輸出:   C,得到的比特分配
% 作者：郭瑩，2004.3
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

dex=zeros(N,1);
x=zeros(N,1);
H=abs(H);
for a=1:N
   x(a)=sum((H>H(a)))+1;
   for t=1:20
   if dex(x(a))~=0
       x(a)=x(a)+1;
   else break
   end
  end
   dex(x(a))=a;
end

C=R/N*ones(N,1);
for m=1:N-1
    for n=N:-1:m+1
        if C(dex(m))>3
            break
        end
        if C(dex(n))==0
           continue
        end
        a=H(dex(m))^2;
        b=H(dex(n))^2; 
        switch (C(dex(n)))
          case 1 
             if a>2*b
              C(dex(m))=C(dex(m))+1;
              C(dex(n))=C(dex(n))-1;
            end
          case 2
            if a>3*b
            C(dex(m))=C(dex(m))+1;
            C(dex(n))=C(dex(n))-1;
           end
        end
    end 
end