girthlenth.m

gen_H:利用随机pi矩阵排列而成的LDPC稀疏校验矩阵产生； girthlenth：计算产生的矩阵H的围长

%function y=girthlenth(H)
clear ,clc,
load Hdata;
cnodes=length(H(:,1));
bnodes=length(H(1,:));
dv=sum(H(:,1)); 
dc=sum(H(1,:));

for i=1:bnodes
    
    v_wayback=zeros(dv-1,1); m=1; 
    for j=1:cnodes
        if H(j,i)==1
            v_wayback(m,1)=j;
            m=m+1;
        end
    end
    k=2; bl=0;
    % v_wayback(1,1)所对应的bnodes;
    vp=v_wayback; 
    flag=true;
    while(flag)
       if k==2 
           vplen=1;
       else
           vplen=length(vp);
       end
       if mod(k,2)==0
           vtemp=zeros( vplen*(dc-1),1);
       else
           vtemp=zeros( vplen*(dv-1),1);
       end
       vtemplen=length(vtemp);
       
       q=1;
       for n=1:vplen
          
           if mod(k,2)==0   % from checknodes to find bitnodes;
              if k==2    % the first stage , the prior node is the node i;
                  vlastnode=i;
                  
              else
                  vlastnode=vlast(floor((n-1)/(dv-1))+1);% pay attention to the denominator;
              end
               for r=1:bnodes
                   if H(vp(n),r)==1 & r~=vlastnode  % avoid the prior node;
                       vtemp(q)=r;  q=q+1;
                   end
               end
           else    % from bitnodes to find checknodes;
              
              vlastnode=vlast(floor((n-1)/(dc-1))+1);
              for r=1:cnodes
                   if H(r,vp(n))==1 & r~=vlastnode
                       vtemp(q)=r;q=q+1;
                   end
               end
           end
       end
       vlast=vp; % keep the prior nodes;
       vp=vtemp; % go forward a step;
      
      k=k+1;  % keep the stage ;
      if mod(k,2)==0
          bl=findapath(vtemp,v_wayback); % judge if there exists a path now;
      end
      if bl==1
          girth(i)=k;  % find a path and break out of the current inner loop  (while);
          flag=false;
      end
    end
end

y=min(girth);