function R = sprandsym(arg1, density, rcond, kind)
%SPRANDSYM Sparse random symmetric matrix.
%   R = SPRANDSYM(S) is a symmetric random matrix whose lower triangle
%       and diagonal have the same structure as S.  The elements are
%       normally distributed, with mean 0 and variance 1.
%
%   R = SPRANDSYM(n,density) is a symmetric random, n-by-n, sparse 
%       matrix with approximately density*n*n nonzeros; each entry is
%       the sum of one or more normally distributed random samples.
%
%   R = SPRANDSYM(n,density,rc) also has a reciprocal condition number
%       equal to rc.  The distribution of entries is
%       nonuniform; it is roughly symmetric about 0; all are in [-1,1].
%
%       If rc is a VECTOR of length n, then R has eigenvalues rc.
%       Thus, if rc is a positive (nonnegative) vector then R will 
%       be positive (nonnegative) definite. In either case, R is
%       generated by random Jacobi rotations applied to a diagonal
%       matrix with the given eigenvalues or condition number.  It has
%       a great deal of topological and algebraic structure.
%
%   R = SPRANDSYM(n, density, rc, kind) is positive definite.
%
%       If kind = 1, R is generated by random Jacobi rotation of
%          a positive definite diagonal matrix.
%          R has the desired condition number exactly.
%
%       If kind = 2, R is a shifted sum of outer products.
%          R has the desired condition number only
%          approximately, but has less structure.
%
%   R = SPRANDSYM(S,[],rc,3) has the same structure as the MATRIX S
%       and approximate condition number 1/rc.
%
%   See also SPRANDN, SPRAND.

%   Rob Schreiber, RIACS, and Cleve Moler, MathWorks, 2/5/91.
%   Copyright (c) 1984-98 by The MathWorks, Inc.
%   $Revision: 5.7 $  $Date: 1997/11/21 23:44:50 $

if nargin > 4
  help sprandsym
  error('Too many or not enough input arguments.')
elseif nargin == 1
   S = arg1;
   [m,n] = size(S);
   [ii,jj] = find(S);  ii = ii(:);  %workaround find bug
   lower = find(ii > jj);  %  find lower triangle
   di = find( ii == jj );  %  find diagonal
   nd = length( di );
   nl = length( lower );
   randi = randn( nd, 1 );
   randl = randn( nl, 1 );
   i = [ii(lower); jj(lower); ii(di)];
   j = [jj(lower); ii(lower); ii(di)];
   r = [randl; randl; randi];
   R = sparse(i,j,r,m,m);
elseif nargin == 2,
   n = arg1;
   nl = round( (n*(n+1)/2) * density );
   rands = randn( nl, 1 );
   ii = fix( rand(nl, 1) * n ) + 1;
   jj = fix( rand(nl, 1) * n ) + 1;
   di = find( ii == jj );
   off = find( ii ~= jj );
   nd = length( di );
   no = length( off );
   randi = rands( 1:nd );
   rando = rands( nd+1 : nl );
   i = [ii(off); jj(off); ii(di)];
   j = [jj(off); ii(off); ii(di)];
   r = [rando; rando; randi];
   R = sparse(i,j,r,n,n);
else % nargin > 2
   lrc = length(rcond);
   if (lrc == 1)
      if( rcond <= 0 | rcond > 1 )
         error('rc must be > 0 and <= 1.')
      end
   end
   if(nargin == 3 )
      n = arg1;
      nnzwanted = round(density*n*n);
      if (lrc == 1)   %   rcond is given
         ratio = ((-1)^nargin) * (rcond ^ (1/(n-1)));
         anz = (ratio .^ ( 0:(n-1)));
      else            % eigenvalues explicitly given
         anz = rcond;
      end
      R = sparse(1:n,1:n, anz);
      % for nargin == 3,  ratio < 0
      % so we start with an indefinite R;
      nnzr = n;
      %
      %   random jacobi rotations
      %
      while ( nnzr < .95*nnzwanted )
          R = rjr(R);
          nnzr = nnz(R);
      end
   else  % nargin == 4
    if(kind==3)
%
%  add a 2-by-2 rank-1 for each off diagonal
%
      [ii,jj] = find(arg1); ii = ii(:);  % workaround
      [m, n] = size(arg1);
      lower = find(ii > jj);
      nl = length(lower);
      ratio = rcond ^ (1/(nl-1));
      ii = ii(lower);
      jj = jj(lower);
      i = zeros(n + 4*nl,1); 
      j = zeros(n + 4*nl,1); 
      anz = zeros(n + 4*nl,1); 
      [ignore,p] = sort(rand(n,1));
      i(1:n) = p;
      j(1:n) = p;
      en = sqrt((28/45) * (1 - ratio^(2*nl)) / (1 - ratio^2));
      anz(1:n) = rcond * en * ones(n,1);
      nnza = n;
      how_big = 1;
      for iouter = 1:nl
         rv = rand(2,1);
         op = how_big * rv * rv';
         op = op(:);
         ik = ii(iouter);
         jk = jj(iouter);
         i(nnza+1:nnza+4) = [ik; jk; ik; jk];
         j(nnza+1:nnza+4) = [ik; ik; jk; jk];
         anz(nnza+1:nnza+4) = op;
         nnza = nnza + 4;
         how_big = how_big * ratio;
      end
      R = sparse(i(1:nnza),j(1:nnza),anz(1:nnza));
    %  end of kind == 3
    else  %  kind == 1 or 2
      n = arg1;  [mn,nn]=size(n);
      if (mn > 1 | nn > 1)
         error('First argument must be a scalar');
      end
      nnzwanted = round(density*n*n);
      ratio = ((-1)^nargin) * (rcond ^ (1/(n-1)));
      if (kind == 1)
        R = sparse(1:n,1:n, (ratio .^ ( 0:(n-1)))  );
        nnzr = n;
        %   random jacobi rotations
        while ( nnzr < .95*nnzwanted )
            R = rjr(R);
            nnzr = nnz(R);
        end
      elseif(kind==2)   %  positive definite via sum of outer products
        MEMFAC = 2;
        NZF = 1.25;
        i = zeros(fix(nnzwanted*MEMFAC),1); 
        j = zeros(fix(nnzwanted*MEMFAC),1); 
        anz = zeros(fix(nnzwanted*MEMFAC),1); 
        nnza = 0;
        %
        %   To start, put a random nonzero in every column / row
        %
        [ignore,p] = sort(rand(n,1));
        i(1:n) = p;
        j(1:n) = p;
        %
        % en <- Frobenius norm and max eval estimate for sprandsym
        %  Theory:  A is the sum of 2n terms with weight ratio^(-j)
        %  each has (density*n*n)/2n nonzeros with variance 1/3
        %  assuming all are separate nonzers, we calculate, by summing
        %  the series that the expected value of ||A||^2 is approximately
        %  en = (n*density/6) * (1-ratio^4n)/(1-ratio^2).  Further,
        %  we approximate max(eig(a)) by ||A||.  We note that
        %  the initial diagonal value is a very close approx to min(eig(a)).
        %  Thus, with the choice rcond * en we get the right cond(a) on
        %  average.
        %
        en = n * density / 6;
        en = en * (1 - ratio^(4*n)) / (1 - ratio^2);
        en = sqrt(en);
        %
        anz(1:n) = rcond * en * ones(n,1);
        nnza = n;   nnztrue = n;
        %
        %   Sum of scaled symmetric outer products with random structure
        %
        how_big = 1;
        for iouter = 1:2*n, 
           nzpc = fix( 2 + sqrt((NZF*nnzwanted - nnza) / (2*n-iouter+1)));
           bk = sprandn(n,1, nzpc/n);
           [ik,jk,anzk] = find(how_big * (bk * bk'));
           lk = length(ik);
           i(nnza+1:nnza+lk) = ik; 
           j(nnza+1:nnza+lk) = jk; 
           anz(nnza+1:nnza+lk) = anzk;
           nnza = nnza + lk;
           nnztrue = nnztrue + (lk - nnz(bk));
           how_big = how_big * ratio;
           if (nnztrue > (nnzwanted*1.1)) break; end;
        end
        R = sparse(i(1:nnza),j(1:nnza),anz(1:nnza));
      end  %  kind == 2
    end   %  kind == 1 or 2
   end   %  nargin == 4
end      %  nargin > 2
R = .5 * (R + R');    % make absolutely symmetric
