diff pyCSalgos/BP/l1qc.py @ 2:735a0e24575c

Organized folders: added tests, apps, matlab, docs folders. Added __init__.py files

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/pyCSalgos/BP/l1qc.py	Fri Oct 21 13:53:49 2011 +0000
@@ -0,0 +1,705 @@
+
+import numpy as np
+import scipy.linalg
+import math
+
+
+#function [x, res, iter] = cgsolve(A, b, tol, maxiter, verbose)
+def cgsolve(A, b, tol, maxiter, verbose=1):
+    # Solve a symmetric positive definite system Ax = b via conjugate gradients.
+    #
+    # Usage: [x, res, iter] = cgsolve(A, b, tol, maxiter, verbose)
+    #
+    # A - Either an NxN matrix, or a function handle.
+    #
+    # b - N vector
+    #
+    # tol - Desired precision.  Algorithm terminates when 
+    #    norm(Ax-b)/norm(b) < tol .
+    #
+    # maxiter - Maximum number of iterations.
+    #
+    # verbose - If 0, do not print out progress messages.
+    #    If and integer greater than 0, print out progress every 'verbose' iters.
+    #
+    # Written by: Justin Romberg, Caltech
+    # Email: [e-mail]
+    # Created: October 2005
+    #
+
+    #---------------------
+    # Original Matab code:
+    #        
+    #if (nargin < 5), verbose = 1; end
+    #
+    #implicit = isa(A,'function_handle');
+    #
+    #x = zeros(length(b),1);
+    #r = b;
+    #d = r;
+    #delta = r'*r;
+    #delta0 = b'*b;
+    #numiter = 0;
+    #bestx = x;
+    #bestres = sqrt(delta/delta0); 
+    #while ((numiter < maxiter) && (delta > tol^2*delta0))
+    #
+    #  # q = A*d
+    #  if (implicit), q = A(d);  else  q = A*d;  end
+    # 
+    #  alpha = delta/(d'*q);
+    #  x = x + alpha*d;
+    #  
+    #  if (mod(numiter+1,50) == 0)
+    #    # r = b - Aux*x
+    #    if (implicit), r = b - A(x);  else  r = b - A*x;  end
+    #  else
+    #    r = r - alpha*q;
+    #  end
+    #  
+    #  deltaold = delta;
+    #  delta = r'*r;
+    #  beta = delta/deltaold;
+    #  d = r + beta*d;
+    #  numiter = numiter + 1;
+    #  if (sqrt(delta/delta0) < bestres)
+    #    bestx = x;
+    #    bestres = sqrt(delta/delta0);
+    #  end    
+    #  
+    #  if ((verbose) && (mod(numiter,verbose)==0))
+    #    disp(sprintf('cg: Iter = #d, Best residual = #8.3e, Current residual = #8.3e', ...
+    #      numiter, bestres, sqrt(delta/delta0)));
+    #  end
+    #  
+    #end
+    #
+    #if (verbose)
+    #  disp(sprintf('cg: Iterations = #d, best residual = #14.8e', numiter, bestres));
+    #end
+    #x = bestx;
+    #res = bestres;
+    #iter = numiter;
+
+    # End of original Matab code
+    #----------------------------
+    
+    #if (nargin < 5), verbose = 1; end
+    # Optional argument
+    
+    #implicit = isa(A,'function_handle');
+    if hasattr(A, '__call__'):
+        implicit = True
+    else:
+        implicit = False
+    
+    x = np.zeros(b.size)
+    r = b.copy()
+    d = r.copy()
+    delta = np.vdot(r,r)
+    delta0 = np.vdot(b,b)
+    numiter = 0
+    bestx = x.copy()
+    bestres = math.sqrt(delta/delta0)
+    while (numiter < maxiter) and (delta > tol**2*delta0):
+    
+      # q = A*d
+      #if (implicit), q = A(d);  else  q = A*d;  end
+      if implicit:
+          q = A(d)
+      else:
+          q = np.dot(A,d)
+     
+      alpha = delta/np.vdot(d,q)
+      x = x + alpha*d
+      
+      if divmod(numiter+1,50)[1] == 0:
+        # r = b - Aux*x
+        #if (implicit), r = b - A(x);  else  r = b - A*x;  end
+        if implicit:
+            r = b - A(x)
+        else:
+            r = b - np.dot(A,x)
+      else:
+        r = r - alpha*q
+      #end
+      
+      deltaold = delta;
+      delta = np.vdot(r,r)
+      beta = delta/deltaold;
+      d = r + beta*d
+      numiter = numiter + 1
+      if (math.sqrt(delta/delta0) < bestres):
+        bestx = x.copy()
+        bestres = math.sqrt(delta/delta0)
+      #end    
+      
+      if ((verbose) and (divmod(numiter,verbose)[1]==0)):
+        #disp(sprintf('cg: Iter = #d, Best residual = #8.3e, Current residual = #8.3e', ...
+        #  numiter, bestres, sqrt(delta/delta0)));
+        print 'cg: Iter = ',numiter,', Best residual = ',bestres,', Current residual = ',math.sqrt(delta/delta0)
+      #end
+      
+    #end
+    
+    if (verbose):
+      #disp(sprintf('cg: Iterations = #d, best residual = #14.8e', numiter, bestres));
+      print 'cg: Iterations = ',numiter,', best residual = ',bestres
+    #end
+    x = bestx.copy()
+    res = bestres
+    iter = numiter
+    
+    return x,res,iter
+
+
+
+#function [xp, up, niter] = l1qc_newton(x0, u0, A, At, b, epsilon, tau, newtontol, newtonmaxiter, cgtol, cgmaxiter) 
+def l1qc_newton(x0, u0, A, At, b, epsilon, tau, newtontol, newtonmaxiter, cgtol, cgmaxiter):
+    # Newton algorithm for log-barrier subproblems for l1 minimization
+    # with quadratic constraints.
+    #
+    # Usage: 
+    # [xp,up,niter] = l1qc_newton(x0, u0, A, At, b, epsilon, tau, 
+    #                             newtontol, newtonmaxiter, cgtol, cgmaxiter)
+    #
+    # x0,u0 - starting points
+    #
+    # A - Either a handle to a function that takes a N vector and returns a K 
+    #     vector , or a KxN matrix.  If A is a function handle, the algorithm
+    #     operates in "largescale" mode, solving the Newton systems via the
+    #     Conjugate Gradients algorithm.
+    #
+    # At - Handle to a function that takes a K vector and returns an N vector.
+    #      If A is a KxN matrix, At is ignored.
+    #
+    # b - Kx1 vector of observations.
+    #
+    # epsilon - scalar, constraint relaxation parameter
+    #
+    # tau - Log barrier parameter.
+    #
+    # newtontol - Terminate when the Newton decrement is <= newtontol.
+    #         Default = 1e-3.
+    #
+    # newtonmaxiter - Maximum number of iterations.
+    #         Default = 50.
+    #
+    # cgtol - Tolerance for Conjugate Gradients; ignored if A is a matrix.
+    #     Default = 1e-8.
+    #
+    # cgmaxiter - Maximum number of iterations for Conjugate Gradients; ignored
+    #     if A is a matrix.
+    #     Default = 200.
+    #
+    # Written by: Justin Romberg, Caltech
+    # Email: [e-mail]
+    # Created: October 2005
+    #
+    
+    #---------------------
+    # Original Matab code:
+    
+    ## check if the matrix A is implicit or explicit
+    #largescale = isa(A,'function_handle');
+    #
+    ## line search parameters
+    #alpha = 0.01;
+    #beta = 0.5;  
+    #
+    #if (~largescale), AtA = A'*A; end
+    #
+    ## initial point
+    #x = x0;
+    #u = u0;
+    #if (largescale), r = A(x) - b; else  r = A*x - b; end
+    #fu1 = x - u;
+    #fu2 = -x - u;
+    #fe = 1/2*(r'*r - epsilon^2);
+    #f = sum(u) - (1/tau)*(sum(log(-fu1)) + sum(log(-fu2)) + log(-fe));
+    #
+    #niter = 0;
+    #done = 0;
+    #while (~done)
+    #  
+    #  if (largescale), atr = At(r); else  atr = A'*r; end
+    #  
+    #  ntgz = 1./fu1 - 1./fu2 + 1/fe*atr;
+    #  ntgu = -tau - 1./fu1 - 1./fu2;
+    #  gradf = -(1/tau)*[ntgz; ntgu];
+    #  
+    #  sig11 = 1./fu1.^2 + 1./fu2.^2;
+    #  sig12 = -1./fu1.^2 + 1./fu2.^2;
+    #  sigx = sig11 - sig12.^2./sig11;
+    #    
+    #  w1p = ntgz - sig12./sig11.*ntgu;
+    #  if (largescale)
+    #    h11pfun = @(z) sigx.*z - (1/fe)*At(A(z)) + 1/fe^2*(atr'*z)*atr;
+    #    [dx, cgres, cgiter] = cgsolve(h11pfun, w1p, cgtol, cgmaxiter, 0);
+    #    if (cgres > 1/2)
+    #      disp('Cannot solve system.  Returning previous iterate.  (See Section 4 of notes for more information.)');
+    #      xp = x;  up = u;
+    #      return
+    #    end
+    #    Adx = A(dx);
+    #  else
+    #    H11p = diag(sigx) - (1/fe)*AtA + (1/fe)^2*atr*atr';
+    #    opts.POSDEF = true; opts.SYM = true;
+    #    [dx,hcond] = linsolve(H11p, w1p, opts);
+    #    if (hcond < 1e-14)
+    #      disp('Matrix ill-conditioned.  Returning previous iterate.  (See Section 4 of notes for more information.)');
+    #      xp = x;  up = u;
+    #      return
+    #    end
+    #    Adx = A*dx;
+    #  end
+    #  du = (1./sig11).*ntgu - (sig12./sig11).*dx;  
+    # 
+    #  # minimum step size that stays in the interior
+    #  ifu1 = find((dx-du) > 0); ifu2 = find((-dx-du) > 0);
+    #  aqe = Adx'*Adx;   bqe = 2*r'*Adx;   cqe = r'*r - epsilon^2;
+    #  smax = min(1,min([...
+    #    -fu1(ifu1)./(dx(ifu1)-du(ifu1)); -fu2(ifu2)./(-dx(ifu2)-du(ifu2)); ...
+    #    (-bqe+sqrt(bqe^2-4*aqe*cqe))/(2*aqe)
+    #    ]));
+    #  s = (0.99)*smax;
+    #  
+    #  # backtracking line search
+    #  suffdec = 0;
+    #  backiter = 0;
+    #  while (~suffdec)
+    #    xp = x + s*dx;  up = u + s*du;  rp = r + s*Adx;
+    #    fu1p = xp - up;  fu2p = -xp - up;  fep = 1/2*(rp'*rp - epsilon^2);
+    #    fp = sum(up) - (1/tau)*(sum(log(-fu1p)) + sum(log(-fu2p)) + log(-fep));
+    #    flin = f + alpha*s*(gradf'*[dx; du]);
+    #    suffdec = (fp <= flin);
+    #    s = beta*s;
+    #    backiter = backiter + 1;
+    #    if (backiter > 32)
+    #      disp('Stuck on backtracking line search, returning previous iterate.  (See Section 4 of notes for more information.)');
+    #      xp = x;  up = u;
+    #      return
+    #    end
+    #  end
+    #  
+    #  # set up for next iteration
+    #  x = xp; u = up;  r = rp;
+    #  fu1 = fu1p;  fu2 = fu2p;  fe = fep;  f = fp;
+    #  
+    #  lambda2 = -(gradf'*[dx; du]);
+    #  stepsize = s*norm([dx; du]);
+    #  niter = niter + 1;
+    #  done = (lambda2/2 < newtontol) | (niter >= newtonmaxiter);
+    #  
+    #  disp(sprintf('Newton iter = #d, Functional = #8.3f, Newton decrement = #8.3f, Stepsize = #8.3e', ...
+    #    niter, f, lambda2/2, stepsize));
+    #  if (largescale)
+    #    disp(sprintf('                CG Res = #8.3e, CG Iter = #d', cgres, cgiter));
+    #  else
+    #    disp(sprintf('                  H11p condition number = #8.3e', hcond));
+    #  end
+    #      
+    #end
+    
+    # End of original Matab code
+    #----------------------------
+    
+    # check if the matrix A is implicit or explicit
+    #largescale = isa(A,'function_handle');
+    if hasattr(A, '__call__'):
+        largescale = True
+    else:
+        largescale = False    
+    
+    # line search parameters
+    alpha = 0.01
+    beta = 0.5
+    
+    #if (~largescale), AtA = A'*A; end
+    if not largescale:
+        AtA = np.dot(A.T,A)
+    
+    # initial point
+    x = x0.copy()
+    u = u0.copy()
+    #if (largescale), r = A(x) - b; else  r = A*x - b; end
+    if largescale:
+        r = A(x) - b
+    else:
+        r = np.dot(A,x) - b
+        
+    fu1 = x - u
+    fu2 = -x - u
+    fe = 1/2*(np.vdot(r,r) - epsilon**2)
+    f = u.sum() - (1/tau)*(np.log(-fu1).sum() + np.log(-fu2).sum() + math.log(-fe))
+    
+    niter = 0
+    done = 0
+    while not done:
+      
+      #if (largescale), atr = At(r); else  atr = A'*r; end
+      if largescale:
+          atr = At(r)
+      else:
+          atr = np.dot(A.T,r)
+      
+      #ntgz = 1./fu1 - 1./fu2 + 1/fe*atr;
+      ntgz = 1.0/fu1 - 1.0/fu2 + 1.0/fe*atr
+      #ntgu = -tau - 1./fu1 - 1./fu2;
+      ntgu = -tau - 1.0/fu1 - 1.0/fu2
+      #gradf = -(1/tau)*[ntgz; ntgu];
+      gradf = -(1.0/tau)*np.concatenate((ntgz, ntgu),0)
+      
+      #sig11 = 1./fu1.^2 + 1./fu2.^2;
+      sig11 = 1.0/(fu1**2) + 1.0/(fu2**2)
+      #sig12 = -1./fu1.^2 + 1./fu2.^2;
+      sig12 = -1.0/(fu1**2) + 1.0/(fu2**2)
+      #sigx = sig11 - sig12.^2./sig11;
+      sigx = sig11 - (sig12**2)/sig11
+        
+      #w1p = ntgz - sig12./sig11.*ntgu;
+      w1p = ntgz - sig12/sig11*ntgu
+      if largescale:
+        #h11pfun = @(z) sigx.*z - (1/fe)*At(A(z)) + 1/fe^2*(atr'*z)*atr;
+        h11pfun = lambda z: sigx*z - (1.0/fe)*At(A(z)) + 1.0/(fe**2)*np.dot(np.dot(atr.T,z),atr)
+        dx,cgres,cgiter = cgsolve(h11pfun, w1p, cgtol, cgmaxiter, 0)
+        if (cgres > 1/2):
+          print 'Cannot solve system.  Returning previous iterate.  (See Section 4 of notes for more information.)'
+          xp = x.copy()
+          up = u.copy()
+          return xp,up,niter
+        #end
+        Adx = A(dx)
+      else:
+        #H11p = diag(sigx) - (1/fe)*AtA + (1/fe)^2*atr*atr';
+        H11p = np.diag(sigx) - (1/fe)*AtA + (1/fe)**2*np.dot(atr,atr.T)
+        #opts.POSDEF = true; opts.SYM = true;
+        #[dx,hcond] = linsolve(H11p, w1p, opts);
+        #if (hcond < 1e-14)
+        #  disp('Matrix ill-conditioned.  Returning previous iterate.  (See Section 4 of notes for more information.)');
+        #  xp = x;  up = u;
+        #  return
+        #end
+        try:
+            dx = scipy.linalg.solve(H11p, w1p, sym_pos=True)
+            hcond = 1.0/scipy.linalg.cond(H11p)
+        except scipy.linalg.LinAlgError:
+            print 'Matrix ill-conditioned.  Returning previous iterate.  (See Section 4 of notes for more information.)'
+            xp = x.copy()
+            up = u.copy()
+            return xp,up,niter
+        if hcond < 1e-14:
+            print 'Matrix ill-conditioned.  Returning previous iterate.  (See Section 4 of notes for more information.)'
+            xp = x.copy()
+            up = u.copy()
+            return xp,up,niter
+        
+        #Adx = A*dx;
+        Adx = np.dot(A,dx)
+      #end
+      #du = (1./sig11).*ntgu - (sig12./sig11).*dx;  
+      du = (1.0/sig11)*ntgu - (sig12/sig11)*dx;
+     
+      # minimum step size that stays in the interior
+      #ifu1 = find((dx-du) > 0); ifu2 = find((-dx-du) > 0);
+      ifu1 = np.nonzero((dx-du)>0)
+      ifu2 = np.nonzero((-dx-du)>0)
+      #aqe = Adx'*Adx;   bqe = 2*r'*Adx;   cqe = r'*r - epsilon^2;
+      aqe = np.dot(Adx.T,Adx)
+      bqe = 2*np.dot(r.T,Adx)
+      cqe = np.vdot(r,r) - epsilon**2
+      #smax = min(1,min([...
+      #  -fu1(ifu1)./(dx(ifu1)-du(ifu1)); -fu2(ifu2)./(-dx(ifu2)-du(ifu2)); ...
+      #  (-bqe+sqrt(bqe^2-4*aqe*cqe))/(2*aqe)
+      #  ]));
+      smax = min(1,np.concatenate((-fu1[ifu1]/(dx[ifu1]-du[ifu1]) , -fu2[ifu2]/(-dx[ifu2]-du[ifu2]) , (-bqe + math.sqrt(bqe**2-4*aqe*cqe))/(2*aqe)),0).min())
+        
+      s = (0.99)*smax
+      
+      # backtracking line search
+      suffdec = 0
+      backiter = 0
+      while not suffdec:
+        #xp = x + s*dx;  up = u + s*du;  rp = r + s*Adx;
+        xp = x + s*dx
+        up = u + s*du
+        rp = r + s*Adx
+        #fu1p = xp - up;  fu2p = -xp - up;  fep = 1/2*(rp'*rp - epsilon^2);
+        fu1p = xp - up
+        fu2p = -xp - up
+        fep = 0.5*(np.vdot(r,r) - epsilon**2)
+        #fp = sum(up) - (1/tau)*(sum(log(-fu1p)) + sum(log(-fu2p)) + log(-fep));
+        fp = up.sum() - (1.0/tau)*(np.log(-fu1p).sum() + np.log(-fu2p).sum() + log(-fep))
+        #flin = f + alpha*s*(gradf'*[dx; du]);
+        flin = f + alpha*s*np.dot(gradf.T , np.concatenate((dx,du),0))
+        #suffdec = (fp <= flin);
+        if fp <= flin:
+            suffdec = True
+        else:
+            suffdec = False
+        
+        s = beta*s
+        backiter = backiter + 1
+        if (backiter > 32):
+          print 'Stuck on backtracking line search, returning previous iterate.  (See Section 4 of notes for more information.)'
+          xp = x.copy()
+          up = u.copy()
+          return xp,up,niter
+        #end
+      #end
+      
+      # set up for next iteration
+      #x = xp; u = up;  r = rp;
+      x = xp.copy()
+      u = up.copy()
+      r = rp.copy()
+      #fu1 = fu1p;  fu2 = fu2p;  fe = fep;  f = fp;
+      fu1 = fu1p.copy()
+      fu2 = fu2p.copy()
+      fe = fep
+      f = fp
+      
+      #lambda2 = -(gradf'*[dx; du]);
+      lambda2 = -np.dot(gradf , np.concatenate((dx,du),0))
+      #stepsize = s*norm([dx; du]);
+      stepsize = s * np.linalg.norm(np.concatenate((dx,du),0))
+      niter = niter + 1
+      #done = (lambda2/2 < newtontol) | (niter >= newtonmaxiter);
+      if lambda2/2.0 < newtontol or niter >= newtonmaxiter:
+          done = 1
+      else:
+          done = 0
+      
+      #disp(sprintf('Newton iter = #d, Functional = #8.3f, Newton decrement = #8.3f, Stepsize = #8.3e', ...
+      print 'Newton iter = ',niter,', Functional = ',f,', Newton decrement = ',lambda2/2.0,', Stepsize = ',stepsize
+
+      if largescale:
+        print '                CG Res = ',cgres,', CG Iter = ',cgiter
+      else:
+        print '                  H11p condition number = ',hcond
+      #end
+          
+    #end
+    return xp,up,niter
+
+#function xp = l1qc_logbarrier(x0, A, At, b, epsilon, lbtol, mu, cgtol, cgmaxiter)
+def l1qc_logbarrier(x0, A, At, b, epsilon, lbtol=1e-3, mu=10, cgtol=1e-8, cgmaxiter=200):
+    # Solve quadratically constrained l1 minimization:
+    # min ||x||_1   s.t.  ||Ax - b||_2 <= \epsilon
+    #
+    # Reformulate as the second-order cone program
+    # min_{x,u}  sum(u)   s.t.    x - u <= 0,
+    #                            -x - u <= 0,
+    #      1/2(||Ax-b||^2 - \epsilon^2) <= 0
+    # and use a log barrier algorithm.
+    #
+    # Usage:  xp = l1qc_logbarrier(x0, A, At, b, epsilon, lbtol, mu, cgtol, cgmaxiter)
+    #
+    # x0 - Nx1 vector, initial point.
+    #
+    # A - Either a handle to a function that takes a N vector and returns a K 
+    #     vector , or a KxN matrix.  If A is a function handle, the algorithm
+    #     operates in "largescale" mode, solving the Newton systems via the
+    #     Conjugate Gradients algorithm.
+    #
+    # At - Handle to a function that takes a K vector and returns an N vector.
+    #      If A is a KxN matrix, At is ignored.
+    #
+    # b - Kx1 vector of observations.
+    #
+    # epsilon - scalar, constraint relaxation parameter
+    #
+    # lbtol - The log barrier algorithm terminates when the duality gap <= lbtol.
+    #         Also, the number of log barrier iterations is completely
+    #         determined by lbtol.
+    #         Default = 1e-3.
+    #
+    # mu - Factor by which to increase the barrier constant at each iteration.
+    #      Default = 10.
+    #
+    # cgtol - Tolerance for Conjugate Gradients; ignored if A is a matrix.
+    #     Default = 1e-8.
+    #
+    # cgmaxiter - Maximum number of iterations for Conjugate Gradients; ignored
+    #     if A is a matrix.
+    #     Default = 200.
+    #
+    # Written by: Justin Romberg, Caltech
+    # Email: [e-mail]
+    # Created: October 2005
+    #
+
+    #---------------------
+    # Original Matab code:
+    
+    #largescale = isa(A,'function_handle');
+    #
+    #if (nargin < 6), lbtol = 1e-3; end
+    #if (nargin < 7), mu = 10; end
+    #if (nargin < 8), cgtol = 1e-8; end
+    #if (nargin < 9), cgmaxiter = 200; end
+    #
+    #newtontol = lbtol;
+    #newtonmaxiter = 50;
+    #
+    #N = length(x0);
+    #
+    ## starting point --- make sure that it is feasible
+    #if (largescale)
+    #  if (norm(A(x0)-b) > epsilon)
+    #    disp('Starting point infeasible; using x0 = At*inv(AAt)*y.');
+    #    AAt = @(z) A(At(z));
+    #    w = cgsolve(AAt, b, cgtol, cgmaxiter, 0);
+    #    if (cgres > 1/2)
+    #      disp('A*At is ill-conditioned: cannot find starting point');
+    #      xp = x0;
+    #      return;
+    #    end
+    #    x0 = At(w);
+    #  end
+    #else
+    #  if (norm(A*x0-b) > epsilon)
+    #    disp('Starting point infeasible; using x0 = At*inv(AAt)*y.');
+    #    opts.POSDEF = true; opts.SYM = true;
+    #    [w, hcond] = linsolve(A*A', b, opts);
+    #    if (hcond < 1e-14)
+    #      disp('A*At is ill-conditioned: cannot find starting point');
+    #      xp = x0;
+    #      return;
+    #    end
+    #    x0 = A'*w;
+    #  end  
+    #end
+    #x = x0;
+    #u = (0.95)*abs(x0) + (0.10)*max(abs(x0));
+    #
+    #disp(sprintf('Original l1 norm = #.3f, original functional = #.3f', sum(abs(x0)), sum(u)));
+    #
+    ## choose initial value of tau so that the duality gap after the first
+    ## step will be about the origial norm
+    #tau = max((2*N+1)/sum(abs(x0)), 1);
+    #                                                                                                                          
+    #lbiter = ceil((log(2*N+1)-log(lbtol)-log(tau))/log(mu));
+    #disp(sprintf('Number of log barrier iterations = #d\n', lbiter));
+    #
+    #totaliter = 0;
+    #
+    ## Added by Nic
+    #if lbiter == 0
+    #    xp = zeros(size(x0));
+    #end
+    #
+    #for ii = 1:lbiter
+    #
+    #  [xp, up, ntiter] = l1qc_newton(x, u, A, At, b, epsilon, tau, newtontol, newtonmaxiter, cgtol, cgmaxiter);
+    #  totaliter = totaliter + ntiter;
+    #  
+    #  disp(sprintf('\nLog barrier iter = #d, l1 = #.3f, functional = #8.3f, tau = #8.3e, total newton iter = #d\n', ...
+    #    ii, sum(abs(xp)), sum(up), tau, totaliter));
+    #  
+    #  x = xp;
+    #  u = up;
+    # 
+    #  tau = mu*tau;
+    #  
+    #end
+    #
+    # End of original Matab code
+    #----------------------------
+    
+    #largescale = isa(A,'function_handle');
+    if hasattr(A, '__call__'):
+        largescale = True
+    else:
+        largescale = False
+    
+    #    if (nargin < 6), lbtol = 1e-3; end
+    #    if (nargin < 7), mu = 10; end
+    #    if (nargin < 8), cgtol = 1e-8; end
+    #    if (nargin < 9), cgmaxiter = 200; end
+    # Nic: added them as optional parameteres
+    
+    newtontol = lbtol
+    newtonmaxiter = 50
+    
+    #N = length(x0);
+    N = x0.size()
+    
+    # starting point --- make sure that it is feasible
+    if largescale:
+      if np.linalg.norm(A(x0) - b) > epsilon:
+        print 'Starting point infeasible; using x0 = At*inv(AAt)*y.'
+        #AAt = @(z) A(At(z));
+        AAt = lambda z: A(At(z))
+        # TODO: implement cgsolve
+        w,cgres,cgiter = cgsolve(AAt, b, cgtol, cgmaxiter, 0)
+        if (cgres > 1/2):
+          print 'A*At is ill-conditioned: cannot find starting point'
+          xp = x0.copy()
+          return xp
+        #end
+        x0 = At(w)
+      #end
+    else:
+      if np.linalg.norm( np.dot(A,x0) - b ) > epsilon:
+        print 'Starting point infeasible; using x0 = At*inv(AAt)*y.'
+        #opts.POSDEF = true; opts.SYM = true;
+        #[w, hcond] = linsolve(A*A', b, opts);
+        #if (hcond < 1e-14)
+        #  disp('A*At is ill-conditioned: cannot find starting point');
+        #  xp = x0;
+        #  return;
+        #end
+        try:
+            w = scipy.linalg.solve(np.dot(A,A.T), b, sym_pos=True)
+            hcond = 1.0/scipy.linalg.cond(np.dot(A,A.T))
+        except scipy.linalg.LinAlgError:
+            print 'A*At is ill-conditioned: cannot find starting point'
+            xp = x0.copy()
+            return xp
+        if hcond < 1e-14:
+            print 'A*At is ill-conditioned: cannot find starting point'
+            xp = x0.copy()
+            return xp           
+        #x0 = A'*w;
+        x0 = np.dot(A.T, w)
+      #end  
+    #end
+    x = x0.copy()
+    u = (0.95)*np.abs(x0) + (0.10)*np.abs(x0).max()
+    
+    #disp(sprintf('Original l1 norm = #.3f, original functional = #.3f', sum(abs(x0)), sum(u)));
+    print 'Original l1 norm = ',np.abs(x0).sum(),'original functional = ',u.sum()
+    
+    # choose initial value of tau so that the duality gap after the first
+    # step will be about the origial norm
+    tau = max(((2*N+1)/np.abs(x0).sum()), 1)
+                                                                                                                              
+    lbiter = math.ceil((math.log(2*N+1)-math.log(lbtol)-math.log(tau))/math.log(mu))
+    #disp(sprintf('Number of log barrier iterations = #d\n', lbiter));
+    print 'Number of log barrier iterations = ',lbiter
+    
+    totaliter = 0
+    
+    # Added by Nic, to fix some crashing
+    if lbiter == 0:
+        xp = np.zeros(x0.size)
+    #end
+    
+    #for ii = 1:lbiter
+    for ii in np.arange(lbiter):
+    
+      xp,up,ntiter = l1qc_newton(x, u, A, At, b, epsilon, tau, newtontol, newtonmaxiter, cgtol, cgmaxiter)
+      totaliter = totaliter + ntiter
+      
+      #disp(sprintf('\nLog barrier iter = #d, l1 = #.3f, functional = #8.3f, tau = #8.3e, total newton iter = #d\n', ...
+      #  ii, sum(abs(xp)), sum(up), tau, totaliter));
+      print 'Log barrier iter = ',ii,', l1 = ',np.abs(xp).sum(),', functional = ',up.sum(),', tau = ',tau,', total newton iter = ',totaliter
+      x = xp.copy()
+      u = up.copy()
+     
+      tau = mu*tau
+      
+    #end
+    return xp
+