def ADMM_ConstrBP(A,s,epsilon): """ Alternating Direction Method of Multipliers for Constrained Basis Pursuit Solve the problem: min ||x||_1 s.t. ||Ax-s||_2 <= epsilon i.e., find the minimum-norm vector s.t. the reconstruction error is under epsilon. The problem is transformed into: min ||y||_1 + 1/2||z-s||_2 s.t x = y, Ax = z """ (M,N) = A.shape x,y,z,u,v = zeros((N,1)), zeros((N,1)), zeros((M,1)), zeros((N,1)), zeros((M,1)) max_it = 200 gamma = epsilon sigma = epsilon k = 0 xks,yks,zks,uks,vks = [x],[y],[z],[u],[v] prim_resid = 100 dual_resid = 100 prim_epsi = 0 dual_epsi = 0 epsi_rel = 0.001 epsi_abs = 0.0001 while k < max_it: #TODO stopping criterion # Update x rhs = gamma*(yks[k] - uks[k]) + sigma*dot(A.T,(zks[k]-vks[k])) x_star = solve.lemma_solve(A,rhs,gamma,PL,U) xks.append(x_star) # Update y t = xks[k+1] + uks[k] y_star = sparse.shrinkage(t, 1/gamma) yks.append(y_star) # Update z b = dot(A,xks[k+1]) + vks[k] if linalg.norm(b-s) > epsilon: z_star = s + epsilon*(b - s)/linalg.norm(b-s) else: z_star = b zks.append(z_star) # Update u u_star = uks[k] + xks[k+1] - yks[k+1] uks.append(u_star) # Update v v_star = vks[k] + dot(A,xks[k+1]) - zks[k+1] vks.append(v_star) prim_resid = linalg.norm(xks[k+1] - yks[k+1]) dual_resid = linalg.norm(rho*(yks[k] - yks[k+1])) print k, linalg.norm(x_star,1) print linalg.norm(dot(A,x_star)-s) k += 1 prim_epsi = sqrt(N)*epsi_abs + epsi_rel*max(linalg.norm(xks[k]),linalg.norm(yks[k])) dual_epsi = sqrt(N)*epsi_abs + epsi_rel*linalg.norm(uks[k]) print prim_resid, dual_resid print prim_epsi, dual_epsi return xks[k]
def ADMM_BPDN(A,s,l, max_it=1000): """ Alternating Direction Method of Multipliers for Basis Pursuit Denoising Solve the problem: min 1/2 ||Ax-s||^2 + lambda*||x||_1 transformed into: min 1/2 ||Ax-s||^2 + lambda*||y||_1 s.t. x = y """ (M,N) = A.shape x,y,u = zeros((N,1)), zeros((N,1)), zeros((N,1)) rho = l k = 0 xks,yks,uks = [x],[y],[u] assert(s.shape == (M,1)) # Cached factorizations when M < N if M < N: C = dot(A,A.T) + rho*eye(M) PL,U = scipy.linalg.lu(C, permute_l=True) As = dot(A.T,s) prim_resid = 100 dual_resid = 100 prim_epsi = 0 dual_epsi = 0 epsi_rel = 0.001 epsi_abs = 0.0001 while k < max_it and (prim_resid >= prim_epsi or dual_resid >= dual_epsi): # Update x rhs = As + rho*(yks[k] - uks[k]) if M < N: x_star = solve.lemma_solve(A,rhs,rho,PL,U) else: x_star = solve.direct_solve(A,rhs,rho) xks.append(x_star) # Update y z = xks[k+1] + uks[k] y_star = sparse.shrinkage(z, l/rho) yks.append(y_star) # Update u u_star = uks[k] + xks[k+1] - yks[k+1] uks.append(u_star) prim_resid = linalg.norm(xks[k+1] - yks[k+1]) dual_resid = linalg.norm(rho*(yks[k] - yks[k+1])) #TODO change rho to balance primal/dual residual print k, 1/2*linalg.norm(dot(A,x_star)-s) + l*linalg.norm(x_star,1) k += 1 prim_epsi = sqrt(N)*epsi_abs + epsi_rel*max(linalg.norm(xks[k]),linalg.norm(yks[k])) dual_epsi = sqrt(N)*epsi_abs + epsi_rel*linalg.norm(uks[k]) print prim_resid, dual_resid print prim_epsi, dual_epsi return xks[k]