ADMM.hpp

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#ifndef ZONOOPT_ADMM_HPP_
#define ZONOOPT_ADMM_HPP_
 
#include <vector>
#include <chrono>
#include <stdexcept>
#include <iostream>
#include <sstream>
#include <memory>
#include <set>
#include <cmath>
#include <random>
#include <atomic>
 
#include "Eigen/Dense"
#include "Eigen/Sparse"
#include "CholeskyUtilities.hpp"
#include "Intervals.hpp"
#include "SparseMatrixUtilities.hpp"
#include "SolverDataStructures.hpp"
 
/* 
    Primary reference: 
    Boyd, Stephen, et al. 
    "Distributed optimization and statistical learning via the alternating direction method of multipliers." 
    Foundations and Trends® in Machine learning 3.1 (2011): 1-122.
*/
 
namespace ZonoOpt::detail {
    struct ADMM_data : std::enable_shared_from_this<ADMM_data>
    {
        Eigen::SparseMatrix<zono_float> P, A, AT;
        Eigen::SparseMatrix<zono_float, Eigen::RowMajor> A_rm;
        Eigen::Vector<zono_float, -1> q, b;
        Eigen::Vector<zono_float, 1> c;
        LDLT_data ldlt_data_M, ldlt_data_AAT;
        int n_x, n_cons;
        zono_float sqrt_n_x;
        std::shared_ptr<Box> x_box;
        OptSettings settings;
 
        // constructor
        ADMM_data() = default;
 
        ADMM_data(const Eigen::SparseMatrix<zono_float>& P, const Eigen::Vector<zono_float, -1>& q,
            const Eigen::SparseMatrix<zono_float>& A, const Eigen::Vector<zono_float, -1>& b,
            const Eigen::Vector<zono_float, -1>& x_l, const Eigen::Vector<zono_float, -1>& x_u,
            const zono_float c=0, const OptSettings& settings= OptSettings())
        {
            set(P, q, A, b, x_l, x_u, c, settings);
        }
 
        // set method
        void set(const Eigen::SparseMatrix<zono_float>& P, const Eigen::Vector<zono_float, -1>& q,
            const Eigen::SparseMatrix<zono_float>& A, const Eigen::Vector<zono_float, -1>& b,
            const Eigen::Vector<zono_float, -1>& x_l, const Eigen::Vector<zono_float, -1>& x_u,
            const zono_float c=0, const OptSettings& settings= OptSettings())
        {
            this->P = P;
            this->q = q;
            this->A = A;
            this->AT = A.transpose();
            this->A_rm = A;
            this->b = b;
            this->c(0) = c;
 
            this->n_x = static_cast<int>(P.rows());
            this->n_cons = static_cast<int>(A.rows());
            this->sqrt_n_x = std::sqrt(static_cast<zono_float>(this->n_x));
 
            this->x_box = std::make_shared<Box>(x_l, x_u);
 
            if (!settings.settings_valid()) throw std::invalid_argument("ADMM data: invalid settings.");
            this->settings = settings;
        }
 
        // clone method
        ADMM_data* clone() const
        {
            const auto new_data = new ADMM_data(*this);
            new_data->x_box = std::make_shared<Box>(*this->x_box);
            return new_data;
        }
    };
 
    // print function
    inline void print_str(std::stringstream &ss)
    {
        #ifdef IS_PYTHON_ENV
        py::print(ss.str());
        #else
        std::cout << ss.str() << std::endl;
        #endif
        ss.str("");
    }
 
    // utilities
 
    class ADMM_solver
    {
    public:
 
        explicit ADMM_solver(const ADMM_data& data)
        {
            // store data and settings
            this->data = std::make_shared<ADMM_data>(data);
            this->eps_prim = data.settings.eps_prim;
            this->eps_dual = data.settings.eps_dual;
 
            // flags
            this->is_warmstarted = false;
        }
 
        explicit ADMM_solver(const std::shared_ptr<ADMM_data>& data)
        {
            // store data and settings
            this->data = data;
            this->eps_prim = data->settings.eps_prim;
            this->eps_dual = data->settings.eps_dual;
 
            // flags
            this->is_warmstarted = false;
        }
 
        ADMM_solver(const ADMM_solver& other)
        {
            this->data = other.data;
            this->x0 = other.x0;
            this->u0 = other.u0;
            this->is_warmstarted = other.is_warmstarted;
            this->eps_dual = other.eps_dual;
            this->eps_prim = other.eps_prim;
        }
 
        virtual ~ADMM_solver() = default;
 
        virtual void warmstart(const Eigen::Vector<zono_float, -1>& x0,
            const Eigen::Vector<zono_float, -1>& u0)
        {
            // copy in warm start variables
            this->x0 = x0;
            this->u0 = u0;
 
            // set flag
            this->is_warmstarted = true;
        }
 
        virtual void factorize()
        {
            auto t0 = std::chrono::high_resolution_clock::now();
            double run_time;
            std::stringstream ss;
            if (!this->data->ldlt_data_M.factorized)
            {
                t0 = std::chrono::high_resolution_clock::now();
                this->factorize_M();
                if (this->data->settings.verbose)
                {
                    run_time = 1e-6 * static_cast<double>(std::chrono::duration_cast<std::chrono::microseconds>(
                        std::chrono::high_resolution_clock::now() - t0).count());
                    ss << "M factorization time = " << run_time << " sec";
                    print_str(ss);
                }
            }
            if (!this->data->ldlt_data_AAT.factorized)
            {
                t0 = std::chrono::high_resolution_clock::now();
                this->factorize_AAT();
                if (this->data->settings.verbose)
                {
                    run_time = 1e-6 * static_cast<double>(std::chrono::duration_cast<std::chrono::microseconds>(
                        std::chrono::high_resolution_clock::now() - t0).count());
                    ss << "A*A^T factorization time = " << run_time << " sec";
                    print_str(ss);
                }
            }
        }
 
        OptSolution solve(std::atomic<bool>* stop)
        {
            // declare solution
            OptSolution solution;
 
            // startup
            if (const bool contractor_feasible = this->startup(*this->data->x_box, solution); !contractor_feasible)
            {
                return solution;
            }
 
            // solve
            solve_core(*this->data->x_box, solution, stop);
            return solution;
        }
        OptSolution solve() { return this->solve(nullptr); }
 
    protected:
 
        // protected fields
        std::shared_ptr<ADMM_data> data;
        zono_float eps_prim=static_cast<zono_float>(1e-3), eps_dual=static_cast<zono_float>(1e-3);
 
        // startup method
        bool startup(Box& x_box, OptSolution& solution, const std::set<int>& contract_inds=std::set<int>())
        {
            // start timer
            const auto start = std::chrono::high_resolution_clock::now();
 
            // reset verbosity
            std::stringstream ss;
 
            // check that problem data is consistent
            if (!this->check_problem_dimensions())
            {
                throw std::invalid_argument("ADMM solve: inconsistent problem data dimensions.");
            }
            if (this->data->settings.verbose)
            {
                ss << "Solving ADMM problem with " << this->data->n_x << " variables and " << this->data->n_cons << " constraints.";
                print_str(ss);
            }
 
            // factorize if not already done
            this->factorize();
 
            // apply contractor
            bool contractor_feasible = true;
            if (this->data->settings.use_interval_contractor)
            {
                const auto t0 = std::chrono::high_resolution_clock::now();
                if (contract_inds.empty())
                {
                    contractor_feasible = x_box.contract(this->data->A_rm, this->data->b, this->data->settings.contractor_iter);
                }
                else
                {
                    contractor_feasible = x_box.contract_subset(this->data->A_rm, this->data->b, this->data->settings.contractor_iter,
                        this->data->A, contract_inds, this->data->settings.contractor_tree_search_depth);
                }
 
                if (this->data->settings.verbose)
                {
                    const double run_time = 1e-6 * static_cast<double>(std::chrono::duration_cast<std::chrono::microseconds>(
                        std::chrono::high_resolution_clock::now() - t0).count());
                    ss << "Interval contractor time = " << run_time << " sec";
                    print_str(ss);
                }
            }
 
            // log startup time
            const double startup_time = 1e-6 * static_cast<double>(std::chrono::duration_cast<std::chrono::microseconds>(
                std::chrono::high_resolution_clock::now() - start).count());
            solution.startup_time = startup_time;
            solution.run_time = startup_time;
 
            // early exit if contractor is infeasible
            if (!contractor_feasible)
            {
                if (this->data->settings.verbose)
                {
                    ss << "Infeasibility detected via interval contractor";
                    print_str(ss);
                }
 
                solution.infeasible = true;
                solution.iter = 0;
                solution.converged = false;
                solution.primal_residual = std::numeric_limits<zono_float>::infinity();
                solution.dual_residual = std::numeric_limits<zono_float>::infinity();
                solution.J = std::numeric_limits<zono_float>::infinity();
                solution.x = Eigen::Vector<zono_float, -1>::Zero(this->data->n_x);
                solution.z = solution.x; // 0
                solution.u = solution.x; // 0
            }
            return contractor_feasible;
        }
 
        // core solve method
        virtual void solve_core(const Box& x_box, OptSolution& solution, std::atomic<bool>* stop)
        {
            // start clock
            auto start = std::chrono::high_resolution_clock::now();
            std::stringstream ss;
 
            // initial values
            Eigen::Vector<zono_float, -1> xk, zk, uk, zkm1, rhs, x_nu;
            if (this->is_warmstarted)
            {
                xk = this->x0;
                uk = this->u0;
            }
            else
            {
                xk = 0.5*(x_box.lower() + x_box.upper());
                uk = Eigen::Vector<zono_float, -1>::Zero(this->data->n_x);
            }
            zk = xk;
            rhs = Eigen::Vector<zono_float, -1>::Zero(this->data->n_x + this->data->n_cons);
            rhs.segment(this->data->n_x, this->data->n_cons) = this->data->b; // unchanging
            zkm1 = zk;
 
            // init residuals
            zono_float rp_k=std::numeric_limits<zono_float>::infinity(), rd_k=std::numeric_limits<zono_float>::infinity();
 
            // init loop
            int k = 0;
            double run_time = 1e-6 * static_cast<double>(std::chrono::duration_cast<std::chrono::microseconds>(
                std::chrono::high_resolution_clock::now() - start).count());
            bool converged = false, infeasible = false;
 
            while ((k < this->data->settings.k_max_admm) && (run_time+solution.startup_time < this->data->settings.t_max) && !converged && !infeasible
                && !(stop && (*stop)))
            {
                // x update
                rhs.segment(0, this->data->n_x) = -this->data->q + this->data->settings.rho*(zk - uk);
                x_nu = solve_LDLT(this->data->ldlt_data_M, rhs);
                xk = x_nu.segment(0, this->data->n_x);
 
                // z update
                zk = xk + uk;
                x_box.project(zk);
 
                // u update
                uk += xk - zk;
 
                // check for infeasibility certificate
                if (k % this->data->settings.k_inf_check == 0)
                {
                    infeasible = this->is_infeasibility_certificate(zk - xk, xk, x_box);
                    if (this->data->settings.verbose && infeasible)
                    {
                        ss << "Infeasibility certificate detected at iteration " << k;
                        print_str(ss);
                    }
                }
 
                // check convergence
                if (this->data->settings.inf_norm_conv)
                {
                    rp_k = (xk - zk).cwiseAbs().maxCoeff();
                    rd_k = this->data->settings.rho*(zk - zkm1).cwiseAbs().maxCoeff();
                    converged = (rp_k < this->eps_prim && rd_k < this->eps_dual);
                }
                else
                {
                    rp_k = (xk - zk).norm();
                    rd_k = this->data->settings.rho*(zk - zkm1).norm();
                    converged = (rp_k < this->data->sqrt_n_x*this->eps_prim && rd_k < this->data->sqrt_n_x*this->eps_dual);
                }
 
                // increment
                zkm1 = zk;
                ++k;
 
                // get time
                run_time = 1e-6 * static_cast<double>(std::chrono::duration_cast<std::chrono::microseconds>(
                    std::chrono::high_resolution_clock::now() - start).count());
 
                // verbosity
                if (this->data->settings.verbose && (k % this->data->settings.verbosity_interval == 0))
                {
                    ss << "k = " << k << ": primal residual = " << rp_k << ", dual residual = "
                        << rd_k << ", run time = " << run_time << " sec";
                    print_str(ss);
                }
            }
 
            // verbosity
            if (this->data->settings.verbose)
            {
                if (converged)
                {
                    ss << "ADMM converged in " << k << " iterations.";
                    print_str(ss);
                }
                else if (infeasible)
                {
                    ss << "ADMM detected infeasibility.";
                    print_str(ss);
                }
                else
                {
                    ss << "ADMM did not converge in " << k << " iterations.";
                    print_str(ss);
                }
            }
 
            // reset flags
            this->is_warmstarted = false;
 
            // build output
            solution.x = xk;
            solution.z = zk;
            solution.u = uk;
            solution.J = (0.5*zk.transpose()*this->data->P*zk + this->data->q.transpose()*zk + this->data->c)(0);
            solution.primal_residual = rp_k;
            solution.dual_residual = rd_k;
            solution.run_time = run_time+solution.startup_time;
            solution.iter = k;
            solution.converged = converged;
            solution.infeasible = infeasible;
        }
 
        // warm start
        Eigen::Vector<zono_float, -1> x0, u0;
 
        // flags
        bool is_warmstarted = false;
 
        // factor problem data
        void factorize_M() const
        {
            // system matrix
            Eigen::SparseMatrix<zono_float> M (this->data->n_x + this->data->n_cons, this->data->n_x + this->data->n_cons);
 
            Eigen::SparseMatrix<zono_float> I (this->data->n_x, this->data->n_x);
            I.setIdentity();
            Eigen::SparseMatrix<zono_float> Phi = this->data->P + this->data->settings.rho*I;
 
            std::vector<Eigen::Triplet<zono_float>> triplets;
            get_triplets_offset<zono_float>(Phi, triplets, 0, 0);
            get_triplets_offset<zono_float>(this->data->A, triplets, this->data->n_x, 0);
            get_triplets_offset<zono_float>(this->data->AT, triplets, 0, this->data->n_x);
            M.setFromTriplets(triplets.begin(), triplets.end());
 
            // factorize system matrix
            Eigen::SimplicialLDLT<Eigen::SparseMatrix<zono_float>> ldlt_solver_M;
            ldlt_solver_M.compute(M);
            if (ldlt_solver_M.info() != Eigen::Success)
                throw std::runtime_error("ADMM: factorization of problem data failed, most likely A is not full row rank");
 
            get_LDLT_data(ldlt_solver_M, this->data->ldlt_data_M);
        }
 
        void factorize_AAT() const
        {
            // factorize A*AT
            const Eigen::SparseMatrix<zono_float> AAT = this->data->A*this->data->AT;
            Eigen::SimplicialLDLT<Eigen::SparseMatrix<zono_float>> ldlt_solver_AAT;
            ldlt_solver_AAT.compute(AAT);
            if (ldlt_solver_AAT.info() != Eigen::Success)
                throw std::runtime_error("ADMM: factorization of A*A^T failed, most likely A is not full row rank");
            get_LDLT_data(ldlt_solver_AAT, this->data->ldlt_data_AAT);
        }
 
        // check for infeasibility certificate
        bool is_infeasibility_certificate(const Eigen::Vector<zono_float, -1>& ek,
            const Eigen::Vector<zono_float, -1>& xk, const Box& x_box) const
        {
            // project ek onto row space of A (i.e. column space of AT)
            Eigen::Vector<zono_float, -1> A_e = this->data->A*ek;
            Eigen::Vector<zono_float, -1> AAT_inv_A_e = solve_LDLT(this->data->ldlt_data_AAT, A_e);
            Eigen::Vector<zono_float, -1> ek_proj = this->data->AT*AAT_inv_A_e;
 
            // check if this is an infeasibility certificate
            const zono_float e_x = ek_proj.dot(xk);
            const Interval e_box = x_box.dot(ek_proj);
            return !e_box.contains(e_x);
        }
 
        bool check_problem_dimensions() const
        {
            const bool prob_data_consistent = (this->data->P.rows() == this->data->n_x && this->data->P.cols() == this->data->n_x &&
                this->data->q.size() == this->data->n_x && this->data->A.rows() == this->data->n_cons &&
                this->data->A.cols() == this->data->n_x && this->data->b.size() == this->data->n_cons &&
                this->data->x_box->size() == static_cast<size_t>(this->data->n_x));
 
            bool warm_start_consistent;
            if (this->is_warmstarted)
            {
                warm_start_consistent = (this->x0.size() == this->data->n_x && this->u0.size() == this->data->n_x &&
                    this->data->x_box->size() == static_cast<size_t>(this->data->n_x));
            }
            else
                warm_start_consistent = true;
 
            return prob_data_consistent && warm_start_consistent;
        }
    };
 
} // end namespace ZonoOpt::detail
 
#endif