# dlib C++ Library - optimization_solve_qp_using_smo.h

    // Copyright (C) 2010  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_OPTIMIZATION_SOLVE_QP_UsING_SMO_Hh_
    #define DLIB_OPTIMIZATION_SOLVE_QP_UsING_SMO_Hh_

    #include "optimization_solve_qp_using_smo_abstract.h"
    #include "../matrix.h"
    #include <map>
    #include "../unordered_pair.h"

    namespace dlib
    {

    // ----------------------------------------------------------------------------------------

        /*
            The algorithm defined in the solve_qp_using_smo() function below can be
            derived by using an important theorem from the theory of constrained optimization.
            This theorem tells us that any optimal point of a constrained function must
            satisfy what are called the KKT conditions (also sometimes called just the KT
            conditions, especially in older literature).  A very good book to consult
            regarding this topic is Practical Methods of Optimization (second edition) by
            R. Fletcher.  Below I will try to explain the general idea of how this is
            applied.

            Let e == ones_matrix(alpha.size(),1)

            First, note that the function below solves the following quadratic program.
                Minimize: f(alpha) == 0.5*trans(alpha)*Q*alpha - trans(alpha)*b
                subject to the following constraints:
                    - trans(e)*alpha == C (i.e. the sum of alpha values doesn't change)
                    - min(alpha) >= 0 (i.e. all alpha values are nonnegative)
                Where f is convex.  This means that Q should be positive-semidefinite.

            To get from this problem formulation to the algorithm below we have to
            consider the KKT conditions.  They tell us that any solution to the above
            problem must satisfy the following 5 conditions:
                1. trans(e)*alpha == C
                2. min(alpha) >= 0

                3. Let L(alpha, x, y) == f(alpha) - trans(x)*alpha - y*(trans(e)*alpha - C)
                   Where x is a vector of length alpha.size() and y is a single scalar.
                   Then the derivative of L with respect to alpha must == 0
                   So we get the following as our 3rd condition:
                   f'(alpha) - x - y*e == 0

                4. min(x) >= 0 (i.e. all x values are nonnegative)
                5. pointwise_multiply(x, alpha) == 0
                   (i.e. only one member of each x(i) and alpha(i) pair can be non-zero)

            From 3 we can easily obtain this rule:
                for all i: f'(alpha)(i) - x(i) == y

            If we then consider 4 and 5 we see that we can infer that the following
            must also be the case:
                - if (alpha(i) > 0) then
                    - x(i) == 0
                    - f'(alpha)(i) == y
                - else
                    - x(i) == some nonnegative number
                    - f'(alpha)(i) >= y

            The important thing to take away is the final rule.  It tells us that at the
            optimal solution all elements of the gradient of f have the same value if
            their corresponding alpha is non-zero.  It also tells us that all the other
            gradient values are bigger than y.  We can use this information to help us
            pick which alpha variables to optimize at each iteration.
        */

    // ----------------------------------------------------------------------------------------

        template <
            typename EXP1,
            typename EXP2,
            typename T, long NR, long NC, typename MM, typename L
            >
        unsigned long solve_qp_using_smo (
            const matrix_exp<EXP1>& Q,
            const matrix_exp<EXP2>& b,
            matrix<T,NR,NC,MM,L>& alpha,
            T eps,
            unsigned long max_iter
        )
        {
            // make sure requires clause is not broken
            DLIB_ASSERT(Q.nr() == Q.nc() &&
                         is_col_vector(b) &&
                         is_col_vector(alpha) &&
                         b.size() == alpha.size() &&
                         b.size() == Q.nr() &&
                         alpha.size() > 0 &&
                         min(alpha) >= 0 &&
                         eps > 0 &&
                         max_iter > 0,
                         "\t unsigned long solve_qp_using_smo()"
                         << "\n\t Invalid arguments were given to this function"
                         << "\n\t Q.nr():               " << Q.nr()
                         << "\n\t Q.nc():               " << Q.nc()
                         << "\n\t is_col_vector(b):     " << is_col_vector(b)
                         << "\n\t is_col_vector(alpha): " << is_col_vector(alpha)
                         << "\n\t b.size():             " << b.size()
                         << "\n\t alpha.size():         " << alpha.size()
                         << "\n\t Q.nr():               " << Q.nr()
                         << "\n\t min(alpha):           " << min(alpha)
                         << "\n\t eps:                  " << eps
                         << "\n\t max_iter:             " << max_iter
            );

            const T C = sum(alpha);

            // Compute f'(alpha) (i.e. the gradient of f(alpha)) for the current alpha.
            matrix<T,NR,NC,MM,L> df = Q*alpha - b;

            const T tau = 1000*std::numeric_limits<T>::epsilon();

            T big, little;
            unsigned long iter = 0;
            for (; iter < max_iter; ++iter)
            {
                // Find the two elements of df that satisfy the following:
                //    - little_idx == index_of_min(df)
                //    - big_idx   == the index of the largest element in df such that alpha(big_idx) > 0
                // These two indices will tell us which two alpha values are most in violation of the KKT
                // optimality conditions.
                big = -std::numeric_limits<T>::max();
                long big_idx = 0;
                little = std::numeric_limits<T>::max();
                long little_idx = 0;
                for (long i = 0; i < df.nr(); ++i)
                {
                    if (df(i) > big && alpha(i) > 0)
                    {
                        big = df(i);
                        big_idx = i;
                    }
                    if (df(i) < little)
                    {
                        little = df(i);
                        little_idx = i;
                    }
                }

                // Check if the KKT conditions are still violated and stop if so.
                //if (alpha(little_idx) > 0 && (big - little) < eps)
                //    break;

                // Check how big the duality gap is and stop when it goes below eps.
                // The duality gap is the gap between the objective value of the function
                // we are optimizing and the value of its primal form.  This value is always
                // greater than or equal to the distance to the optimum solution so it is a
                // good way to decide if we should stop.   See the book referenced above for
                // more information.  In particular, see the part about the Wolfe Dual.
                if (trans(alpha)*df - C*little < eps)
                    break;

                // Save these values, we will need them later.
                const T old_alpha_big = alpha(big_idx);
                const T old_alpha_little = alpha(little_idx);

                // Now optimize the two variables we just picked.
                T quad_coef = Q(big_idx,big_idx) + Q(little_idx,little_idx) - 2*Q(big_idx, little_idx);
                if (quad_coef <= tau)
                    quad_coef = tau;
                const T delta = (big - little)/quad_coef;
                alpha(big_idx) -= delta;
                alpha(little_idx) += delta;

                // Make sure alpha stays feasible.  That is, make sure the updated alpha doesn't
                // violate the non-negativity constraint.
                if (alpha(big_idx) < 0)
                {
                    // Since an alpha can't be negative we will just set it to 0 and shift all the
                    // weight to the other alpha.
                    alpha(big_idx) = 0;
                    alpha(little_idx) = old_alpha_big + old_alpha_little;
                }

                // Every 300 iterations
                if ((iter%300) == 299)
                {
                    // Perform this form of the update every so often because doing so can help
                    // avoid the buildup of numerical errors you get with the alternate update
                    // below.
                    df = Q*alpha - b;
                }
                else
                {
                    // Now update the gradient. We will perform the equivalent of: df = Q*alpha - b;
                    const T delta_alpha_big   = alpha(big_idx) - old_alpha_big;
                    const T delta_alpha_little = alpha(little_idx) - old_alpha_little;

                    for(long k = 0; k < df.nr(); ++k)
                        df(k) += Q(big_idx,k)*delta_alpha_big + Q(little_idx,k)*delta_alpha_little;;
                }
            }

            /*
            using namespace std;
            cout << "SMO: " << endl;
            cout << "   duality gap: "<< trans(alpha)*df - C*min(df) << endl;
            cout << "   KKT gap:     "<< big-little << endl;
            cout << "   iter:        "<< iter+1 << endl;
            cout << "   eps:         "<< eps << endl;
            */

            return iter+1;
        }

    // ----------------------------------------------------------------------------------------

        template <
            typename EXP1,
            typename EXP2,
            typename EXP3,
            typename EXP4,
            typename T, long NR, long NC, typename MM, typename L,
            long NR2, long NC2
            >
        unsigned long solve_qp4_using_smo (
            const matrix_exp<EXP1>& A,
            const matrix_exp<EXP2>& Q,
            const matrix_exp<EXP3>& b,
            const matrix_exp<EXP4>& d,
            matrix<T,NR,NC,MM,L>& alpha,
            matrix<T,NR2,NC2,MM,L>& lambda,
            T eps,
            unsigned long max_iter,
            T max_lambda = std::numeric_limits<T>::infinity()
        )
        {
            // make sure requires clause is not broken
            DLIB_ASSERT(A.nc() == alpha.size() &&
                         Q.nr() == Q.nc() &&
                         is_col_vector(b) &&
                         is_col_vector(alpha) &&
                         b.size() == alpha.size() &&
                         b.size() == Q.nr() &&
                         alpha.size() > 0 &&
                         min(alpha) >= 0 &&
                         eps > 0 &&
                         max_iter > 0,
                         "\t void solve_qp4_using_smo()"
                         << "\n\t Invalid arguments were given to this function"
                         << "\n\t A.nc():               " << A.nc()
                         << "\n\t Q.nr():               " << Q.nr()
                         << "\n\t Q.nc():               " << Q.nc()
                         << "\n\t is_col_vector(b):     " << is_col_vector(b)
                         << "\n\t is_col_vector(alpha): " << is_col_vector(alpha)
                         << "\n\t b.size():             " << b.size()
                         << "\n\t alpha.size():         " << alpha.size()
                         << "\n\t Q.nr():               " << Q.nr()
                         << "\n\t min(alpha):           " << min(alpha)
                         << "\n\t eps:                  " << eps
                         << "\n\t max_iter:             " << max_iter
            );
            DLIB_ASSERT(is_col_vector(d) == true &&
                         max_lambda >= 0 &&
                         d.size() == A.nr(),
                         "\t void solve_qp4_using_smo()"
                         << "\n\t Invalid arguments were given to this function"
                         << "\n\t A.nr():     " << A.nr()
                         << "\n\t d.size():   " << d.size()
                         << "\n\t max_lambda: " << max_lambda
            );

            const T C = sum(alpha);

            /*
                For this optimization problem, it is the case that the optimal
                value of lambda is given by a simple closed form expression if we
                know the optimal alpha.  So what we will do is to just optimize
                alpha and every now and then we will update lambda with its optimal
                value.  Therefore, we use essentially the same method as the
                solve_qp_using_smo() routine.
            */

            const bool d_is_zero = d==zeros_matrix(d);

            // compute optimal lambda for current alpha
            if (d_is_zero)
                lambda = A*alpha;
            else
                lambda = A*alpha + d;
            lambda = dlib::clamp(lambda, 0, max_lambda);

            // Compute f'(alpha) (i.e. the gradient of f(alpha) with respect to alpha) for the current alpha.
            matrix<T,NR,NC,MM,L> df = Q*alpha - b - trans(A)*lambda;

            const T tau = 1000*std::numeric_limits<T>::epsilon();

            T big, little;
            unsigned long iter = 0;
            for (; iter < max_iter; ++iter)
            {
                // Find the two elements of df that satisfy the following:
                //    - little_idx == index_of_min(df)
                //    - big_idx   == the index of the largest element in df such that alpha(big_idx) > 0
                // These two indices will tell us which two alpha values are most in violation of the KKT
                // optimality conditions.
                big = -std::numeric_limits<T>::max();
                long big_idx = 0;
                little = std::numeric_limits<T>::max();
                long little_idx = 0;
                for (long i = 0; i < df.nr(); ++i)
                {
                    if (df(i) > big && alpha(i) > 0)
                    {
                        big = df(i);
                        big_idx = i;
                    }
                    if (df(i) < little)
                    {
                        little = df(i);
                        little_idx = i;
                    }
                }

                // Check how big the duality gap is and stop when it goes below eps.
                // The duality gap is the gap between the objective value of the function
                // we are optimizing and the value of its primal form.  This value is always
                // greater than or equal to the distance to the optimum solution so it is a
                // good way to decide if we should stop.
                if (trans(alpha)*df - C*little < eps)
                {
                    // compute optimal lambda and recheck the duality gap to make
                    // sure we have really converged.
                    if (d_is_zero)
                        lambda = A*alpha;
                    else
                        lambda = A*alpha + d;
                    lambda = dlib::clamp(lambda, 0, max_lambda);
                    df = Q*alpha - b - trans(A)*lambda;

                    if (trans(alpha)*df - C*min(df) < eps)
                        break;
                    else
                        continue;
                }

                // Save these values, we will need them later.
                const T old_alpha_big = alpha(big_idx);
                const T old_alpha_little = alpha(little_idx);

                // Now optimize the two variables we just picked.
                T quad_coef = Q(big_idx,big_idx) + Q(little_idx,little_idx) - 2*Q(big_idx, little_idx);
                if (quad_coef <= tau)
                    quad_coef = tau;
                const T delta = (big - little)/quad_coef;
                alpha(big_idx) -= delta;
                alpha(little_idx) += delta;

                // Make sure alpha stays feasible.  That is, make sure the updated alpha doesn't
                // violate the non-negativity constraint.
                if (alpha(big_idx) < 0)
                {
                    // Since an alpha can't be negative we will just set it to 0 and shift all the
                    // weight to the other alpha.
                    alpha(big_idx) = 0;
                    alpha(little_idx) = old_alpha_big + old_alpha_little;
                }

                // Every 300 iterations
                if ((iter%300) == 299)
                {
                    // compute the optimal lambda for the current alpha
                    if (d_is_zero)
                        lambda = A*alpha;
                    else
                        lambda = A*alpha + d;
                    lambda = dlib::clamp(lambda, 0, max_lambda);

                    // Perform this form of the update every so often because doing so can help
                    // avoid the buildup of numerical errors you get with the alternate update
                    // below.
                    df = Q*alpha - b - trans(A)*lambda;
                }
                else
                {
                    // Now update the gradient. We will perform the equivalent of: df = Q*alpha - b;
                    const T delta_alpha_big   = alpha(big_idx) - old_alpha_big;
                    const T delta_alpha_little = alpha(little_idx) - old_alpha_little;

                    for(long k = 0; k < df.nr(); ++k)
                        df(k) += Q(big_idx,k)*delta_alpha_big + Q(little_idx,k)*delta_alpha_little;;
                }
            }

            /*
            using namespace std;
            cout << "SMO: " << endl;
            cout << "   duality gap: "<< trans(alpha)*df - C*min(df) << endl;
            cout << "   KKT gap:     "<< big-little << endl;
            cout << "   iter:        "<< iter+1 << endl;
            cout << "   eps:         "<< eps << endl;
            */

            return iter+1;
        }

    // ----------------------------------------------------------------------------------------

        template <
            typename EXP1,
            typename EXP2,
            typename T, long NR, long NC, typename MM, typename L
            >
        unsigned long solve_qp_box_constrained (
            const matrix_exp<EXP1>& Q,
            const matrix_exp<EXP2>& b,
            matrix<T,NR,NC,MM,L>& alpha,
            const matrix<T,NR,NC,MM,L>& lower,
            const matrix<T,NR,NC,MM,L>& upper,
            T eps = 1e-10,
            unsigned long max_iter = 30000
        )
        {
            // make sure requires clause is not broken
            DLIB_ASSERT(Q.nr() == Q.nc() &&
                         alpha.size() == lower.size() &&
                         alpha.size() == upper.size() &&
                         is_col_vector(b) &&
                         is_col_vector(alpha) &&
                         is_col_vector(lower) &&
                         is_col_vector(upper) &&
                         b.size() == alpha.size() &&
                         b.size() == Q.nr() &&
                         alpha.size() > 0 &&
                         0 <= min(alpha-lower) &&
                         0 <= max(upper-alpha) &&
                         eps > 0 &&
                         max_iter > 0,
                         "\t unsigned long solve_qp_box_constrained()"
                         << "\n\t Invalid arguments were given to this function"
                         << "\n\t Q.nr():               " << Q.nr()
                         << "\n\t Q.nc():               " << Q.nc()
                         << "\n\t is_col_vector(b):     " << is_col_vector(b)
                         << "\n\t is_col_vector(alpha): " << is_col_vector(alpha)
                         << "\n\t is_col_vector(lower): " << is_col_vector(lower)
                         << "\n\t is_col_vector(upper): " << is_col_vector(upper)
                         << "\n\t b.size():             " << b.size()
                         << "\n\t alpha.size():         " << alpha.size()
                         << "\n\t lower.size():         " << lower.size()
                         << "\n\t upper.size():         " << upper.size()
                         << "\n\t Q.nr():               " << Q.nr()
                         << "\n\t min(alpha-lower):     " << min(alpha-lower)
                         << "\n\t max(upper-alpha):     " << max(upper-alpha)
                         << "\n\t eps:                  " << eps
                         << "\n\t max_iter:             " << max_iter
            );

            // Compute f'(alpha) (i.e. the gradient of f(alpha)) for the current alpha.
            matrix<T,NR,NC,MM,L> df = Q*alpha + b;
            matrix<T,NR,NC,MM,L> QQ = reciprocal_max(diag(Q));

            // First we use a coordinate descent method to initialize alpha.
            double max_df = 0;
            for (long iter = 0; iter < alpha.size()*2; ++iter)
            {
                max_df = 0;
                long best_r =0;
                // find the best alpha to optimize.
                for (long r = 0; r < Q.nr(); ++r)
                {
                    if (alpha(r) <= lower(r) && df(r) > 0)
                        ;//alpha(r) = lower(r);
                    else if (alpha(r) >= upper(r) && df(r) < 0)
                        ;//alpha(r) = upper(r);
                    else if (std::abs(df(r)) > max_df)
                    {
                        best_r = r;
                        max_df = std::abs(df(r));
                    }
                }

                // now optimize alpha(best_r)
                const long r = best_r;
                const T old_alpha = alpha(r);
                alpha(r) = -(df(r)-Q(r,r)*alpha(r))*QQ(r);
                if (alpha(r) < lower(r))
                    alpha(r) = lower(r);
                else if (alpha(r) > upper(r))
                    alpha(r) = upper(r);

                const T delta = old_alpha-alpha(r);

                // Now update the gradient. We will perform the equivalent of: df = Q*alpha + b;
                for(long k = 0; k < df.nr(); ++k)
                    df(k) -= Q(r,k)*delta;
            }
            //cout << "max_df: " << max_df << endl;
            //cout << "objective value: " << 0.5*trans(alpha)*Q*alpha + trans(b)*alpha << endl;

            // Now do the main iteration block of this solver.  The coordinate descent method
            // we used above can improve the objective rapidly in the beginning.  However,
            // Nesterov's method has more rapid convergence once it gets going so this is what
            // we use for the main iteration.
            matrix<T,NR,NC,MM,L> v, v_old;
            v = alpha;
            // We need to get an upper bound on the Lipschitz constant for this QP. Since that
            // is just the max eigenvalue of Q we can do it using Gershgorin disks.
            const T lipschitz_bound = max(diag(Q) + (sum_cols(abs(Q)) - abs(diag(Q))));
            double lambda = 1;
            unsigned long iter;
            for (iter = 0; iter < max_iter; ++iter)
            {
                const double next_lambda = (1 + std::sqrt(1+4*lambda*lambda))/2;
                const double gamma = (1-lambda)/next_lambda;
                lambda = next_lambda;

                v_old = v;

                // now take a projected gradient step using Nesterov's method.
                v = clamp(alpha - 1.0/lipschitz_bound * df, lower, upper);
                alpha = dlib::clamp((1-gamma)*v + gamma*v_old, lower, upper);

                df = Q*alpha + b;

                // check for convergence every 10 iterations
                if (iter%10 == 0)
                {
                    max_df = 0;
                    double absalpha = 0;
                    double thealpha = 0;
                    for (long r = 0; r < Q.nr(); ++r)
                    {
                        absalpha += std::abs(alpha(r));
                        if (alpha(r) <= lower(r) && df(r) > 0)
                            ;//alpha(r) = lower(r);
                        else if (alpha(r) >= upper(r) && df(r) < 0)
                            ;//alpha(r) = upper(r);
                        else if (std::abs(df(r)) > max_df)
                        {
                            max_df = std::abs(df(r));
                            thealpha = std::abs(alpha(r));
                        }
                    }
                    absalpha /= Q.nr();
                    // Stop when the magnitude of the changes we are making to alpha are eps
                    // smaller than the typical alpha.
                    if (max_df/lipschitz_bound <= eps*std::min(thealpha,absalpha))
                        break;
                }
            }

            //cout << "max_df: " << max_df << endl;
            //cout << "objective value: " << 0.5*trans(alpha)*Q*alpha + trans(b)*alpha << endl;
            return iter+1;
        }

    // ----------------------------------------------------------------------------------------
    // ----------------------------------------------------------------------------------------

        namespace impl
        {
            // Check if each vector in Q_offdiag is actually a constant times the 1s vector.
            template <
                typename T, long NR, long NC, typename MM, typename L
                >
            bool has_uniform_offdiag_vectors(
                const std::map<unordered_pair<size_t>, matrix<T,NR,NC,MM,L>>& Q_offdiag
            )
            {
                for (auto& x : Q_offdiag)
                {
                    auto ref = x.second(0);
                    for (auto& y : x.second)
                        if (ref != y)
                            return false;
                }
                return true;
            }

            template <
                typename T, long NR, long NC, typename MM, typename L
                >
            matrix<T,0,0,MM,L> compact_offdiag(
                const size_t& num_blocks,
                const std::map<unordered_pair<size_t>, matrix<T,NR,NC,MM,L>>& Q_offdiag
            )
            {
                matrix<T,0,0,MM,L> temp;
                // we can only compact the offdiag information if they are uniform vectors
                if (!has_uniform_offdiag_vectors(Q_offdiag))
                    return temp;

                temp.set_size(num_blocks, num_blocks);
                temp = 0;

                for (auto& x : Q_offdiag)
                {
                    long r = x.first.first;
                    long c = x.first.second;
                    temp(r,c) = x.second(0);
                    temp(c,r) = x.second(0);
                }

                return temp;
            }
        }

    // ----------------------------------------------------------------------------------------

        template <
            typename T, long NR, long NC, typename MM, typename L
            >
        unsigned long solve_qp_box_constrained_blockdiag (
            const std::vector<matrix<T,NR,NR,MM,L>>& Q_blocks,
            const std::vector<matrix<T,NR,NC,MM,L>>& bs,
            const std::map<unordered_pair<size_t>, matrix<T,NR,NC,MM,L>>& Q_offdiag,
            std::vector<matrix<T,NR,NC,MM,L>>& alphas,
            const std::vector<matrix<T,NR,NC,MM,L>>& lowers,
            const std::vector<matrix<T,NR,NC,MM,L>>& uppers,
            T eps = 1e-10,
            unsigned long max_iter = 30000
        )
        {
            // make sure requires clause is not broken
            DLIB_CASSERT(Q_blocks.size() > 0);
            DLIB_CASSERT(Q_blocks.size() == bs.size() &&
                         Q_blocks.size() == alphas.size() &&
                         Q_blocks.size() == lowers.size() &&
                         Q_blocks.size() == uppers.size(),
                       "Q_blocks.size():  "<< Q_blocks.size() << "\n" <<
                       "bs.size():        "<< bs.size() << "\n" <<
                       "alphas.size():    "<< alphas.size() << "\n" <<
                       "lowers.size():    "<< lowers.size() << "\n" <<
                       "uppers.size():    "<< uppers.size() << "\n"
                       );
            for (auto& Q : Q_blocks)
            {
                DLIB_CASSERT(Q.nr() == Q.nc(), "All the matrices in Q_blocks have the same dimensions.");
                DLIB_CASSERT(Q.size() > 0, "All the matrices in Q_blocks must be non-empty and have the same dimensions.");
                DLIB_CASSERT(Q.nr() == Q_blocks[0].nr() && Q.nc() == Q_blocks[0].nc(), "All the matrices in Q_blocks have the same dimensions.");
            }
    #ifdef ENABLE_ASSERTS
            for (size_t i = 0; i < alphas.size(); ++i)
            {
                DLIB_CASSERT(is_col_vector(bs[i]) && bs[i].size() == Q_blocks[0].nr(),
                    "is_col_vector(bs["<<i<<"]): " << is_col_vector(bs[i]) << "\n" <<
                    "bs["<<i<<"].size():         " << bs[i].size() << "\n" <<
                    "Q_blocks[0].nr():           " << Q_blocks[0].nr());

                for (auto& Qoffdiag : Q_offdiag)
                {
                    auto& Q_offdiag_element = Qoffdiag.second;
                    long r = Qoffdiag.first.first;
                    long c = Qoffdiag.first.second;
                    DLIB_CASSERT(is_col_vector(Q_offdiag_element) && Q_offdiag_element.size() == Q_blocks[0].nr(),
                        "is_col_vector(Q_offdiag["<<r<<","<<c<<"]): " << is_col_vector(Q_offdiag_element) << "\n" <<
                        "Q_offdiag["<<r<<","<<c<<"].size():         " << Q_offdiag_element.size() << "\n" <<
                        "Q_blocks[0].nr():                  " << Q_blocks[0].nr());
                }

                DLIB_CASSERT(is_col_vector(alphas[i]) && alphas[i].size() == Q_blocks[0].nr(),
                    "is_col_vector(alphas["<<i<<"]): " << is_col_vector(alphas[i]) << "\n" <<
                    "alphas["<<i<<"].size():         " << alphas[i].size() << "\n" <<
                    "Q_blocks[0].nr():               " << Q_blocks[0].nr());

                DLIB_CASSERT(is_col_vector(lowers[i]) && lowers[i].size() == Q_blocks[0].nr(),
                    "is_col_vector(lowers["<<i<<"]): " << is_col_vector(lowers[i]) << "\n" <<
                    "lowers["<<i<<"].size():         " << lowers[i].size() << "\n" <<
                    "Q_blocks[0].nr():               " << Q_blocks[0].nr());

                DLIB_CASSERT(is_col_vector(uppers[i]) && uppers[i].size() == Q_blocks[0].nr(),
                    "is_col_vector(uppers["<<i<<"]): " << is_col_vector(uppers[i]) << "\n" <<
                    "uppers["<<i<<"].size():         " << uppers[i].size() << "\n" <<
                    "Q_blocks[0].nr():               " << Q_blocks[0].nr());

                DLIB_CASSERT(0 <= min(alphas[i]-lowers[i]), "min(alphas["<<i<<"]-lowers["<<i<<"]): " << min(alphas[i]-lowers[i]));
                DLIB_CASSERT(0 <= max(uppers[i]-alphas[i]), "max(uppers["<<i<<"]-alphas["<<i<<"]): " << max(uppers[i]-alphas[i]));
            }
            DLIB_CASSERT(eps > 0 && max_iter > 0, "eps: " << eps << "\nmax_iter: "<< max_iter);
    #endif // ENABLE_ASSERTS

            const auto offdiag_compact = impl::compact_offdiag(Q_blocks.size(), Q_offdiag);
            matrix<T,0,0,MM,L> temp, alphas_compact;

            // Compute f'(alpha) (i.e. the gradient of f(alpha)) for the current alpha.
            std::vector<matrix<T,NR,NC,MM,L>> df;// = Q*alpha + b;
            auto compute_df = [&]()
            {
                df.resize(Q_blocks.size());
                for (size_t i = 0; i < df.size(); ++i)
                    df[i] = Q_blocks[i]*alphas[i] + bs[i];

                // Don't forget to include the Q_offdiag terms in the computation.  Note that
                // we have two options for how we can compute this part.  If Q_offdiag is
                // uniform and can be compacted into a simple matrix and there are a lot of off
                // diagonal entries then it's faster to do it as a matrix multiply.  Otherwise
                // we do the more general computation.
                if (offdiag_compact.size() != 0 && Q_offdiag.size() > Q_blocks.size()*5)
                {
                    // Do it as a matrix multiply (with a bit of data shuffling)
                    alphas_compact.set_size(alphas[0].size(), offdiag_compact.nr());
                    for (long c = 0; c < alphas_compact.nc(); ++c)
                        set_colm(alphas_compact,c) = alphas[c];
                    temp = alphas_compact*offdiag_compact;
                    for (size_t i = 0; i < df.size(); ++i)
                        df[i] += colm(temp,i);
                }
                else
                {
                    // Do the fully general computation that allows for non-uniform values in
                    // the off diagonal vectors.
                    for (auto& p : Q_offdiag)
                    {
                        long r = p.first.first;
                        long c = p.first.second;
                        df[r] += pointwise_multiply(p.second, alphas[c]);
                        if (r != c)
                            df[c] += pointwise_multiply(p.second, alphas[r]);
                    }
                }
            };
            compute_df();

            std::vector<matrix<T,NR,NC,MM,L>> Q_diag, Q_ggd;
            std::vector<matrix<T,NR,NC,MM,L>> QQ;// = reciprocal_max(diag(Q));
            QQ.resize(Q_blocks.size());
            Q_diag.resize(Q_blocks.size());
            Q_ggd.resize(Q_blocks.size());

            // We need to get an upper bound on the Lipschitz constant for this QP. Since that
            // is just the max eigenvalue of Q we can do it using Gershgorin disks.
            //const T lipschitz_bound = max(diag(Q) + (sum_cols(abs(Q)) - abs(diag(Q))));
            for (size_t i = 0; i < QQ.size(); ++i)
            {
                auto f = Q_offdiag.find(make_unordered_pair(i,i));
                if (f != Q_offdiag.end())
                    Q_diag[i] = diag(Q_blocks[i]) + f->second;
                else
                    Q_diag[i] = diag(Q_blocks[i]);
                QQ[i] = reciprocal_max(Q_diag[i]);

                Q_ggd[i] = Q_diag[i] + (sum_cols(abs(Q_blocks[i]))-abs(diag(Q_blocks[i])));
            }
            for (auto& p : Q_offdiag)
            {
                long r = p.first.first;
                long c = p.first.second;
                if (r != c)
                {
                    Q_ggd[r] += abs(p.second);
                    Q_ggd[c] += abs(p.second);
                }
            }
            T lipschitz_bound = -std::numeric_limits<T>::infinity();
            for (auto& x : Q_ggd)
                lipschitz_bound = std::max(lipschitz_bound, max(x));

            const long num_variables = alphas.size()*alphas[0].size();

            // First we use a coordinate descent method to initialize alpha.
            double max_df = 0;
            for (long iter = 0; iter < num_variables*2; ++iter)
            {
                max_df = 0;
                long best_r =0;
                size_t best_r2 =0;
                // find the best alpha to optimize.
                for (size_t r2 = 0; r2 < alphas.size(); ++r2)
                {
                    auto& alpha = alphas[r2];
                    auto& df_ = df[r2];
                    auto& lower = lowers[r2];
                    auto& upper = uppers[r2];
                    for (long r = 0; r < alpha.nr(); ++r)
                    {
                        if (alpha(r) <= lower(r) && df_(r) > 0)
                            ;//alpha(r) = lower(r);
                        else if (alpha(r) >= upper(r) && df_(r) < 0)
                            ;//alpha(r) = upper(r);
                        else if (std::abs(df_(r)) > max_df)
                        {
                            best_r = r;
                            best_r2 = r2;
                            max_df = std::abs(df_(r));
                        }
                    }
                }

                // now optimize alphas[best_r2](best_r)
                const long r = best_r;
                auto& alpha = alphas[best_r2];
                auto& lower = lowers[best_r2];
                auto& upper = uppers[best_r2];
                auto& df_ = df[best_r2];
                const T old_alpha = alpha(r);
                alpha(r) = -(df_(r)-Q_diag[best_r2](r)*alpha(r))*QQ[best_r2](r);
                if (alpha(r) < lower(r))
                    alpha(r) = lower(r);
                else if (alpha(r) > upper(r))
                    alpha(r) = upper(r);

                const T delta = old_alpha-alpha(r);

                // Now update the gradient. We will perform the equivalent of: df = Q*alpha +
                // b; except we only need to compute one column of the matrix multiply because
                // only one element of alpha changed.
                auto& Q = Q_blocks[best_r2];
                for(long k = 0; k < df_.nr(); ++k)
                    df_(k) -= Q(r,k)*delta;
                for(size_t j = 0; j < Q_blocks.size(); ++j)
                {
                    auto f = Q_offdiag.find(make_unordered_pair(best_r2, j));
                    if (f != Q_offdiag.end())
                        df[j](r) -= f->second(r)*delta;
                }
            }

            std::vector<matrix<T,NR,NC,MM,L>> v(alphas), v_old(alphas.size());
            double lambda = 1;
            unsigned long iter;
            // Now do the main iteration block of this solver.  The coordinate descent method
            // we used above can improve the objective rapidly in the beginning.  However,
            // Nesterov's method has more rapid convergence once it gets going so this is what
            // we use for the main iteration.
            for (iter = 0; iter < max_iter; ++iter)
            {
                const double next_lambda = (1 + std::sqrt(1+4*lambda*lambda))/2;
                const double gamma = (1-lambda)/next_lambda;
                lambda = next_lambda;

                v_old.swap(v);

                // now take a projected gradient step using Nesterov's method.
                for (size_t j = 0; j < alphas.size(); ++j)
                {
                    v[j] = clamp(alphas[j] - 1.0/lipschitz_bound * df[j], lowers[j], uppers[j]);
                    alphas[j] = clamp((1-gamma)*v[j] + gamma*v_old[j], lowers[j], uppers[j]);
                }

                //df = Q*alpha + b;
                compute_df();

                // check for convergence every 10 iterations
                if (iter%10 == 0)
                {
                    max_df = 0;
                    double absalpha = 0;
                    double thealpha = 0;
                    for (size_t r2 = 0; r2 < alphas.size(); ++r2)
                    {
                        auto& alpha = alphas[r2];
                        auto& df_ = df[r2];
                        auto& lower = lowers[r2];
                        auto& upper = uppers[r2];
                        for (long r = 0; r < alpha.nr(); ++r)
                        {
                            absalpha += std::abs(alpha(r));
                            if (alpha(r) <= lower(r) && df_(r) > 0)
                                ;//alpha(r) = lower(r);
                            else if (alpha(r) >= upper(r) && df_(r) < 0)
                                ;//alpha(r) = upper(r);
                            else if (std::abs(df_(r)) > max_df)
                            {
                                max_df = std::abs(df_(r));
                                thealpha = std::abs(alpha(r));
                            }
                        }
                    }
                    absalpha /= num_variables;
                    // Stop when the magnitude of the changes we are making to alpha are eps
                    // smaller than the typical alpha.
                    if (max_df/lipschitz_bound <= eps*std::min(thealpha,absalpha))
                        break;
                }
            }

            return iter+1;
        }

    // ----------------------------------------------------------------------------------------

        template <
            typename EXP1,
            typename EXP2,
            typename T, long NRa, long NRb
            >
        unsigned long find_gap_between_convex_hulls (
            const matrix_exp<EXP1>& A,
            const matrix_exp<EXP2>& B,
            matrix<T,NRa,1>& cA,
            matrix<T,NRb,1>& cB,
            const double eps,
            const unsigned long max_iter = 1000
        )
        {
            DLIB_CASSERT(A.size() != 0);
            DLIB_CASSERT(B.size() != 0);
            DLIB_CASSERT(A.nr() == B.nr(), "The dimensionality of the points in both convex hull sets must match");
            DLIB_CASSERT(eps > 0);
            DLIB_CASSERT(max_iter > 0);

            cA.set_size(A.nc());
            cB.set_size(B.nc());

            // initialize to the centroids of A and B respectively.
            cA = 1.0/cA.size();
            cB = 1.0/cB.size();

            matrix<T> AA, BB, AB, ABb, ABa;

            AA = trans(A)*A;
            BB = trans(B)*B;
            AB = trans(A)*B;

            unsigned long iter = 0;
            for (iter = 0; iter < max_iter; ++iter)
            {
                // find the convex combination of A that is nearest to B*cB
                ABb = AB*cB;
                const auto smo_iter1 = solve_qp_using_smo(AA, ABb, cA, eps, cA.size());

                // now find the convex combination of B that is nearest to A*cA
                ABa = trans(AB)*cA;
                const auto smo_iter2 = solve_qp_using_smo(BB, ABa, cB, eps, cB.size());

                // stop if the QP solvers failed to improve
                if (smo_iter1 == 1 && smo_iter2 == 1)
                    break;
            }

            return iter+1;
        }

    // ----------------------------------------------------------------------------------------

    }

    #endif // DLIB_OPTIMIZATION_SOLVE_QP_UsING_SMO_Hh_