Merge remote-tracking branch 'upstream/master' into cech_optimization

16 files changed, 1421 insertions, 0 deletions

diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Cartesian_product.h b/src/Coxeter_triangulation/include/gudhi/Functions/Cartesian_product.h
new file mode 100644
index 00000000..0533bb83
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Cartesian_product.h
@@ -0,0 +1,157 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_CARTESIAN_PRODUCT_H_
+#define FUNCTIONS_CARTESIAN_PRODUCT_H_
+
+#include <cstdlib>
+#include <tuple>
+#include <type_traits>  // for std::enable_if
+#include <cstdlib>      // for std::size_t
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/* Get the domain dimension of the tuple of functions.
+ */
+template <std::size_t I = 0, typename... T>
+inline typename std::enable_if<I == sizeof...(T), std::size_t>::type get_amb_d(const std::tuple<T...>& tuple) {
+  return 0;
+}
+
+template <std::size_t I = 0, typename... T>
+inline typename std::enable_if<I != sizeof...(T), std::size_t>::type get_amb_d(const std::tuple<T...>& tuple) {
+  return std::get<I>(tuple).amb_d() + get_amb_d<I + 1, T...>(tuple);
+}
+
+/* Get the codomain dimension of the tuple of functions.
+ */
+template <std::size_t I = 0, typename... T>
+inline typename std::enable_if<I == sizeof...(T), std::size_t>::type get_cod_d(const std::tuple<T...>& tuple) {
+  return 0;
+}
+
+template <std::size_t I = 0, typename... T>
+inline typename std::enable_if<I != sizeof...(T), std::size_t>::type get_cod_d(const std::tuple<T...>& tuple) {
+  return std::get<I>(tuple).cod_d() + get_cod_d<I + 1, T...>(tuple);
+}
+
+/* Get the seed of the tuple of functions.
+ */
+template <std::size_t I = 0, typename... T>
+inline typename std::enable_if<I == sizeof...(T), void>::type get_seed(const std::tuple<T...>& tuple,
+                                                                       Eigen::VectorXd& point, std::size_t i = 0) {}
+
+template <std::size_t I = 0, typename... T>
+inline typename std::enable_if<I != sizeof...(T), void>::type get_seed(const std::tuple<T...>& tuple,
+                                                                       Eigen::VectorXd& point, std::size_t i = 0) {
+  const auto& f = std::get<I>(tuple);
+  std::size_t n = f.amb_d();
+  Eigen::VectorXd seed = f.seed();
+  for (std::size_t j = 0; j < n; ++j) point(i + j) = seed(j);
+  get_seed<I + 1, T...>(tuple, point, i + n);
+}
+
+/* Get the seed of the tuple of functions.
+ */
+template <std::size_t I = 0, typename... T>
+inline typename std::enable_if<I == sizeof...(T), void>::type get_value(const std::tuple<T...>& tuple,
+                                                                        const Eigen::VectorXd& x,
+                                                                        Eigen::VectorXd& point, std::size_t i = 0,
+                                                                        std::size_t j = 0) {}
+
+template <std::size_t I = 0, typename... T>
+inline typename std::enable_if<I != sizeof...(T), void>::type get_value(const std::tuple<T...>& tuple,
+                                                                        const Eigen::VectorXd& x,
+                                                                        Eigen::VectorXd& point, std::size_t i = 0,
+                                                                        std::size_t j = 0) {
+  const auto& f = std::get<I>(tuple);
+  std::size_t n = f.amb_d();
+  std::size_t k = f.cod_d();
+  Eigen::VectorXd x_i(n);
+  for (std::size_t l = 0; l < n; ++l) x_i(l) = x(i + l);
+  Eigen::VectorXd res = f(x_i);
+  for (std::size_t l = 0; l < k; ++l) point(j + l) = res(l);
+  get_value<I + 1, T...>(tuple, x, point, i + n, j + k);
+}
+
+/**
+ * \class Cartesian_product
+ * \brief Constructs the function the zero-set of which is the Cartesian product
+ * of the zero-sets of some given functions.
+ *
+ * \tparam Functions A pack template parameter for functions. All functions should be models of
+ * the concept FunctionForImplicitManifold.
+ */
+template <class... Functions>
+struct Cartesian_product {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    Eigen::VectorXd result(cod_d_);
+    get_value(function_tuple_, p, result, 0, 0);
+    return result;
+  }
+
+  /** \brief Returns the domain (ambient) dimension. */
+  std::size_t amb_d() const { return amb_d_; }
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return cod_d_; }
+
+  /** \brief Returns a point on the zero-set. */
+  Eigen::VectorXd seed() const {
+    Eigen::VectorXd result(amb_d_);
+    get_seed(function_tuple_, result, 0);
+    return result;
+  }
+
+  /**
+   * \brief Constructor of the Cartesian product function.
+   *
+   * @param[in] functions The functions the zero-sets of which are factors in the
+   * Cartesian product of the resulting function.
+   */
+  Cartesian_product(const Functions&... functions) : function_tuple_(std::make_tuple(functions...)) {
+    amb_d_ = get_amb_d(function_tuple_);
+    cod_d_ = get_cod_d(function_tuple_);
+  }
+
+ private:
+  std::tuple<Functions...> function_tuple_;
+  std::size_t amb_d_, cod_d_;
+};
+
+/**
+ * \brief Static constructor of a Cartesian product function.
+ *
+ * @param[in] functions The functions the zero-sets of which are factors in the
+ * Cartesian product of the resulting function.
+ *
+ * \tparam Functions A pack template parameter for functions. All functions should be models of
+ * the concept FunctionForImplicitManifold.
+ *
+ * \ingroup coxeter_triangulation
+ */
+template <typename... Functions>
+Cartesian_product<Functions...> make_product_function(const Functions&... functions) {
+  return Cartesian_product<Functions...>(functions...);
+}
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Constant_function.h b/src/Coxeter_triangulation/include/gudhi/Functions/Constant_function.h
new file mode 100644
index 00000000..0603afd8
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Constant_function.h
@@ -0,0 +1,64 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_CONSTANT_FUNCTION_H_
+#define FUNCTIONS_CONSTANT_FUNCTION_H_
+
+#include <cstdlib>  // for std::size_t
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class Constant_function
+ * \brief A class that encodes a constant function from R^d to R^k.
+ * This class does not have any implicit manifold in correspondence.
+ */
+struct Constant_function {
+  /** \brief Value of the function at a specified point. The value is constant.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    return value_;
+  }
+
+  /** \brief Returns the domain dimension. Same as the ambient dimension of the sphere. */
+  std::size_t amb_d() const { return d_; };
+
+  /** \brief Returns the codomain dimension. Same as the codimension of the sphere. */
+  std::size_t cod_d() const { return k_; };
+
+  /** \brief No seed point is available. Throws an exception on evocation. */
+  Eigen::VectorXd seed() const { throw "Seed invoked on a constant function.\n"; }
+
+  Constant_function() {}
+
+  /**
+   * \brief Constructor of a constant function from R^d to R^m.
+   *
+   * @param[in] d The domain dimension.
+   * @param[in] k The codomain dimension.
+   * @param[in] value The constant value of the function.
+   */
+  Constant_function(std::size_t d, std::size_t k, const Eigen::VectorXd& value) : d_(d), k_(k), value_(value) {}
+
+ private:
+  std::size_t d_, k_;
+  Eigen::VectorXd value_;
+};
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Embed_in_Rd.h b/src/Coxeter_triangulation/include/gudhi/Functions/Embed_in_Rd.h
new file mode 100644
index 00000000..e1fe868f
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Embed_in_Rd.h
@@ -0,0 +1,93 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_EMBED_IN_RD_H_
+#define FUNCTIONS_EMBED_IN_RD_H_
+
+#include <cstdlib>  // for std::size_t
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class Embed_in_Rd
+ * \brief Embedding of an implicit manifold in a higher dimension.
+ *
+ * \tparam Function_ The function template parameter. Should be a model of
+ * the concept FunctionForImplicitManifold.
+ */
+template <class Function_>
+struct Embed_in_Rd {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    Eigen::VectorXd x = p;
+    Eigen::VectorXd x_k(fun_.amb_d()), x_rest(d_ - fun_.amb_d());
+    for (std::size_t i = 0; i < fun_.amb_d(); ++i) x_k(i) = x(i);
+    for (std::size_t i = fun_.amb_d(); i < d_; ++i) x_rest(i - fun_.amb_d()) = x(i);
+    Eigen::VectorXd result = fun_(x_k);
+    result.conservativeResize(this->cod_d());
+    for (std::size_t i = fun_.cod_d(); i < this->cod_d(); ++i) result(i) = x_rest(i - fun_.cod_d());
+    return result;
+  }
+
+  /** \brief Returns the domain (ambient) dimension. */
+  std::size_t amb_d() const { return d_; }
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return d_ - (fun_.amb_d() - fun_.cod_d()); }
+
+  /** \brief Returns a point on the zero-set of the embedded function. */
+  Eigen::VectorXd seed() const {
+    Eigen::VectorXd result = fun_.seed();
+    result.conservativeResize(d_);
+    for (std::size_t l = fun_.amb_d(); l < d_; ++l) result(l) = 0;
+    return result;
+  }
+
+  /**
+   * \brief Constructor of the embedding function.
+   *
+   * @param[in] function The function to be embedded in higher dimension.
+   * @param[in] d Embedding dimension.
+   */
+  Embed_in_Rd(const Function_& function, std::size_t d) : fun_(function), d_(d) {}
+
+ private:
+  Function_ fun_;
+  std::size_t d_;
+};
+
+/**
+ * \brief Static constructor of an embedding function.
+ *
+ * @param[in] function The function to be embedded in higher dimension.
+ * @param[in] d Embedding dimension.
+ *
+ * \tparam Function_ The function template parameter. Should be a model of
+ * the concept FunctionForImplicitManifold.
+ *
+ * \ingroup coxeter_triangulation
+ */
+template <class Function_>
+Embed_in_Rd<Function_> make_embedding(const Function_& function, std::size_t d) {
+  return Embed_in_Rd<Function_>(function, d);
+}
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Function_Sm_in_Rd.h b/src/Coxeter_triangulation/include/gudhi/Functions/Function_Sm_in_Rd.h
new file mode 100644
index 00000000..8911f990
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Function_Sm_in_Rd.h
@@ -0,0 +1,110 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_FUNCTION_SM_IN_RD_H_
+#define FUNCTIONS_FUNCTION_SM_IN_RD_H_
+
+#include <cstdlib>  // for std::size_t
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class Function_Sm_in_Rd
+ * \brief A class for the function that defines an m-dimensional implicit sphere embedded
+ * in the d-dimensional Euclidean space.
+ */
+struct Function_Sm_in_Rd {
+  /** \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    Eigen::VectorXd x = p;
+    for (std::size_t i = 0; i < d_; ++i) x(i) -= center_[i];
+    Eigen::VectorXd result = Eigen::VectorXd::Zero(k_);
+    for (std::size_t i = 0; i < m_ + 1; ++i) result(0) += x(i) * x(i);
+    result(0) -= r_ * r_;
+    for (std::size_t j = 1; j < k_; ++j) result(j) = x(m_ + j);
+    return result;
+  }
+
+  /** \brief Returns the domain dimension. Same as the ambient dimension of the sphere. */
+  std::size_t amb_d() const { return d_; };
+
+  /** \brief Returns the codomain dimension. Same as the codimension of the sphere. */
+  std::size_t cod_d() const { return k_; };
+
+  /** \brief Returns a point on the sphere. */
+  Eigen::VectorXd seed() const {
+    Eigen::VectorXd result = Eigen::VectorXd::Zero(d_);
+    result(0) += r_;
+    for (std::size_t i = 0; i < d_; ++i) result(i) += center_[i];
+    return result;
+  }
+
+  /**
+   * \brief Constructor of the function that defines an m-dimensional implicit sphere embedded
+   * in the d-dimensional Euclidean space.
+   *
+   * @param[in] r The radius of the sphere.
+   * @param[in] m The dimension of the sphere.
+   * @param[in] d The ambient dimension of the sphere.
+   * @param[in] center The center of the sphere.
+   */
+  Function_Sm_in_Rd(double r, std::size_t m, std::size_t d, Eigen::VectorXd center)
+      : m_(m), k_(d - m), d_(d), r_(r), center_(center) {}
+
+  /**
+   * \brief Constructor of the function that defines an m-dimensional implicit sphere embedded
+   * in the d-dimensional Euclidean space centered at the origin.
+   *
+   * @param[in] r The radius of the sphere.
+   * @param[in] m The dimension of the sphere.
+   * @param[in] d The ambient dimension of the sphere.
+   */
+  Function_Sm_in_Rd(double r, std::size_t m, std::size_t d)
+      : m_(m), k_(d - m), d_(d), r_(r), center_(Eigen::VectorXd::Zero(d_)) {}
+
+  /**
+   * \brief Constructor of the function that defines an m-dimensional implicit sphere embedded
+   * in the (m+1)-dimensional Euclidean space.
+   *
+   * @param[in] r The radius of the sphere.
+   * @param[in] m The dimension of the sphere.
+   * @param[in] center The center of the sphere.
+   */
+  Function_Sm_in_Rd(double r, std::size_t m, Eigen::VectorXd center)
+      : m_(m), k_(1), d_(m_ + 1), r_(r), center_(center) {}
+
+  /**
+   * \brief Constructor of the function that defines an m-dimensional implicit sphere embedded
+   * in the (m+1)-dimensional Euclidean space centered at the origin.
+   *
+   * @param[in] r The radius of the sphere.
+   * @param[in] m The dimension of the sphere.
+   */
+  Function_Sm_in_Rd(double r, std::size_t m) : m_(m), k_(1), d_(m_ + 1), r_(r), center_(Eigen::VectorXd::Zero(d_)) {}
+
+  Function_Sm_in_Rd(const Function_Sm_in_Rd& rhs) : Function_Sm_in_Rd(rhs.r_, rhs.m_, rhs.d_, rhs.center_) {}
+
+ private:
+  std::size_t m_, k_, d_;
+  double r_;
+  Eigen::VectorXd center_;
+};
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Function_affine_plane_in_Rd.h b/src/Coxeter_triangulation/include/gudhi/Functions/Function_affine_plane_in_Rd.h
new file mode 100644
index 00000000..b29f0906
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Function_affine_plane_in_Rd.h
@@ -0,0 +1,91 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_FUNCTION_AFFINE_PLANE_IN_RD_H_
+#define FUNCTIONS_FUNCTION_AFFINE_PLANE_IN_RD_H_
+
+#include <cstdlib>  // for std::size_t
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class Function_affine_plane_in_Rd
+ * \brief A class for the function that defines an m-dimensional implicit affine plane
+ * embedded in d-dimensional Euclidean space.
+ */
+struct Function_affine_plane_in_Rd {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    Eigen::VectorXd result = normal_matrix_.transpose() * (p - off_);
+    return result;
+  }
+
+  /** \brief Returns the domain dimension. Same as the ambient dimension of the sphere. */
+  std::size_t amb_d() const { return d_; };
+
+  /** \brief Returns the codomain dimension. Same as the codimension of the sphere. */
+  std::size_t cod_d() const { return k_; };
+
+  /** \brief Returns a point on the affine plane. */
+  Eigen::VectorXd seed() const {
+    Eigen::VectorXd result = off_;
+    return result;
+  }
+
+  /**
+   * \brief Constructor of the function that defines an m-dimensional implicit affine
+   * plane in the d-dimensional Euclidean space.
+   *
+   * @param[in] normal_matrix A normal matrix of the affine plane. The number of rows should
+   * correspond to the ambient dimension, the number of columns should corespond to
+   * the size of the normal basis (codimension).
+   * @param[in] offset The offset vector of the affine plane.
+   * The dimension of the vector should be the ambient dimension of the manifold.
+   */
+  Function_affine_plane_in_Rd(const Eigen::MatrixXd& normal_matrix, const Eigen::VectorXd& offset)
+      : normal_matrix_(normal_matrix), d_(normal_matrix.rows()), k_(normal_matrix.cols()), m_(d_ - k_), off_(offset) {
+    normal_matrix_.colwise().normalize();
+  }
+
+  /**
+   * \brief Constructor of the function that defines an m-dimensional implicit affine
+   * plane in the d-dimensional Euclidean space that passes through origin.
+   *
+   * @param[in] normal_matrix A normal matrix of the affine plane. The number of rows should
+   * correspond to the ambient dimension, the number of columns should corespond to
+   * the size of the normal basis (codimension).
+   */
+  Function_affine_plane_in_Rd(const Eigen::MatrixXd& normal_matrix)
+      : normal_matrix_(normal_matrix),
+        d_(normal_matrix.rows()),
+        k_(normal_matrix.cols()),
+        m_(d_ - k_),
+        off_(Eigen::VectorXd::Zero(d_)) {
+    normal_matrix_.colwise().normalize();
+  }
+
+ private:
+  Eigen::MatrixXd normal_matrix_;
+  std::size_t d_, k_, m_;
+  Eigen::VectorXd off_;
+};
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Function_chair_in_R3.h b/src/Coxeter_triangulation/include/gudhi/Functions/Function_chair_in_R3.h
new file mode 100644
index 00000000..620446da
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Function_chair_in_R3.h
@@ -0,0 +1,80 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_FUNCTION_CHAIR_IN_R3_H_
+#define FUNCTIONS_FUNCTION_CHAIR_IN_R3_H_
+
+#include <cstdlib>  // for std::size_t
+#include <cmath>    // for std::pow
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class Function_chair_in_R3
+ * \brief A class that encodes the function, the zero-set of which is a so-called
+ * "chair" surface embedded in R^3.
+ */
+struct Function_chair_in_R3 {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    double x = p(0) - off_[0], y = p(1) - off_[1], z = p(2) - off_[2];
+    Eigen::VectorXd result(cod_d());
+    result(0) = std::pow(x * x + y * y + z * z - a_ * k_ * k_, 2) -
+                b_ * ((z - k_) * (z - k_) - 2 * x * x) * ((z + k_) * (z + k_) - 2 * y * y);
+    return result;
+  }
+
+  /** \brief Returns the domain (ambient) dimension. */
+  std::size_t amb_d() const { return 3; }
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return 1; }
+
+  /** \brief Returns a point on the surface. */
+  Eigen::VectorXd seed() const {
+    double t1 = a_ - b_;
+    double discr = t1 * t1 - (1.0 - b_) * (a_ * a_ - b_);
+    double z0 = k_ * std::sqrt((t1 + std::sqrt(discr)) / (1 - b_));
+    Eigen::Vector3d result(off_[0], off_[1], z0 + off_[2]);
+    return result;
+  }
+
+  /**
+   * \brief Constructor of the function that defines the 'chair' surface
+   * embedded in R^3.
+   *
+   * @param[in] a A numerical parameter.
+   * @param[in] b A numerical parameter.
+   * @param[in] k A numerical parameter.
+   * @param[in] off Offset vector.
+   */
+  Function_chair_in_R3(double a = 0.8, double b = 0.4, double k = 1.0, Eigen::Vector3d off = Eigen::Vector3d::Zero())
+      : a_(a), b_(b), k_(k), off_(off) {}
+
+ protected:
+  double a_, b_, k_;
+  Eigen::Vector3d off_;
+};
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
+
+// (x^2 + y^2 + z^2 - a*k^2)^2 - b*((z-k)^2 - 2*x^2)*((z+k)^2 - 2*y^2)
+// sqrt(k/(1-b))*sqrt(a-b + sqrt((a-b)^2 - (1-b)*(a^2 - b)*k^2))
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Function_iron_in_R3.h b/src/Coxeter_triangulation/include/gudhi/Functions/Function_iron_in_R3.h
new file mode 100644
index 00000000..f73c4280
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Function_iron_in_R3.h
@@ -0,0 +1,69 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_FUNCTION_IRON_IN_R3_H_
+#define FUNCTIONS_FUNCTION_IRON_IN_R3_H_
+
+#include <cstdlib>  // for std::size_t
+#include <cmath>    // for std::pow
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class Function_iron_in_R3
+ * \brief A class that encodes the function, the zero-set of which is a surface
+ * embedded in R^3 that ressembles an iron.
+ */
+struct Function_iron_in_R3 {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    double x = p(0), y = p(1), z = p(2);
+    Eigen::VectorXd result(cod_d());
+    result(0) = -std::pow(x, 6) / 300. - std::pow(y, 6) / 300. - std::pow(z, 6) / 300. + x * y * y * z / 2.1 + y * y +
+                std::pow(z - 2, 4) - 1;
+    return result;
+  }
+
+  /** \brief Returns the domain (ambient) dimension. */
+  std::size_t amb_d() const { return 3; };
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return 1; };
+
+  /** \brief Returns a point on the surface. */
+  Eigen::VectorXd seed() const {
+    Eigen::Vector3d result(std::pow(4500, 1. / 6), 0, 0);
+    return result;
+  }
+
+  /**
+   * \brief Constructor of the function that defines a surface embedded in R^3
+   * that ressembles an iron.
+   *
+   * @param[in] off Offset vector.
+   */
+  Function_iron_in_R3(Eigen::Vector3d off = Eigen::Vector3d::Zero()) : off_(off) {}
+
+ private:
+  Eigen::Vector3d off_;
+};
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Function_lemniscate_revolution_in_R3.h b/src/Coxeter_triangulation/include/gudhi/Functions/Function_lemniscate_revolution_in_R3.h
new file mode 100644
index 00000000..beb41e00
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Function_lemniscate_revolution_in_R3.h
@@ -0,0 +1,85 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_FUNCTION_LEMNISCATE_REVOLUTION_IN_R3_H_
+#define FUNCTIONS_FUNCTION_LEMNISCATE_REVOLUTION_IN_R3_H_
+
+#include <cstdlib>  // for std::size_t
+#include <cmath>    // for std::sqrt
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class Function_lemniscate_revolution_in_R3
+ * \brief A class that encodes the function, the zero-set of which is a surface of revolution
+ * around the x axis based on the lemniscate of Bernoulli embedded in R^3.
+ */
+struct Function_lemniscate_revolution_in_R3 {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    double x = p(0) - off_[0], y = p(1) - off_[1], z = p(2) - off_[2];
+    Eigen::VectorXd result(cod_d());
+    double x2 = x * x, y2 = y * y, z2 = z * z, a2 = a_ * a_;
+    double t1 = x2 + y2 + z2;
+    result(0) = t1 * t1 - 2 * a2 * (x2 - y2 - z2);
+    return result;
+  }
+
+  /** \brief Returns the (ambient) domain dimension.*/
+  std::size_t amb_d() const { return 3; };
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return 1; };
+
+  /** \brief Returns a point on the surface. This seed point is only one of
+   * two necessary seed points for the manifold tracing algorithm.
+   * See the method seed2() for the other point.
+   */
+  Eigen::VectorXd seed() const {
+    Eigen::Vector3d result(std::sqrt(2 * a_) + off_[0], off_[1], off_[2]);
+    return result;
+  }
+
+  /** \brief Returns a point on the surface. This seed point is only one of
+   * two necessary seed points for the manifold tracing algorithm.
+   * See the method seed() for the other point.
+   */
+  Eigen::VectorXd seed2() const {
+    Eigen::Vector3d result(-std::sqrt(2 * a_) + off_[0], off_[1], off_[2]);
+    return result;
+  }
+
+  /**
+   * \brief Constructor of the function that defines a surface of revolution
+   * around the x axis based on the lemniscate of Bernoulli embedded in R^3.
+   *
+   * @param[in] a A numerical parameter.
+   * @param[in] off Offset vector.
+   */
+  Function_lemniscate_revolution_in_R3(double a = 1, Eigen::Vector3d off = Eigen::Vector3d::Zero())
+      : a_(a), off_(off) {}
+
+ private:
+  double a_;
+  Eigen::Vector3d off_;
+};
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Function_moment_curve_in_Rd.h b/src/Coxeter_triangulation/include/gudhi/Functions/Function_moment_curve_in_Rd.h
new file mode 100644
index 00000000..11b379f3
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Function_moment_curve_in_Rd.h
@@ -0,0 +1,79 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_FUNCTION_MOMENT_CURVE_IN_RD_H_
+#define FUNCTIONS_FUNCTION_MOMENT_CURVE_IN_RD_H_
+
+#include <cstdlib>  // for std::size_t
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class Function_moment_curve_in_Rd
+ * \brief A class for the function that defines an implicit moment curve
+ * in the d-dimensional Euclidean space.
+ */
+struct Function_moment_curve_in_Rd {
+  /** \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    Eigen::VectorXd result(k_);
+    for (std::size_t i = 1; i < d_; ++i) result(i - 1) = p(i) - p(0) * p(i - 1);
+    return result;
+  }
+
+  /** \brief Returns the domain (ambient) dimension.. */
+  std::size_t amb_d() const { return d_; };
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return k_; };
+
+  /** \brief Returns a point on the moment curve. */
+  Eigen::VectorXd seed() const {
+    Eigen::VectorXd result = Eigen::VectorXd::Zero(d_);
+    return result;
+  }
+
+  /**
+   * \brief Constructor of the function that defines an implicit moment curve
+   * in the d-dimensional Euclidean space.
+   *
+   * @param[in] r Numerical parameter.
+   * @param[in] d The ambient dimension.
+   */
+  Function_moment_curve_in_Rd(double r, std::size_t d) : m_(1), k_(d - 1), d_(d), r_(r) {}
+
+  /**
+   * \brief Constructor of the function that defines an implicit moment curve
+   * in the d-dimensional Euclidean space.
+   *
+   * @param[in] r Numerical parameter.
+   * @param[in] d The ambient dimension.
+   * @param[in] offset The offset of the moment curve.
+   */
+  Function_moment_curve_in_Rd(double r, std::size_t d, Eigen::VectorXd& offset)
+      : m_(1), k_(d - 1), d_(d), r_(r), off_(offset) {}
+
+ private:
+  std::size_t m_, k_, d_;
+  double r_;
+  Eigen::VectorXd off_;
+};
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Function_torus_in_R3.h b/src/Coxeter_triangulation/include/gudhi/Functions/Function_torus_in_R3.h
new file mode 100644
index 00000000..b54d3c74
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Function_torus_in_R3.h
@@ -0,0 +1,71 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_FUNCTION_TORUS_IN_R3_H_
+#define FUNCTIONS_FUNCTION_TORUS_IN_R3_H_
+
+#include <cstdlib>  // for std::size_t
+#include <cmath>    // for std::sqrt
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class Function_torus_in_R3
+ * \brief A class that encodes the function, the zero-set of which is a torus
+ * surface embedded in R^3.
+ */
+struct Function_torus_in_R3 {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    double x = p(0) - off_[0], y = p(1) - off_[1], z = p(2) - off_[2];
+    Eigen::VectorXd result(cod_d());
+    result(0) = (z * z + (std::sqrt(x * x + y * y) - r_) * (std::sqrt(x * x + y * y) - r_) - R_ * R_);
+    return result;
+  }
+
+  /** \brief Returns the domain (ambient) dimension. */
+  std::size_t amb_d() const { return 3; };
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return 1; };
+
+  /** \brief Returns a point on the surface. */
+  Eigen::VectorXd seed() const {
+    Eigen::Vector3d result(R_ + r_ + off_[0], off_[1], off_[2]);
+    return result;
+  }
+
+  /**
+   * \brief Constructor of the function that defines a torus embedded in R^3.
+   *
+   * @param[in] R The outer radius of the torus.
+   * @param[in] r The inner radius of the torus.
+   * @param[in] off Offset vector.
+   */
+  Function_torus_in_R3(double R = 1, double r = 0.5, Eigen::Vector3d off = Eigen::Vector3d::Zero())
+      : R_(R), r_(r), off_(off) {}
+
+ private:
+  double R_, r_;
+  Eigen::Vector3d off_;
+};
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Function_whitney_umbrella_in_R3.h b/src/Coxeter_triangulation/include/gudhi/Functions/Function_whitney_umbrella_in_R3.h
new file mode 100644
index 00000000..df1f1eec
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Function_whitney_umbrella_in_R3.h
@@ -0,0 +1,78 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_FUNCTION_WHITNEY_UMBRELLA_IN_R3_H_
+#define FUNCTIONS_FUNCTION_WHITNEY_UMBRELLA_IN_R3_H_
+
+#include <cstdlib>  // for std::size_t
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class Function_whitney_umbrella_in_R3
+ * \brief A class that encodes the function, the zero-set of which is the Whitney umbrella
+ * surface embedded in R^3.
+ */
+struct Function_whitney_umbrella_in_R3 {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    double x = p(0) - off_[0], y = p(1) - off_[1], z = p(2) - off_[2];
+    Eigen::VectorXd result(cod_d());
+    result(0) = x * x - y * y * z;
+    return result;
+  }
+
+  /** \brief Returns the (ambient) domain dimension.*/
+  std::size_t amb_d() const { return 3; };
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return 1; };
+
+  /** \brief Returns a point on the surface. This seed point is only one of
+   * two necessary seed points for the manifold tracing algorithm.
+   * See the method seed2() for the other point.
+   */
+  Eigen::VectorXd seed() const {
+    Eigen::Vector3d result(1 + off_[0], 1 + off_[1], 1 + off_[2]);
+    return result;
+  }
+
+  /** \brief Returns a point on the surface. This seed point is only one of
+   * two necessary seed points for the manifold tracing algorithm.
+   * See the method seed() for the other point.
+   */
+  Eigen::VectorXd seed2() const {
+    Eigen::Vector3d result(-1 + off_[0], -1 + off_[1], 1 + off_[2]);
+    return result;
+  }
+
+  /**
+   * \brief Constructor of the function that defines the Whitney umbrella in R^3.
+   *
+   * @param[in] off Offset vector.
+   */
+  Function_whitney_umbrella_in_R3(Eigen::Vector3d off = Eigen::Vector3d::Zero()) : off_(off) {}
+
+ private:
+  Eigen::Vector3d off_;
+};
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Linear_transformation.h b/src/Coxeter_triangulation/include/gudhi/Functions/Linear_transformation.h
new file mode 100644
index 00000000..82e25bb9
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Linear_transformation.h
@@ -0,0 +1,88 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_LINEAR_TRANSFORMATION_H_
+#define FUNCTIONS_LINEAR_TRANSFORMATION_H_
+
+#include <cstdlib>  // for std::size_t
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/** \class Linear_transformation
+ * \brief Transforms the zero-set of the function by a given linear transformation.
+ * The underlying function corresponds to f(M*x), where M is the transformation matrix.
+ *
+ * \tparam Function_ The function template parameter. Should be a model of
+ * the concept FunctionForImplicitManifold.
+ */
+template <class Function_>
+struct Linear_transformation {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    Eigen::VectorXd result = fun_(matrix_.householderQr().solve(p));
+    return result;
+  }
+
+  /** \brief Returns the domain (ambient) dimension. */
+  std::size_t amb_d() const { return fun_.amb_d(); }
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return fun_.cod_d(); }
+
+  /** \brief Returns a point on the zero-set. */
+  Eigen::VectorXd seed() const {
+    Eigen::VectorXd result = fun_.seed();
+    result = matrix_ * result;
+    return result;
+  }
+
+  /**
+   * \brief Constructor of a linearly transformed function.
+   *
+   * @param[in] function The function to be linearly transformed.
+   * @param[in] matrix The transformation matrix. Its dimension should be d*d,
+   * where d is the domain (ambient) dimension of 'function'.
+   */
+  Linear_transformation(const Function_& function, const Eigen::MatrixXd& matrix) : fun_(function), matrix_(matrix) {}
+
+ private:
+  Function_ fun_;
+  Eigen::MatrixXd matrix_;
+};
+
+/**
+ * \brief Static constructor of a linearly transformed function.
+ *
+ * @param[in] function The function to be linearly transformed.
+ * @param[in] matrix The transformation matrix. Its dimension should be d*d,
+ * where d is the domain (ambient) dimension of 'function'.
+ *
+ * \tparam Function_ The function template parameter. Should be a model of
+ * the concept FunctionForImplicitManifold.
+ *
+ * \ingroup coxeter_triangulation
+ */
+template <class Function_>
+Linear_transformation<Function_> make_linear_transformation(const Function_& function, const Eigen::MatrixXd& matrix) {
+  return Linear_transformation<Function_>(function, matrix);
+}
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Negation.h b/src/Coxeter_triangulation/include/gudhi/Functions/Negation.h
new file mode 100644
index 00000000..fdf07f27
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Negation.h
@@ -0,0 +1,84 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_NEGATION_H_
+#define FUNCTIONS_NEGATION_H_
+
+#include <cstdlib>  // for std::size_t
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ *\class Negation
+ * \brief Constructs the "minus" function. The zero-set is the same, but
+ * the values at other points are the negative of their original value.
+ *
+ * \tparam Function_ The function template parameter. Should be a model of
+ * the concept FunctionForImplicitManifold.
+ */
+template <class Function_>
+struct Negation {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    Eigen::VectorXd result = -fun_(p);
+    return result;
+  }
+
+  /** \brief Returns the domain (ambient) dimension. */
+  std::size_t amb_d() const { return fun_.amb_d(); }
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return fun_.cod_d(); }
+
+  /** \brief Returns a point on the zero-set. */
+  Eigen::VectorXd seed() const {
+    Eigen::VectorXd result = fun_.seed();
+    return result;
+  }
+
+  /**
+   * \brief Constructor of the negative function.
+   *
+   * @param[in] function The function to be negated.
+   */
+  Negation(const Function_& function) : fun_(function) {}
+
+ private:
+  Function_ fun_;
+};
+
+/**
+ * \brief Static constructor of the negative function.
+ *
+ * @param[in] function The function to be translated.
+ * domain (ambient) dimension of 'function'.
+ *
+ * \tparam Function_ The function template parameter. Should be a model of
+ * the concept FunctionForImplicitManifold.
+ *
+ * \ingroup coxeter_triangulation
+ */
+template <class Function_>
+Negation<Function_> negation(const Function_& function) {
+  return Negation<Function_>(function);
+}
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/PL_approximation.h b/src/Coxeter_triangulation/include/gudhi/Functions/PL_approximation.h
new file mode 100644
index 00000000..22071d6d
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/PL_approximation.h
@@ -0,0 +1,111 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_PL_APPROXIMATION_H_
+#define FUNCTIONS_PL_APPROXIMATION_H_
+
+#include <cstdlib>  // for std::size_t
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class PL_approximation
+ * \brief Constructs a piecewise-linear approximation of a function induced by
+ * an ambient triangulation.
+ *
+ * \tparam Function_ The function template parameter. Should be a model of
+ * the concept FunctionForImplicitManifold.
+ * \tparam Triangulation The triangulation template parameter. Should be a model of
+ * the concept TriangulationForManifoldTracing.
+ */
+template <class Function_, class Triangulation_>
+struct PL_approximation {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    std::size_t cod_d = this->cod_d();
+    std::size_t amb_d = this->amb_d();
+    auto s = tr_.locate_point(p);
+    Eigen::MatrixXd matrix(cod_d, s.dimension() + 1);
+    Eigen::MatrixXd vertex_matrix(amb_d + 1, s.dimension() + 1);
+    for (std::size_t i = 0; i < s.dimension() + 1; ++i) vertex_matrix(0, i) = 1;
+    std::size_t j = 0;
+    for (auto v : s.vertex_range()) {
+      Eigen::VectorXd pt_v = tr_.cartesian_coordinates(v);
+      Eigen::VectorXd fun_v = fun_(pt_v);
+      for (std::size_t i = 1; i < amb_d + 1; ++i) vertex_matrix(i, j) = pt_v(i - 1);
+      for (std::size_t i = 0; i < cod_d; ++i) matrix(i, j) = fun_v(i);
+      j++;
+    }
+    assert(j == s.dimension() + 1);
+    Eigen::VectorXd z(amb_d + 1);
+    z(0) = 1;
+    for (std::size_t i = 1; i < amb_d + 1; ++i) z(i) = p(i - 1);
+    Eigen::VectorXd lambda = vertex_matrix.colPivHouseholderQr().solve(z);
+    Eigen::VectorXd result = matrix * lambda;
+    return result;
+  }
+
+  /** \brief Returns the domain (ambient) dimension. */
+  std::size_t amb_d() const { return fun_.amb_d(); }
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return fun_.cod_d(); }
+
+  /** \brief Returns a point on the zero-set. */
+  Eigen::VectorXd seed() const {
+    // TODO: not finished. Should use an oracle.
+    return Eigen::VectorXd(amb_d());
+  }
+
+  /**
+   * \brief Constructor of the piecewise-linear approximation of a function
+   * induced by an ambient triangulation.
+   *
+   * @param[in] function The function.
+   * @param[in] triangulation The ambient triangulation.
+   */
+  PL_approximation(const Function_& function, const Triangulation_& triangulation)
+      : fun_(function), tr_(triangulation) {}
+
+ private:
+  Function_ fun_;
+  Triangulation_ tr_;
+};
+
+/**
+ * \brief Static constructor of the piecewise-linear approximation of a function
+ * induced by an ambient triangulation.
+ *
+ * @param[in] function The function.
+ * @param[in] triangulation The ambient triangulation.
+ *
+ * \tparam Function_ The function template parameter. Should be a model of
+ * the concept FunctionForImplicitManifold.
+ *
+ * \ingroup coxeter_triangulation
+ */
+template <class Function_, class Triangulation_>
+PL_approximation<Function_, Triangulation_> make_pl_approximation(const Function_& function,
+                                                                  const Triangulation_& triangulation) {
+  return PL_approximation<Function_, Triangulation_>(function, triangulation);
+}
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/Translate.h b/src/Coxeter_triangulation/include/gudhi/Functions/Translate.h
new file mode 100644
index 00000000..cbe65abe
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/Translate.h
@@ -0,0 +1,89 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_TRANSLATE_H_
+#define FUNCTIONS_TRANSLATE_H_
+
+#include <cstdlib>  // for std::size_t
+
+#include <Eigen/Dense>
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/**
+ * \class Translate
+ * \brief Translates the zero-set of the function by a vector.
+ * The underlying function corresponds to f(x-off), where off is the offset vector.
+ *
+ * \tparam Function_ The function template parameter. Should be a model of
+ * the concept FunctionForImplicitManifold.
+ */
+template <class Function_>
+struct Translate {
+  /**
+   * \brief Value of the function at a specified point.
+   * @param[in] p The input point. The dimension needs to coincide with the ambient dimension.
+   */
+  Eigen::VectorXd operator()(const Eigen::VectorXd& p) const {
+    Eigen::VectorXd result = fun_(p - off_);
+    return result;
+  }
+
+  /** \brief Returns the domain (ambient) dimension. */
+  std::size_t amb_d() const { return fun_.amb_d(); }
+
+  /** \brief Returns the codomain dimension. */
+  std::size_t cod_d() const { return fun_.cod_d(); }
+
+  /** \brief Returns a point on the zero-set. */
+  Eigen::VectorXd seed() const {
+    Eigen::VectorXd result = fun_.seed();
+    result += off_;
+    return result;
+  }
+
+  /**
+   * \brief Constructor of the translated function.
+   *
+   * @param[in] function The function to be translated.
+   * @param[in] off The offset vector. The dimension should correspond to the
+   * domain (ambient) dimension of 'function'.
+   */
+  Translate(const Function_& function, const Eigen::VectorXd& off) : fun_(function), off_(off) {}
+
+ private:
+  Function_ fun_;
+  Eigen::VectorXd off_;
+};
+
+/**
+ * \brief Static constructor of a translated function.
+ *
+ * @param[in] function The function to be translated.
+ * @param[in] off The offset vector. The dimension should correspond to the
+ * domain (ambient) dimension of 'function'.
+ *
+ * \tparam Function_ The function template parameter. Should be a model of
+ * the concept FunctionForImplicitManifold.
+ *
+ * \ingroup coxeter_triangulation
+ */
+template <class Function_>
+Translate<Function_> translate(const Function_& function, Eigen::VectorXd off) {
+  return Translate<Function_>(function, off);
+}
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif
diff --git a/src/Coxeter_triangulation/include/gudhi/Functions/random_orthogonal_matrix.h b/src/Coxeter_triangulation/include/gudhi/Functions/random_orthogonal_matrix.h
new file mode 100644
index 00000000..6a896e94
--- /dev/null
+++ b/src/Coxeter_triangulation/include/gudhi/Functions/random_orthogonal_matrix.h
@@ -0,0 +1,72 @@
+/*    This file is part of the Gudhi Library - https://gudhi.inria.fr/ - which is released under MIT.
+ *    See file LICENSE or go to https://gudhi.inria.fr/licensing/ for full license details.
+ *    Author(s):       Siargey Kachanovich
+ *
+ *    Copyright (C) 2019 Inria
+ *
+ *    Modification(s):
+ *      - YYYY/MM Author: Description of the modification
+ */
+
+#ifndef FUNCTIONS_RANDOM_ORTHOGONAL_MATRIX_H_
+#define FUNCTIONS_RANDOM_ORTHOGONAL_MATRIX_H_
+
+#include <cstdlib>  // for std::size_t
+#include <cmath>    // for std::cos, std::sin
+#include <random>   // for std::uniform_real_distribution, std::random_device
+
+#include <Eigen/Dense>
+#include <Eigen/Sparse>
+#include <Eigen/SVD>
+
+#include <CGAL/Epick_d.h>
+#include <CGAL/point_generators_d.h>
+
+#include <boost/math/constants/constants.hpp>  // for PI value
+
+namespace Gudhi {
+
+namespace coxeter_triangulation {
+
+/** \brief Generates a uniform random orthogonal matrix using the "subgroup algorithm" by
+ * Diaconis & Shashahani.
+ * \details Taken from https://en.wikipedia.org/wiki/Rotation_matrix#Uniform_random_rotation_matrices.
+ * The idea: take a random rotation matrix of dimension d-1, embed it
+ * as a d*d matrix M with the last column (0,...,0,1).
+ * Pick a random vector v on a sphere S^d. rotate the matrix M so that its last column is v.
+ * The determinant of the matrix can be either 1 or -1
+ */
+// Note: the householderQR operation at the end seems to take a lot of time at compilation.
+// The CGAL headers are another source of long compilation time.
+Eigen::MatrixXd random_orthogonal_matrix(std::size_t d) {
+  typedef CGAL::Epick_d<CGAL::Dynamic_dimension_tag> Kernel;
+  typedef typename Kernel::Point_d Point_d;
+  if (d == 1) return Eigen::VectorXd::Constant(1, 1.0);
+  if (d == 2) {
+    // 0. < alpha < 2 Pi
+    std::uniform_real_distribution<double> unif(0., 2 * boost::math::constants::pi<double>());
+    std::random_device rand_dev;
+    std::mt19937 rand_engine(rand_dev());
+    double alpha = unif(rand_engine);
+
+    Eigen::Matrix2d rot;
+    rot << std::cos(alpha), -std::sin(alpha), std::sin(alpha), cos(alpha);
+    return rot;
+  }
+  Eigen::MatrixXd low_dim_rot = random_orthogonal_matrix(d - 1);
+  Eigen::MatrixXd rot(d, d);
+  Point_d v = *CGAL::Random_points_on_sphere_d<Point_d>(d, 1);
+  for (std::size_t i = 0; i < d; ++i) rot(i, 0) = v[i];
+  for (std::size_t i = 0; i < d - 1; ++i)
+    for (std::size_t j = 1; j < d - 1; ++j) rot(i, j) = low_dim_rot(i, j - 1);
+  for (std::size_t j = 1; j < d; ++j) rot(d - 1, j) = 0;
+  rot = rot.householderQr()
+            .householderQ();  // a way to do Gram-Schmidt, see https://forum.kde.org/viewtopic.php?f=74&t=118568#p297246
+  return rot;
+}
+
+}  // namespace coxeter_triangulation
+
+}  // namespace Gudhi
+
+#endif