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+/*    This file is part of the Gudhi Library. The Gudhi library
+ *    (Geometric Understanding in Higher Dimensions) is a generic C++
+ *    library for computational topology.
+ *
+ *    Author(s):       Pawel Dlotko
+ *
+ *    Copyright (C) 2016 INRIA (France)
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the GNU General Public License as published by
+ *    the Free Software Foundation, either version 3 of the License, or
+ *    (at your option) any later version.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    GNU General Public License for more details.
+ *
+ *    You should have received a copy of the GNU General Public License
+ *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef COMMON_PERSISTENCE_REPRESENTATIONS_H_
+#define COMMON_PERSISTENCE_REPRESENTATIONS_H_
+
+#include <utility>
+#include <string>
+#include <cmath>
+
+namespace Gudhi {
+namespace Persistence_representations {
+// this file contain an implementation of some common procedures used in Persistence_representations.
+
+// double epsi = std::numeric_limits<double>::epsilon();
+double epsi = 0.000005;
+
+/**
+ *  A procedure used to compare doubles. Typically given two doubles A and B, comparing A == B is not good idea. In this
+ *case, we use the procedure almostEqual with the epsi defined at
+ *  the top of the file. Setting up the epsi gives the user a tolerance on what should be consider equal.
+**/
+inline bool almost_equal(double a, double b) {
+  if (std::fabs(a - b) < epsi) return true;
+  return false;
+}
+
+// landscapes
+/**
+ * Extra functions needed in construction of barcodes.
+**/
+double minus_length(std::pair<double, double> a) { return a.first - a.second; }
+double birth_plus_deaths(std::pair<double, double> a) { return a.first + a.second; }
+
+// landscapes
+/**
+ * Given two points in R^2, the procedure compute the parameters A and B of the line y = Ax + B that crosses those two
+ *points.
+**/
+std::pair<double, double> compute_parameters_of_a_line(std::pair<double, double> p1, std::pair<double, double> p2) {
+  double a = (p2.second - p1.second) / (p2.first - p1.first);
+  double b = p1.second - a * p1.first;
+  return std::make_pair(a, b);
+}
+
+// landscapes
+/**
+ * This procedure given two points which lies on the opposite sides of x axis, compute x for which the line connecting
+ *those two points crosses x axis.
+**/
+double find_zero_of_a_line_segment_between_those_two_points(std::pair<double, double> p1,
+                                                            std::pair<double, double> p2) {
+  if (p1.first == p2.first) return p1.first;
+  if (p1.second * p2.second > 0) {
+    std::ostringstream errMessage;
+    errMessage << "In function find_zero_of_a_line_segment_between_those_two_points the arguments are: (" << p1.first
+               << "," << p1.second << ") and (" << p2.first << "," << p2.second
+               << "). There is no zero in line between those two points. Program terminated.";
+    std::string errMessageStr = errMessage.str();
+    const char* err = errMessageStr.c_str();
+    throw(err);
+  }
+  // we assume here, that x \in [ p1.first, p2.first ] and p1 and p2 are points between which we will put the line
+  // segment
+  double a = (p2.second - p1.second) / (p2.first - p1.first);
+  double b = p1.second - a * p1.first;
+  return -b / a;
+}
+
+// landscapes
+/**
+ * This method provides a comparison of points that is used in construction of persistence landscapes. The ordering is
+ *lexicographical for the first coordinate, and reverse-lexicographical for the
+ * second coordinate.
+**/
+bool compare_points_sorting(std::pair<double, double> f, std::pair<double, double> s) {
+  if (f.first < s.first) {
+    return true;
+  } else {  // f.first >= s.first
+    if (f.first > s.first) {
+      return false;
+    } else {  // f.first == s.first
+      if (f.second > s.second) {
+        return true;
+      } else {
+        return false;
+      }
+    }
+  }
+}
+
+// landscapes
+/**
+ * This procedure takes two points in R^2 and a double value x. It computes the line parsing through those two points
+ *and return the value of that linear function at x.
+**/
+double function_value(std::pair<double, double> p1, std::pair<double, double> p2, double x) {
+  // we assume here, that x \in [ p1.first, p2.first ] and p1 and p2 are points between which we will put the line
+  // segment
+  double a = (p2.second - p1.second) / (p2.first - p1.first);
+  double b = p1.second - a * p1.first;
+  return (a * x + b);
+}
+
+}  // namespace Persistence_representations
+}  // namespace Gudhi
+
+#endif  // COMMON_PERSISTENCE_REPRESENTATIONS_H_