[apps/regression] Use a new helper to compute linear regression coefficients

apps/regression/Makefile

@@ -13,6 +13,7 @@ app_src += $(addprefix apps/regression/,\
   graph_options_controller.cpp \
   graph_view.cpp \
   initialisation_parameter_controller.cpp \
+  linear_model_helper.cpp \
   regression_context.cpp \
   regression_controller.cpp \
   store.cpp \

apps/regression/linear_model_helper.cpp

@@ -0,0 +1,17 @@
+#include "linear_model_helper.h"
+
+namespace Regression {
+
+namespace LinearModelHelper {
+
+double Slope(double covariance, double variance) {
+  return covariance / variance;
+}
+
+double YIntercept(double meanOfY, double meanOfX, double slope) {
+  return meanOfY - slope * meanOfX;
+}
+
+}
+
+}

apps/regression/linear_model_helper.h

@@ -0,0 +1,15 @@
+#ifndef REGRESSION_LINEAR_MODEL_HELPER
+#define REGRESSION_LINEAR_MODEL_HELPER
+
+namespace Regression {
+
+namespace LinearModelHelper {
+
+double Slope(double covariance, double variance);
+double YIntercept(double meanOfY, double meanOfX, double slope);
+
+}
+
+}
+
+#endif

apps/regression/model/exponential_model.cpp

@@ -1,5 +1,6 @@
 #include "exponential_model.h"
 #include "../store.h"
+#include "../linear_model_helper.h"
 #include <math.h>
 #include <assert.h>
 #include <poincare/code_point_layout.h>
@@ -81,8 +82,9 @@ void ExponentialModel::fit(Store * store, int series, double * modelCoefficients
   const double meanOfXY = sumOfXY / numberOfPoints;
   const double variance = meanOfXX - meanOfX * meanOfX;
   const double covariance = meanOfXY - meanOfX * meanOfY;
-  modelCoefficients[1] = covariance / variance;
-  modelCoefficients[0] = sign * exp(meanOfY - modelCoefficients[1] * meanOfX);
+  modelCoefficients[1] = LinearModelHelper::Slope(covariance, variance);
+  modelCoefficients[0] =
+    sign * exp(LinearModelHelper::YIntercept(meanOfY, meanOfX, modelCoefficients[1]));
 }
 
 double ExponentialModel::partialDerivate(double * modelCoefficients, int derivateCoefficientIndex, double x) const {

apps/regression/store.cpp

@@ -1,4 +1,5 @@
 #include "store.h"
+#include "linear_model_helper.h"
 #include "apps/apps_container.h"
 #include <poincare/preferences.h>
 #include <assert.h>
@@ -238,11 +239,11 @@ double Store::covariance(int series) const {
 }
 
 double Store::slope(int series) const {
-  return covariance(series)/varianceOfColumn(series, 0);
+  return LinearModelHelper::Slope(covariance(series), varianceOfColumn(series, 0));
 }
 
 double Store::yIntercept(int series) const {
-  return meanOfColumn(series, 1) - slope(series)*meanOfColumn(series, 0);
+  return LinearModelHelper::YIntercept(meanOfColumn(series, 1), meanOfColumn(series, 0), slope(series));
 }
 
 double Store::yValueForXValue(int series, double x, Poincare::Context * globalContext) {