From 4653f9ff52077b601434a912ac573c694ba90ea4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C3=89milie=20Feral?= <emilie.feral@numworks.com>
Date: Wed, 19 Oct 2016 15:17:35 +0200
Subject: [PATCH] [apps/graph] Add method to approximate the derivative
 function at a given abscissa

Change-Id: I31bbd99a0469f7fad140a38dae1783b2d2ee414d
---
 apps/graph/function.cpp | 9 ++++++++-
 apps/graph/function.h   | 4 +++-
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/apps/graph/function.cpp b/apps/graph/function.cpp
index cd43159c8..b5c8e7d18 100644
--- a/apps/graph/function.cpp
+++ b/apps/graph/function.cpp
@@ -61,9 +61,16 @@ void Function::setActive(bool active) {
   m_active = active;
 }
 
-float Function::evaluateAtAbscissa(float x, EvaluateContext * context) {
+float Function::evaluateAtAbscissa(float x, EvaluateContext * context) const {
   context->setOverridenValueForSymbolX(x);
   return m_expression->approximate(*context);
 }
 
+float Function::approximateDerivative(float x, EvaluateContext * context) const {
+  float functionPlus = evaluateAtAbscissa(x + k_epsilon, context);
+  float functionMinus = evaluateAtAbscissa(x - k_epsilon, context);
+  float growthRate = (functionPlus - functionMinus)/(2*k_epsilon);
+  return growthRate;
+}
+
 }
diff --git a/apps/graph/function.h b/apps/graph/function.h
index fc8ba57fc..44c1252f5 100644
--- a/apps/graph/function.h
+++ b/apps/graph/function.h
@@ -22,8 +22,10 @@ public:
   void setActive(bool active);
   void setContent(const char * c);
   void setColor(KDColor m_color);
-  float evaluateAtAbscissa(float x, EvaluateContext * context);
+  float evaluateAtAbscissa(float x, EvaluateContext * context) const;
+  float approximateDerivative(float x, EvaluateContext * context) const;
 private:
+  constexpr static float k_epsilon = 0.0001f;
   constexpr static int k_bodyLength = 255;
   char m_text[k_bodyLength];
   const char * m_name;