[Positioner] Measurement: compute the variance

The variance is now computed along with the average. Measurement has
three new attributes:
  - average_mw (average in mW),
  - variance,
  - variance_m2 (variable used for the variance calculation).
In addition, average_ss has been renamed average_dbm (ditto for its
getter).

The computation method of the average has been changed to allow the
computation of the variance simultaneously.

owlps-positioner/src/fbcm.cc

@@ -24,9 +24,9 @@ Result FBCM::compute(const Request &_request)
 float FBCM::estimate_distance(const Measurement &measurement)
 {
   double constant_term = make_constant_term(measurement) ;
-  const float &average_ss = measurement.get_average_ss() ;
+  const float &average_dbm = measurement.get_average_dbm() ;
   const AccessPoint *ap = measurement.get_ap() ;
-  return pow(10, (constant_term - average_ss) /
+  return pow(10, (constant_term - average_dbm) /
              (10 * friis_index(ap))) ;
 }
 

owlps-positioner/src/frbhmbasic.cc

@@ -32,9 +32,9 @@ Result FRBHMBasic::compute(const Request &_request)
 float FRBHMBasic::estimate_distance(const Measurement &measurement)
 {
   double constant_term = make_constant_term(measurement) ;
-  const float &average_ss = measurement.get_average_ss() ;
+  const float &average_dbm = measurement.get_average_dbm() ;
   const AccessPoint *ap = measurement.get_ap() ;
-  return pow(10, (constant_term - average_ss) /
+  return pow(10, (constant_term - average_dbm) /
              (10 * friis_index(ap))) ;
 }
 

owlps-positioner/src/interlinknetworks.cc

@@ -13,6 +13,6 @@ float InterlinkNetworks::
 estimate_distance(const Measurement &measurement)
 {
   double constant_term = make_constant_term(measurement) ;
-  const float &average_ss = measurement.get_average_ss() ;
-  return pow(10, (constant_term - average_ss) / 35) ;
+  const float &average_dbm = measurement.get_average_dbm() ;
+  return pow(10, (constant_term - average_dbm) / 35) ;
 }

owlps-positioner/src/measurement.cc

@@ -39,8 +39,6 @@ Measurement::~Measurement()
 void Measurement::
 add_ss(const pkt_id_t packet_id, const ss_t ss_dbm)
 {
-  unsigned int ss_list_size = ss_list.size() ;
-
   // Add the new value (in dBm) along with the packet identifier:
   pair<pkt_id_t, ss_t> packet(make_pair(packet_id, ss_dbm)) ;
   if (! ss_list.insert(packet).second)
@@ -51,12 +49,8 @@ add_ss(const pkt_id_t packet_id, const ss_t ss_dbm)
       return ;
     }
 
-  // Update the average with the new value
-  double total_ss_mwatts =
-    pow(10, static_cast<double>(ss_dbm) / 10.0) + // New SS in mW
-    (ss_list_size * pow(10, average_ss / 10.0)) ; // Other values in mW
-  // Convert the average back in dBm:
-  average_ss = 10.0 * log10(total_ss_mwatts / ss_list.size()) ;
+  // Update the average & variance with the new value:
+  update_average(ss_dbm) ;
 }
 
 
@@ -74,7 +68,7 @@ add_ss_list(const map<pkt_id_t, ss_t> &_ss_list)
        i = _ss_list.begin() ; i != _ss_list.end() ; ++i)
     ss_list[i->first] = i->second ;
 
-  update_average_ss() ;
+  recalculate_average() ;
 }
 
 
@@ -96,12 +90,15 @@ void Measurement::merge(const Measurement &source)
 /**
  * - #ap is not deleted, only initialised to NULL.
  * - #ss_list is cleared.
- * - #average_ss is initialised to 0.
+ * - #average_dbm, #average_mw and #variance are reset to 0.
  */
 void Measurement::clear()
 {
   ss_list.clear() ;
-  average_ss = 0 ;
+  average_dbm = 0 ;
+  average_mw = 0 ;
+  variance = 0 ;
+  variance_m2 = 0 ;
   ap = NULL ;
 }
 
@@ -110,23 +107,33 @@ void Measurement::clear()
 /* *** Operations *** */
 
 
-void Measurement::update_average_ss()
+void Measurement::recalculate_average()
 {
-  if (ss_list.empty())
-    {
-      average_ss = 0 ;
-      return ;
-    }
-
-  double total_ss_mwatts = 0 ;
+  average_dbm = 0 ;
+  average_mw = 0 ;
+  variance = 0 ;
+  variance_m2 = 0 ;
 
   for (map<pkt_id_t, ss_t>::const_iterator
        i = ss_list.begin() ; i != ss_list.end() ; ++i)
-    // Add the current value in mW to the total
-    total_ss_mwatts += pow(10, static_cast<double>(i->second) / 10.0) ;
+    update_average(i->second) ;
+}
 
-  // Compute the average in mW and convert it to dBm
-  average_ss = 10 * log10(total_ss_mwatts / ss_list.size()) ;
+
+void Measurement::update_average(ss_t ss_dbm)
+{
+  // Convert the new SS in mW:
+  double ss_mw = pow(10, static_cast<double>(ss_dbm) / 10.0) ;
+
+  // Update the average:
+  double delta = ss_mw - average_mw ;
+  average_mw += delta / ss_list.size() ;
+  average_dbm = 10.0 * log10(average_mw) ;
+
+  // Update the variance:
+  variance_m2 += delta * (ss_mw - average_mw) ;
+  if (ss_list.size() > 1)
+    variance = variance_m2 / (ss_list.size() - 1) ;
 }
 
 
@@ -141,7 +148,10 @@ Measurement& Measurement::operator=(const Measurement &m)
 
   ap = m.ap ;
   ss_list = m.ss_list ;
-  average_ss = m.average_ss ;
+  average_dbm = m.average_dbm ;
+  average_mw = m.average_mw ;
+  variance = m.variance ;
+  variance_m2 = m.variance_m2 ;
 
   return *this ;
 }
@@ -154,8 +164,7 @@ bool Measurement::operator==(const Measurement &m) const
 
   return
     ap == m.ap &&
-    ss_list == m.ss_list &&
-    average_ss == m.average_ss ;
+    ss_list == m.ss_list ;
 }
 
 
@@ -206,7 +215,7 @@ ostream &operator<<(ostream &os, const Measurement &m)
         os << static_cast<int_fast16_t>(i->second)
            << '(' << i->first << ')' ;
       }
-  os << " [AVG=" << m.average_ss << "]" ;
+  os << " [AVG=" << m.average_dbm << "]" ;
 
   return os ;
 }

owlps-positioner/src/measurement.hh

@@ -25,39 +25,54 @@ protected:
   AccessPoint *ap ;
   /// List of signal strengths captured (in dBm)
   std::map<pkt_id_t, ss_t> ss_list ;
-  /// Average of all signal strength captured (dBm)
-  double average_ss ;
+  /// Average of all the captured signal strengths (dBm)
+  double average_dbm ;
+  /// Average of all the captured signal strengths (mW)
+  double average_mw ;
+  /// Variance of all the captured signal strengths
+  double variance ;
 
+private:
+  /// Intermediate variable used to compute the variance
+  double variance_m2 ;
+
+
+protected:
   /** @name Operations */
   //@{
-  /// Recalculates #average_ss from #ss_list
-  void update_average_ss(void) ;
+  /// Recalculates entirely the average and variance from #ss_list
+  void recalculate_average(void) ;
+  /// Update the average and variance with a new SS
+  void update_average(ss_t ss_dbm) ;
   //@}
 
 
 public:
   Measurement(const AccessPoint *_ap = NULL):
-    ap(const_cast<AccessPoint*>(_ap)), average_ss(0) {}
+    ap(const_cast<AccessPoint*>(_ap)), average_dbm(0),
+    average_mw(0), variance(0), variance_m2(0) {}
 
   Measurement(const Measurement &source):
     ap(source.ap), ss_list(source.ss_list),
-    average_ss(source.average_ss) {}
+    average_dbm(source.average_dbm), average_mw(source.average_mw),
+    variance(source.variance), variance_m2(0) {}
 
   ~Measurement(void) ;
 
   /** @name Read accessors */
   //@{
   AccessPoint* get_ap() const ;
-  double get_average_ss() const ;
+  double get_average_dbm() const ;
+  double get_variance() const ;
   int get_ss_list_size() const ;
   //@}
 
   /** @name Write accessors */
   //@{
   void set_ap(const AccessPoint *_ap) ;
-  /// Adds a signal strength to #ss_list
+  /// Adds a signal strength (in dBm) to #ss_list
   void add_ss(const pkt_id_t packet_id, const ss_t ss_dbm) ;
-  /// Adds several signal strengths to #ss_list
+  /// Adds several signal strengths (in dBm) to #ss_list
   void add_ss_list(const std::map<pkt_id_t, ss_t> &_ss_list) ;
   /// Merges a given Measurement into the current Measurement
   void merge(const Measurement &source) ;
@@ -68,8 +83,8 @@ public:
   //@{
   /// Computes the distance to another Measurement in SS space
   float ss_square_distance(const Measurement &source) const ;
-  /// Computes the distance to another SS value
-  float ss_square_distance(const float &ss) const ;
+  /// Computes the distance to another SS value (in dBm)
+  float ss_square_distance(const float &ss_dbm) const ;
   //@}
 
   /** @name Operators */
@@ -101,9 +116,15 @@ inline AccessPoint* Measurement::get_ap() const
 }
 
 
-inline double Measurement::get_average_ss() const
+inline double Measurement::get_average_dbm() const
 {
-  return average_ss ;
+  return average_dbm ;
+}
+
+
+inline double Measurement::get_variance() const
+{
+  return variance ;
 }
 
 
@@ -130,13 +151,13 @@ inline void Measurement::set_ap(const AccessPoint *_ap)
 inline float Measurement::
 ss_square_distance(const Measurement &source) const
 {
-  return ss_square_distance(source.average_ss) ;
+  return ss_square_distance(source.average_dbm) ;
 }
 
 
-inline float Measurement::ss_square_distance(const float &ss) const
+inline float Measurement::ss_square_distance(const float &ss_dbm) const
 {
-  return ((ss - average_ss) * (ss - average_ss)) ;
+  return ((ss_dbm - average_dbm) * (ss_dbm - average_dbm)) ;
 }
 
 

owlps-positioner/src/referencepoint.cc

@@ -44,7 +44,7 @@ average_measurements(const std::string &mac_transmitter) const
          measurements.begin() ;
        i != measurements.end() ; ++i)
     {
-      avg += i->second.get_average_ss() ;
+      avg += i->second.get_average_dbm() ;
       ++n_ss ;
     }
 
@@ -257,7 +257,7 @@ float ReferencePoint::friis_indexes_for_ap(
         measurements.find(ap_mac) ;
       if (measurement != measurements.end())
         {
-          double ss = measurement->second.get_average_ss() ;
+          double ss = measurement->second.get_average_dbm() ;
           assert((*request)->get_mobile()) ;
           float mobile_gain =
             (*request)->get_mobile()->get_antenna_gain() ;

owlps-positioner/tests/measurement_test.hh

@@ -30,7 +30,7 @@ public:
     Measurement m1(&ap1, vi1) ;
     TS_ASSERT_EQUALS(m1.get_ap(), &ap1) ;
     TS_ASSERT_EQUALS(m1.get_ss_list(), vi1) ;
-    TS_ASSERT_EQUALS(m1.get_average_ss(), 0) ;
+    TS_ASSERT_EQUALS(m1.get_average_dbm(), 0) ;
 
     // Write & read accessors
     AccessPoint ap2 ;
@@ -61,7 +61,7 @@ public:
      * Back to dBm:
      * 0.0028148 mW = -25.505481 dBm
      */
-    TS_ASSERT_DELTA(m1.get_average_ss(), -25.505481, 0.0001) ;
+    TS_ASSERT_DELTA(m1.get_average_dbm(), -25.505481, 0.0001) ;
 
     m1.clear() ;
     std::vector<int> vi2 ;
@@ -69,7 +69,7 @@ public:
     vi2.push_back(-1) ;
     m1.set_ss_list(vi2) ;
     TS_ASSERT_EQUALS(m1.get_ss_list(), vi2) ;
-    TS_ASSERT_DELTA(m1.get_average_ss(), -4.0102782, 0.0001) ;
+    TS_ASSERT_DELTA(m1.get_average_dbm(), -4.0102782, 0.0001) ;
 
     m1.clear() ;
     Measurement m2 ;

owlps-positioner/tests/testutil.cc

@@ -345,8 +345,8 @@ bool TestUtil::measurement_equals(const Measurement &first,
   if (first == second)
     return true ;
 
-  // Compare average_ss
-  if (first.get_average_ss() != second.get_average_ss())
+  // Compare average_dbm
+  if (first.get_average_dbm() != second.get_average_dbm())
     return false ;
 
   // Compare ap values