From ac090a01a8202e37971a67af117eeeed0470c042 Mon Sep 17 00:00:00 2001
From: Chris Schlaeger <chris@linux.com>
Date: Sun, 22 Nov 2015 06:18:24 +0100
Subject: Adding support for heart rate variability score.

---
 lib/postrunner/ActivitySummary.rb |   6 +++
 lib/postrunner/HRV_Analyzer.rb    | 105 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 111 insertions(+)
 create mode 100644 lib/postrunner/HRV_Analyzer.rb

diff --git a/lib/postrunner/ActivitySummary.rb b/lib/postrunner/ActivitySummary.rb
index c9234ac..b01dfed 100644
--- a/lib/postrunner/ActivitySummary.rb
+++ b/lib/postrunner/ActivitySummary.rb
@@ -14,6 +14,7 @@ require 'fit4ruby'
 
 require 'postrunner/FlexiTable'
 require 'postrunner/ViewFrame'
+require 'postrunner/HRV_Analyzer'
 
 module PostRunner
 
@@ -112,6 +113,11 @@ module PostRunner
       vo2max = @fit_activity.vo2max
       t.row([ 'VO2max:', vo2max ? vo2max : '-' ])
 
+      hrv = HRV_Analyzer.new(@fit_activity)
+      if hrv.has_hrv_data?
+        t.row([ 'HRV Score:', hrv.lnrmssdx20 ])
+      end
+
       t
     end
 
diff --git a/lib/postrunner/HRV_Analyzer.rb b/lib/postrunner/HRV_Analyzer.rb
new file mode 100644
index 0000000..60d6134
--- /dev/null
+++ b/lib/postrunner/HRV_Analyzer.rb
@@ -0,0 +1,105 @@
+#!/usr/bin/env ruby -w
+# encoding: UTF-8
+#
+# = HRV_Analyzer.rb -- PostRunner - Manage the data from your Garmin sport devices.
+#
+# Copyright (c) 2015 by Chris Schlaeger <cs@taskjuggler.org>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of version 2 of the GNU General Public License as
+# published by the Free Software Foundation.
+#
+
+module PostRunner
+
+  # This class analyzes the heart rate variablity based on the R-R intervals
+  # in the given FIT file. It can compute RMSSD and a HRV score if the data
+  # quality is good enough.
+  class HRV_Analyzer
+
+    def initialize(fit_file)
+      @fit_file = fit_file
+      collect_rr_intervals
+    end
+
+    # The method can be used to check if we have valid HRV data. The FIT file
+    # must have HRV data, it must have correctly measured 90% of the beats and
+    # the measurement duration must be at least 30 seconds.
+    def has_hrv_data?
+      !@fit_file.hrv.empty? &&
+        @missed_beats < (0.1 * @rr_intervals.length) &&
+        total_duration > 30.0
+    end
+
+    # Return the total duration of all measured intervals in seconds.
+    def total_duration
+      @rr_intervals.inject(:+)
+    end
+
+    # root mean square of successive differences
+    def rmssd
+      last_i = nil
+      sum = 0.0
+      @rr_intervals.each do |i|
+        if last_i
+          sum += (last_i - i) ** 2.0
+        end
+        last_i = i
+      end
+      Math.sqrt(sum / (@rr_intervals.length - 1))
+    end
+
+    # The RMSSD value is not very easy to memorize. Alternatively, we can
+    # multiply the natural logarithm of RMSSD by -20. This usually results in
+    # values between 40 (for untrained) and 100 (for higly trained) athletes.
+    def lnrmssdx20
+      (-20.0 * Math.log(rmssd)).round.to_i
+    end
+
+    private
+
+    def collect_rr_intervals
+      raw_rr_intervals = []
+      @fit_file.hrv.each do |hrv|
+        raw_rr_intervals += hrv.time.compact
+      end
+
+      prev_dts = []
+      avg_dt = nil
+      @missed_beats = 0
+      @rr_intervals = []
+      raw_rr_intervals.each do |dt|
+        # Sometimes the hrv data is missing one or more beats. The next
+        # detected beat is then listed with a time interval since the last
+        # detected beat. We try to detect these skipped beats by looking for
+        # time intervals that are 1.8 or more times larger than the average of
+        # the last 5 good intervals.
+        if avg_dt && dt > 1.8 * avg_dt
+          # If we have found skipped beats we calcluate how many beats were
+          # skipped.
+          skip = (dt / avg_dt).round.to_i
+          # We count the total number of skipped beats. We don't use the HRV
+          # data if too many beats were skipped.
+          @missed_beats += skip
+          # Insert skip times the average skipped beat intervals.
+          skip.times do
+            new_dt = dt / skip
+            @rr_intervals << new_dt
+            prev_dts << new_dt
+            prev_dts.shift if prev_dts.length > 5
+          end
+        else
+          @rr_intervals << dt
+          # We keep a list of the previous 5 good intervals and compute the
+          # average value of them.
+          prev_dts << dt
+          prev_dts.shift if prev_dts.length > 5
+        end
+        avg_dt = prev_dts.inject(:+) / prev_dts.length
+      end
+    end
+
+  end
+
+end
+
-- 
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