#!/usr/bin/env ruby
# The above line makes the script executable - don't forget to chmod +x the file as well.
#
# Usage:
# './parseruby.rb gpxfilename'
#
# Uses the hpricot library to parse gpx files
#
require 'rubygems'
require 'hpricot'

# Trackpt class contains points with latitude, longitude and elevation
#
class Trackpt
attr_accessor :lat, :lon, :ele

def initialize(lat,lon,ele)
	@lat = lat
	@lon = lon
	@ele = ele
end
end

# Calculate distance on globe between two Coord points; result in m; using earth radius of 6371 km
#
def calc_distance(lat1, lon1, lat2, lon2)
    diff = 2 * Math.asin(Math.sqrt((Math.sin((lat1 - lat2)/2))**2 + Math.cos(lat1)*Math.cos(lat2)*(Math.sin((lon1-lon2)/2))**2))
    diff *= 6371.0 * 1000
    return diff
end	

# Some variables; elarray contains the raw elevation data, elfiltered the low-pass filtered elevation data (simple discrete filter based on Euler differentation),
# elfilteredhyst the filtered and hysteresis-quantized elevation data, diff the metric distance between the trackpoints
#
lonarray = Array.new
latarray = Array.new
elarray  = Array.new
elfiltered = Array.new
elfilteredhyst = Array.new
ptarray = Array.new
diff = Array.new
meter2ft = 3.28084
km2miles = 0.62137

# Uses command line parameter as filename; alternatively, use something like 
#  puts 'Enter filename:' 
#  filename = gets.chomp
# here to prompt for filename
#
filename = ARGV[0]

# Open GPX file and parse it
#
begin
doc = open(filename) { |f| Hpricot::XML(f) }

# Rescue loop that prompts for filename if file not found or not entered correctly as command-line argument
#
rescue
  begin
  puts 'File not found; please enter filename:'
  filename = gets.chomp
  doc = open(filename) { |f| Hpricot::XML(f) }
  rescue
    retry
  end
end

(doc/:trkpt).each do |trkpt|
    latarray.push(trkpt.attributes['lat'])
	lonarray.push(trkpt.attributes['lon'])
	elarray.push(trkpt.at("ele").innerHTML)
  end
  
# Some initialization
#
prev_ele = (elarray[1].to_f * meter2ft).round
prev_pt = Trackpt.new(latarray[1].to_f * Math::PI/180,lonarray[1].to_f * Math::PI/180,prev_ele)
prev_ele_hyst = prev_ele
dist = 0.0
eltotal = 0
desctotal = 0
# 'Smoothing' constant - similar to the rc time constant in first order systems but here in the spatial domain
rc = 40
# 'Hysteresis' constant (in feet)
hyst = 3
 
# Calculate distance array, filter elevation data, calculate ascent and descent totals
#
 elarray.each_index do |i| 
	ptarray[i] = Trackpt.new(latarray[i].to_f * Math::PI/180, lonarray[i].to_f * Math::PI/180, (elarray[i].to_f * meter2ft).round)
	diff[i] = calc_distance(ptarray[i].lat, ptarray[i].lon, prev_pt.lat, prev_pt.lon) * meter2ft
	if (diff[i] + rc) == 0: elfiltered[i] = prev_ele
	else 
	  alpha = diff[i]/(diff[i] + rc)
	  elfiltered[i] = (alpha * ptarray[i].ele + (1-alpha) * prev_ele)
	end
	elfilteredhyst[i] = case
		when elfiltered[i] > (prev_ele_hyst + hyst): elfiltered[i].round
		when elfiltered[i] < (prev_ele_hyst - hyst): elfiltered[i].round
		else prev_ele_hyst
	end
	
	if (elfilteredhyst[i]-prev_ele_hyst) > 0: eltotal += elfilteredhyst[i]-prev_ele_hyst
	elsif (elfilteredhyst[i]-prev_ele_hyst) < 0: desctotal += prev_ele_hyst-elfilteredhyst[i]
	end
		
	prev_pt	= ptarray[i]
	prev_ele = elfiltered[i]
	prev_ele_hyst = elfilteredhyst[i]
	dist += diff[i]

  end
 
# Output totals in miles/feet, write distance + elevation + filtered + filtered-quantized elevation data in text format (separated by tabs) to output file named 'outputdata.txt'
# ...includes some messing around to get the miles in two digits after the decimal point
#
puts "Total distance: #{(dist/(10 * meter2ft) * km2miles).round/100.to_f} miles"
puts "Total elevation gain: #{eltotal} feet"
puts "Total elevation loss: #{desctotal} feet"

outputfile = File.new('outputdata.txt','w')

diff.each_index do |i|
outputfile.puts "#{diff[i].round}\t#{ptarray[i].ele}\t#{elfiltered[i].round}\t#{elfilteredhyst[i]}"
end

outputfile.close