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# PI = 3.1415926535
RAD_PER_DEG = 0.017453293  #  PI/180

# the great circle distance d will be in whatever units R is in

Rmiles = 3956           # radius of the great circle in miles
Rkm = 6371              # radius in kilometers...some algorithms use 6367
Rfeet = Rmiles * 5282   # radius in feet
Rmeters = Rkm * 1000    # radius in meters

@distances = Hash.new   # this is global because if computing lots of track point distances, it didn't make
        # sense to new a Hash each time over potentially 100's of thousands of points

=begin rdoc
given two lat/lon points, compute the distance between the two points using the haversine formula
the result will be a Hash of distances which are key'd by 'mi','km','ft', and 'm'
=end

def haversine_distance( lat1, lon1, lat2, lon2 )
    dlon = lon2 - lon1
    dlat = lat2 - lat1

    dlon_rad = dlon * RAD_PER_DEG 
    dlat_rad = dlat * RAD_PER_DEG

    lat1_rad = lat1 * RAD_PER_DEG
    lon1_rad = lon1 * RAD_PER_DEG

    lat2_rad = lat2 * RAD_PER_DEG
    lon2_rad = lon2 * RAD_PER_DEG

    # puts "dlon: #{dlon}, dlon_rad: #{dlon_rad}, dlat: #{dlat}, dlat_rad: #{dlat_rad}"

    a = (Math.sin(dlat_rad/2))**2 + Math.cos(lat1_rad) * Math.cos(lat2_rad) * (Math.sin(dlon_rad/2))**2
    c = 2 * Math.atan2( Math.sqrt(a), Math.sqrt(1-a))

    dMi = Rmiles * c          # delta between the two points in miles
    dKm = Rkm * c             # delta in kilometers
    dFeet = Rfeet * c         # delta in feet
    dMeters = Rmeters * c     # delta in meters

    @distances["mi"] = dMi
    @distances["km"] = dKm
    @distances["ft"] = dFeet
    @distances["m"] = dMeters
end

def test_haversine
    lon1 = -104.88544
    lat1 = 39.06546

    lon2 = -104.80
    lat2 = lat1

    haversine_distance( lat1, lon1, lat2, lon2 )

    puts "the distance from  #{lat1}, #{lon1} to #{lat2}, #{lon2} is:"
    puts "#{@distances['mi']} mi"
    puts "#{@distances['km']} km"
    puts "#{@distances['ft']} ft"
    puts "#{@distances['m']} m"

    if ( @distances['km'].to_s.match(/7\.376*/) != nil )
      puts "Test: Success"
    else
      puts "Test: Failed"
    end
end