class SRTM

# File lib/SRTMGem.rb, line 4
def initialize(input_file)
  @input_file = File.open(input_file, 'rb')
end

# File lib/SRTMGem.rb, line 9
def get_elevation(longitude, latitude)
  d_longitude = longitude
  d_latitude = latitude
  i_longitude = d_longitude.to_i
  i_latitude = d_latitude.to_i
  intervals = 1200

  if d_longitude < 0
    i_longitude = (i_longitude - 1) * -1
    d_longitude = (i_longitude + d_longitude) + i_longitude.to_f
  end

  if d_latitude < 0
    i_latitude = (i_latitude - 1) * -1
    d_latitude = (i_latitude.to_f + d_latitude) + i_latitude.to_f
  end


  i_longitude_index = ((d_longitude - i_longitude.to_f) * intervals).to_i
  i_latitude_index = ((d_latitude - i_latitude.to_f) * intervals).to_i


  if i_longitude_index >= intervals
    i_longitude_index = intervals - 1
  end

  if i_latitude_index >= intervals
    i_latitude_index = intervals - 1
  end


  d_longitude_offset = d_longitude - i_longitude.to_f
  d_latitude_offset = d_latitude - i_latitude.to_f


  position = (((intervals - i_latitude_index) - 1) * (intervals + 1)) + i_longitude_index
  d_left_top = read_bin(position)
  position = ((intervals - i_latitude_index) * (intervals + 1)) + i_longitude_index
  d_left_bottom = read_bin(position)
  position = (((intervals - i_latitude_index) - 1) * (intervals + 1)) + i_longitude_index +1
  d_right_top = read_bin(position)
  position = ((intervals - i_latitude_index) * (intervals + 1)) + i_longitude_index + 1
  d_right_bottom = read_bin(position)


  d_delta_longitude = d_longitude_offset - (i_longitude_index.to_f * (1 / intervals.to_f))
  d_delta_latitude = d_latitude_offset - (i_latitude_index.to_f * (1 / intervals.to_f))

  d_longitude_height_left = d_left_bottom - calc_elev(d_left_bottom - d_left_top, 1 / intervals.to_f, d_delta_latitude)
  d_longitude_height_right = d_right_bottom - calc_elev(d_right_bottom - d_right_top, 1 / intervals.to_f, d_delta_latitude)

  d_elevation = d_longitude_height_left - calc_elev(d_longitude_height_left - d_longitude_height_right, 1 / intervals.to_f, d_delta_longitude)
  return d_elevation + 0.5
end

# File lib/SRTMGem.rb, line 73
def calc_elev(height, length, diff)
  return (height * diff) / length
end

# File lib/SRTMGem.rb, line 65
def read_bin(position)
  @input_file.seek(position * 2)
  bytes = @input_file.read(2)
  first_byte = bytes[0].ord
  second_byte = bytes[1].ord
  return first_byte * 256 + second_byte
end