class ActiveSupport::Duration::ISO8601Serializer

Serializes duration to string according to ISO 8601 Duration format.

def initialize(duration, precision: nil) 

def initialize(duration, precision: nil)
  @duration = duration
  @precision = precision
end

def normalize

If all parts are negative it will negate all of them and return minus as a sign.
Zero parts are removed as not significant.
Parts are summarized (as they can become repetitive due to addition, etc).
Return pair of duration's parts and whole duration sign. 
def normalize
  parts = @duration.parts.each_with_object(Hash.new(0)) do |(k,v),p|
    p[k] += v  unless v.zero?
  end
  # If all parts are negative - let's make a negative duration
  sign = ''
  if parts.values.all? { |v| v < 0 }
    sign = '-'
    parts.transform_values!(&:-@)
  end
  [parts, sign]
end

def serialize

Builds and returns output string. 
def serialize
  output = 'P'
  parts, sign = normalize
  output << "#{parts[:years]}Y"   if parts.key?(:years)
  output << "#{parts[:months]}M"  if parts.key?(:months)
  output << "#{parts[:weeks]}W"   if parts.key?(:weeks)
  output << "#{parts[:days]}D"    if parts.key?(:days)
  time = ''
  time << "#{parts[:hours]}H"     if parts.key?(:hours)
  time << "#{parts[:minutes]}M"   if parts.key?(:minutes)
  if parts.key?(:seconds)
    time << "#{sprintf(@precision ? "%0.0#{@precision}f" : '%g', parts[:seconds])}S"
  end
  output << "T#{time}"  if time.present?
  "#{sign}#{output}"
end