`ActiveSupport::Duration` supports ISO8601 formatting and parsing.

```ruby
ActiveSupport::Duration.parse('P3Y6M4DT12H30M5S')

(3.years + 3.days).iso8601
```

Inspired by Arnau Siches' [ISO8601 gem](https://github.com/arnau/ISO8601/)
and rewritten by Andrey Novikov with suggestions from Andrew White. Test
data from the ISO8601 gem redistributed under MIT license.

(Will be used to support the PostgreSQL interval data type.)

2 files changed, 173 insertions, 0 deletions

diff --git a/activesupport/lib/active_support/duration/iso8601_parser.rb b/activesupport/lib/active_support/duration/iso8601_parser.rb
new file mode 100644
index 0000000000..07af58ad99
--- /dev/null
+++ b/activesupport/lib/active_support/duration/iso8601_parser.rb
@@ -0,0 +1,122 @@
+require 'strscan'
+
+module ActiveSupport
+  class Duration
+    # Parses a string formatted according to ISO 8601 Duration into the hash.
+    #
+    # See {ISO 8601}[http://en.wikipedia.org/wiki/ISO_8601#Durations] for more information.
+    #
+    # This parser allows negative parts to be present in pattern.
+    class ISO8601Parser # :nodoc:
+      class ParsingError < ::ArgumentError; end
+
+      PERIOD_OR_COMMA = /\.|,/
+      PERIOD = '.'.freeze
+      COMMA = ','.freeze
+
+      SIGN_MARKER = /\A\-|\+|/
+      DATE_MARKER = /P/
+      TIME_MARKER = /T/
+      DATE_COMPONENT = /(\-?\d+(?:[.,]\d+)?)(Y|M|D|W)/
+      TIME_COMPONENT = /(\-?\d+(?:[.,]\d+)?)(H|M|S)/
+
+      DATE_TO_PART = { 'Y' => :years, 'M' => :months, 'W' => :weeks, 'D' => :days }
+      TIME_TO_PART = { 'H' => :hours, 'M' => :minutes, 'S' => :seconds }
+
+      DATE_COMPONENTS = [:years, :months, :days]
+      TIME_COMPONENTS = [:hours, :minutes, :seconds]
+
+      attr_reader :parts, :scanner
+      attr_accessor :mode, :sign
+
+      def initialize(string)
+        @scanner = StringScanner.new(string)
+        @parts = {}
+        @mode = :start
+        @sign = 1
+      end
+
+      def parse!
+        while !finished?
+          case mode
+            when :start
+              if scan(SIGN_MARKER)
+                self.sign = (scanner.matched == '-') ? -1 : 1
+                self.mode = :sign
+              else
+                raise_parsing_error
+              end
+
+            when :sign
+              if scan(DATE_MARKER)
+                self.mode = :date
+              else
+                raise_parsing_error
+              end
+
+            when :date
+              if scan(TIME_MARKER)
+                self.mode = :time
+              elsif scan(DATE_COMPONENT)
+                parts[DATE_TO_PART[scanner[2]]] = number * sign
+              else
+                raise_parsing_error
+              end
+
+            when :time
+              if scan(TIME_COMPONENT)
+                parts[TIME_TO_PART[scanner[2]]] = number * sign
+              else
+                raise_parsing_error
+              end
+
+          end
+        end
+
+        validate!
+        parts
+      end
+
+      private
+
+      def finished?
+        scanner.eos?
+      end
+
+      # Parses number which can be a float with either comma or period.
+      def number
+        scanner[1] =~ PERIOD_OR_COMMA ? scanner[1].tr(COMMA, PERIOD).to_f : scanner[1].to_i
+      end
+
+      def scan(pattern)
+        scanner.scan(pattern)
+      end
+
+      def raise_parsing_error(reason = nil)
+        raise ParsingError, "Invalid ISO 8601 duration: #{scanner.string.inspect} #{reason}".strip
+      end
+
+      # Checks for various semantic errors as stated in ISO 8601 standard.
+      def validate!
+        raise_parsing_error('is empty duration') if parts.empty?
+
+        # Mixing any of Y, M, D with W is invalid.
+        if parts.key?(:weeks) && (parts.keys & DATE_COMPONENTS).any?
+          raise_parsing_error('mixing weeks with other date parts not allowed')
+        end
+
+        # Specifying an empty T part is invalid.
+        if mode == :time && (parts.keys & TIME_COMPONENTS).empty?
+          raise_parsing_error('time part marker is present but time part is empty')
+        end
+
+        fractions = parts.values.reject(&:zero?).select { |a| (a % 1) != 0 }
+        unless fractions.empty? || (fractions.size == 1 && fractions.last == @parts.values.reject(&:zero?).last)
+          raise_parsing_error '(only last part can be fractional)'
+        end
+
+        return true
+      end
+    end
+  end
+end
diff --git a/activesupport/lib/active_support/duration/iso8601_serializer.rb b/activesupport/lib/active_support/duration/iso8601_serializer.rb
new file mode 100644
index 0000000000..05c6a083a9
--- /dev/null
+++ b/activesupport/lib/active_support/duration/iso8601_serializer.rb
@@ -0,0 +1,51 @@
+require 'active_support/core_ext/object/blank'
+require 'active_support/core_ext/hash/transform_values'
+
+module ActiveSupport
+  class Duration
+    # Serializes duration to string according to ISO 8601 Duration format.
+    class ISO8601Serializer
+      def initialize(duration, precision: nil)
+        @duration = duration
+        @precision = precision
+      end
+
+      # 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
+
+      private
+
+        # Return pair of duration's parts and whole duration sign.
+        # Parts are summarized (as they can become repetitive due to addition, etc).
+        # Zero parts are removed as not significant.
+        # If all parts are negative it will negate all of them and return minus as a 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
+    end
+  end
+end