`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.)

1 files changed, 51 insertions, 0 deletions

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