Use appropriate style for serialized strings

When psych parses YAML with string in the folded style as:

    a: >
      some
      inline
      content

and serializes it back to YAML,
then it renders the string in the literal style instead:

    a: |
      some inline content

This patch fixes this behaviour. When a string doesn't contain newlines
(blank line at the end is ignored) and it's not necessary to be quoted,
then it will use plain style or folded style according to its length and
the `line_width` option.

    # line_width = 100
    a: some inline content

    # line_width = 11
    a: >
      some inline
      content

This change was originally proposed by @Iazel.

2 files changed, 34 insertions, 5 deletions

diff --git a/lib/psych/visitors/yaml_tree.rb b/lib/psych/visitors/yaml_tree.rb
index e24b655..976ea8d 100644
--- a/lib/psych/visitors/yaml_tree.rb
+++ b/lib/psych/visitors/yaml_tree.rb
@@ -309,19 +309,20 @@ module Psych
           style = Nodes::Scalar::LITERAL
           plain = false
           quote = false
-        elsif o =~ /\n/
+        elsif o =~ /\n[^\Z]/  # match \n except blank line at the end of string
           style = Nodes::Scalar::LITERAL
         elsif o == '<<'
           style = Nodes::Scalar::SINGLE_QUOTED
           tag   = 'tag:yaml.org,2002:str'
           plain = false
           quote = false
+        elsif line_width != 0 && o.length > line_width
+          style = Nodes::Scalar::FOLDED
+          o += "\n" unless o =~ /\n\Z/  # to avoid non-default chomping indicator
         elsif o =~ /^[^[:word:]][^"]*$/
           style = Nodes::Scalar::DOUBLE_QUOTED
-        else
-          unless String === @ss.tokenize(o)
-            style = Nodes::Scalar::SINGLE_QUOTED
-          end
+        elsif not String === @ss.tokenize(o)
+          style = Nodes::Scalar::SINGLE_QUOTED
         end
 
         ivars = find_ivars o
@@ -588,6 +589,10 @@ module Psych
           accept target.instance_variable_get(iv)
         end
       end
+
+      def line_width
+        @line_width ||= (@options[:line_width] || 0)
+      end
     end
   end
 end
diff --git a/test/psych/test_string.rb b/test/psych/test_string.rb
index 26a4e20..3188b5c 100644
--- a/test/psych/test_string.rb
+++ b/test/psych/test_string.rb
@@ -34,6 +34,30 @@ module Psych
       assert_match(/---\s*"/, yaml)
     end
 
+    def test_plain_when_shorten_than_line_width
+      str = "Lorem ipsum"
+      yaml = Psych.dump str, line_width: 12
+      assert_match /---\s*[^>|]+\n/, yaml
+    end
+
+    def test_folded_when_longer_than_line_width_and_no_newlines
+      str = "Lorem ipsum dolor sit amet, consectetur"
+      yaml = Psych.dump str, line_width: 12
+      assert_match /---\s*>\n(.*\n){3}\Z/, yaml
+    end
+
+    def test_folded_when_longer_than_line_width_and_trailing_newline
+      str = "Lorem ipsum dolor sit\n"
+      yaml = Psych.dump str, line_width: 12
+      assert_match /---\s*>\n(.*\n){2}\Z/, yaml
+    end
+
+    def test_literal_when_inner_newline
+      str = "Lorem ipsum\ndolor\n"
+      yaml = Psych.dump str, line_width: 12
+      assert_match /---\s*|\n(.*\n){2}\Z/, yaml
+    end
+
     def test_cycle_x
       str = X.new 'abc'
       assert_cycle str