new-lines: add `type: platform` config option

The new option infers the correct newline character(s) from the
operating system running yamllint.

tests/rules/test_new_lines.py

@@ -13,6 +13,8 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+from unittest import mock
+
 from tests.common import RuleTestCase
 
 
@@ -58,3 +60,37 @@ class NewLinesTestCase(RuleTestCase):
         self.check('\r\n', conf)
         self.check('---\ntext\n', conf, problem=(1, 4))
         self.check('---\r\ntext\r\n', conf)
+
+    def test_platform_type(self):
+        conf = ('new-line-at-end-of-file: disable\n'
+                'new-lines: {type: platform}\n')
+
+        self.check('', conf)
+
+        # mock the Linux new-line-character
+        with mock.patch('yamllint.rules.new_lines.linesep', '\n'):
+            self.check('\n', conf)
+            self.check('\r\n', conf, problem=(1, 1))
+            self.check('---\ntext\n', conf)
+            self.check('---\r\ntext\r\n', conf, problem=(1, 4))
+            self.check('---\r\ntext\n', conf, problem=(1, 4))
+            # FIXME: the following tests currently don't work
+            # because only the first line is checked for line-endings
+            # see: issue #475
+            # ---
+            # self.check('---\ntext\r\nfoo\n', conf, problem=(2, 4))
+            # self.check('---\ntext\r\n', conf, problem=(2, 4))
+
+        # mock the Windows new-line-character
+        with mock.patch('yamllint.rules.new_lines.linesep', '\r\n'):
+            self.check('\r\n', conf)
+            self.check('\n', conf, problem=(1, 1))
+            self.check('---\r\ntext\r\n', conf)
+            self.check('---\ntext\n', conf, problem=(1, 4))
+            self.check('---\ntext\r\n', conf, problem=(1, 4))
+            # FIXME: the following tests currently don't work
+            # because only the first line is checked for line-endings
+            # see: issue #475
+            # ---
+            # self.check('---\r\ntext\nfoo\r\n', conf, problem=(2, 4))
+            # self.check('---\r\ntext\n', conf, problem=(2, 4))

yamllint/rules/new_lines.py

@@ -18,8 +18,11 @@ Use this rule to force the type of new line characters.
 
 .. rubric:: Options
 
-* Set ``type`` to ``unix`` to use UNIX-typed new line characters (``\\n``), or
-  ``dos`` to use DOS-typed new line characters (``\\r\\n``).
+* Set ``type`` to ``unix`` to enforce UNIX-typed new line characters (``\\n``),
+  set ``type`` to ``dos`` to enforce DOS-typed new line characters
+  (``\\r\\n``), or set ``type`` to ``platform`` to infer the type from the
+  system running yamllint (``\\n`` on POSIX / UNIX / Linux / Mac OS systems or
+  ``\\r\\n`` on DOS / Windows systems).
 
 .. rubric:: Default values (when enabled)
 
@@ -30,19 +33,22 @@ Use this rule to force the type of new line characters.
      type: unix
 """
 
+from os import linesep
 
 from yamllint.linter import LintProblem
 
 
 ID = 'new-lines'
 TYPE = 'line'
-CONF = {'type': ('unix', 'dos')}
+CONF = {'type': ('unix', 'dos', 'platform')}
 DEFAULT = {'type': 'unix'}
 
 
 def check(conf, line):
     if conf['type'] == 'unix':
         newline_char = '\n'
+    elif conf['type'] == 'platform':
+        newline_char = linesep
     elif conf['type'] == 'dos':
         newline_char = '\r\n'