add extension to swap satin column rails

inx/inkstitch_flip.inx

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<inkscape-extension xmlns="http://www.inkscape.org/namespace/inkscape/extension">
+    <_name>Flip Satin Columns</_name>
+    <id>org.inkstitch.flip_satins</id>
+    <dependency type="executable" location="extensions">inkstitch.py</dependency>
+    <dependency type="executable" location="extensions">inkex.py</dependency>
+    <param name="extension" type="string" gui-hidden="true">flip</param>
+    <effect>
+        <object-type>all</object-type>
+        <effects-menu>
+            <submenu _name="Embroidery"/>
+        </effects-menu>
+    </effect>
+    <script>
+        <command reldir="extensions" interpreter="python">inkstitch.py</command>
+    </script>
+</inkscape-extension>

lib/extensions/__init__.py

@@ -6,3 +6,4 @@ from simulate import Simulate
 from input import Input
 from output import Output
 from zip import Zip
+from flip import Flip

lib/extensions/flip.py

@@ -0,0 +1,34 @@
+import sys
+import inkex
+import cubicsuperpath
+
+from .base import InkstitchExtension
+from ..i18n import _
+from ..elements import SatinColumn
+
+class Flip(InkstitchExtension):
+    def flip(self, satin):
+        csp = cubicsuperpath.parsePath(satin.node.get("d"))
+
+        if len(csp) > 1:
+            # find the rails (the two longest paths) and swap them
+            indices = range(len(csp))
+            indices.sort(key=lambda i: len(csp[i]), reverse=True)
+
+            first = indices[0]
+            second = indices[1]
+            csp[first], csp[second] = csp[second], csp[first]
+
+            satin.node.set("d", cubicsuperpath.formatPath(csp))
+
+    def effect(self):
+        if not self.get_elements():
+            return
+
+        if not self.selected:
+            inkex.errormsg(_("Please select one or more satin columns to flip."))
+            return
+
+        for element in self.elements:
+            if isinstance(element, SatinColumn):
+                self.flip(element)

messages.po

@@ -8,7 +8,7 @@ msgid ""
 msgstr ""
 "Project-Id-Version: PROJECT VERSION\n"
 "Report-Msgid-Bugs-To: EMAIL@ADDRESS\n"
-"POT-Creation-Date: 2018-06-28 20:32-0400\n"
+"POT-Creation-Date: 2018-06-30 13:02-0400\n"
 "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
 "Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
 "Language-Team: LANGUAGE <LL@li.org>\n"
@@ -198,6 +198,9 @@ msgid ""
 "Seeing a 'no such option' message?  Please restart Inkscape to fix."
 msgstr ""
 
+msgid "Please select one or more satin columns to flip."
+msgstr ""
+
 msgid ""
 "Ink/Stitch can install files (\"add-ons\") that make it easier to use "
 "Inkscape to create machine embroidery designs.  These add-ons will be "