fix(stringx) rpartition returned bad order args if not found

fixes #299

CHANGELOG.md

@@ -32,6 +32,7 @@
   - `dir.copyfile`, `dir.movefile`, and `dir.makepath` create the new file/path with
     the requested casing, and no longer force lowercase (only impacted Windows)
   - added a missing assertion on `path.getmtime` [#291](https://github.com/stevedonovan/Penlight/pull/291)
+  - `stringx.rpartition` returned bad resuklts on a not-found [#299](https://github.com/stevedonovan/Penlight/pull/299)
 
 ## 1.6.0 (2018-11-23)
 

lua/pl/stringx.lua

@@ -442,7 +442,11 @@ end
 function stringx.rpartition(s,ch)
     assert_string(1,s)
     assert_nonempty_string(2,ch)
-    return _partition(s,ch,stringx.rfind)
+    local a,b,c = _partition(s,ch,stringx.rfind)
+    if a == s then -- no match found
+        return c,b,a
+    end
+    return a,b,c
 end
 
 --- return the 'character' at the index.

tests/test-stringx.lua

@@ -244,9 +244,12 @@ asserteq(T(stringx.partition('a', 'a')), T('', 'a', ''))
 asserteq(T(stringx.partition('abc', 'b')), T('a', 'b', 'c'))
 asserteq(T(stringx.partition('abc', '.+')), T('abc','',''))
 asserteq(T(stringx.partition('a,b,c', ',')), T('a',',','b,c'))
+asserteq(T(stringx.partition('abc', '/')), T('abc', '', ''))
 -- rpartition
 asserteq(T(stringx.rpartition('a/b/c', '/')), T('a/b', '/', 'c'))
 asserteq(T(stringx.rpartition('abc', 'b')), T('a', 'b', 'c'))
+asserteq(T(stringx.rpartition('a', 'a')), T('', 'a', ''))
+asserteq(T(stringx.rpartition('abc', '/')), T('', '', 'abc'))
 
 
 -- at (works like s:sub(idx,idx), so negative indices allowed