fix splitOn when sep appears at end (digital-asset#5872)

Fixes digital-asset#5786. CHANGELOG_BEGIN [DAML Standard Lib] ``DA.Text.splitOn`` will now correctly handle the case where the separator appears at the end but should not be matched, as in ``splitOn "aa" "aaa" == ["", "a"]`` (previously this was erroneously returning ``["", "a", ""]``). See digital-asset#5786 for more details. CHANGELOG_END
kgeonw · May 7, 2020 · e582e61 · e582e61
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diff --git a/compiler/damlc/daml-stdlib-src/DA/Text.daml b/compiler/damlc/daml-stdlib-src/DA/Text.daml
@@ -205,14 +205,20 @@ dropWhile p = implode . P.dropWhile p . explode
 dropWhileEnd : (Text -> Bool) -> Text -> Text
 dropWhileEnd p = implode . L.dropWhileEnd p . explode
 
+-- Separate function because recursion is only allowed at the top-level and we
+-- don't want to pay for many implode/explode operations.
+-- This shoud not be exported.
+splitOnHelper : [Text] -> [Text] -> [Text]
+splitOnHelper s t =
+    case L.stripInfix s t of
+        None -> [implode t]
+        Some (before, after) -> implode before :: splitOnHelper s after
+
 -- | Break a text into pieces separated by the first text argument
 -- (which cannot be empty), consuming the delimiter.
 splitOn : Text -> Text -> [Text]
 splitOn "" t' = [t']
-splitOn _ "" = [""]
-splitOn (explode -> s) (explode -> t) =
-  let fixit = if L.isSuffixOf s t then (++ [""]) else identity
-  in fixit $ implode <$> L.repeatedly (fmap (L.dropPrefix s) . L.breakOn s) t
+splitOn s t = splitOnHelper (explode s) (explode t)
 
 -- | Split a text before a given position so that for `0 <= n <= length t`,
 -- `length (fst (splitAt n t)) == n`.

diff --git a/compiler/damlc/tests/daml-test-files/Text.daml b/compiler/damlc/tests/daml-test-files/Text.daml
@@ -139,6 +139,10 @@ testSplitOn = scenario do
   [""] === T.splitOn "" ""
   ["", "", ""] === T.splitOn "." ".."
   ["1", "2", ",3"] === T.splitOn ",," "1,,2,,,3"
+  ["", "a"] === T.splitOn "aa" "aaa"
+  ["", ""] === T.splitOn "aa" "aa"
+  ["", "a"] === T.splitOn "aa" "aaa"
+  ["", "", ""] === T.splitOn "aa" "aaaa"
 
 testSplitAt = scenario do
   ("", "test") === T.splitAt 0 "test"