improve performance of stripPrefix/stripSuffix; add tests

Author: michaelficarra

src/Data/String/CodeUnits.purs

@@ -44,10 +44,9 @@ import Data.String.Unsafe as U
 -- | stripPrefix (Pattern "http:") "https://purescript.org" == Nothing
 -- | ```
 stripPrefix :: Pattern -> String -> Maybe String
-stripPrefix prefix@(Pattern prefixS) str =
-  case indexOf prefix str of
-    Just 0 -> Just $ drop (length prefixS) str
-    _ -> Nothing
+stripPrefix (Pattern prefix) str =
+  let { before, after } = splitAt (length prefix) str in
+  if before == prefix then Just after else Nothing
 
 -- | If the string ends with the given suffix, return the portion of the
 -- | string left after removing it, as a `Just` value. Otherwise, return
@@ -58,10 +57,9 @@ stripPrefix prefix@(Pattern prefixS) str =
 -- | stripSuffix (Pattern ".exe") "psc" == Nothing
 -- | ```
 stripSuffix :: Pattern -> String -> Maybe String
-stripSuffix suffix@(Pattern suffixS) str =
-  case lastIndexOf suffix str of
-    Just x | x == length str - length suffixS -> Just $ take x str
-    _ -> Nothing
+stripSuffix (Pattern suffix) str =
+  let { before, after } = splitAt (length str - length suffix) str in
+  if after == suffix then Just before else Nothing
 
 -- | Checks whether the pattern appears in the given string.
 -- |

test/Test/Data/String.purs

@@ -17,25 +17,17 @@ testString = do
   assert $ not (S.null "a")
 
   log "stripPrefix"
+  -- this is a re-export from Data.String.CodeUnits, so the majority of tests are in there
   assertEqual
-    { actual: S.stripPrefix (Pattern "") ""
-    , expected: Just ""
-    }
-  assertEqual
-    { actual: S.stripPrefix (Pattern "") "abc"
-    , expected: Just "abc"
-    }
-  assertEqual
-    { actual: S.stripPrefix (Pattern "a") "abc"
-    , expected: Just "bc"
-    }
-  assertEqual
-    { actual: S.stripPrefix (Pattern "!") "abc"
-    , expected: Nothing
+    { actual: S.stripPrefix (Pattern "𝕒𝕓𝕔") "𝕒𝕓𝕔𝕕𝕖"
+    , expected: Just "𝕕𝕖"
     }
+
+  log "stripSuffix"
+  -- this is a re-export from Data.String.CodeUnits, so the majority of tests are in there
   assertEqual
-    { actual: S.stripPrefix (Pattern "!") ""
-    , expected: Nothing
+    { actual: S.stripSuffix (Pattern "𝕔𝕕𝕖") "𝕒𝕓𝕔𝕕𝕖"
+    , expected: Just "𝕒𝕓"
     }
 
   log "contains"

test/Test/Data/String/CodeUnits.purs

@@ -12,6 +12,58 @@ import Test.Assert (assert, assertEqual)
 
 testStringCodeUnits :: Effect Unit
 testStringCodeUnits = do
+  log "stripPrefix"
+  assertEqual
+    { actual: SCU.stripPrefix (Pattern "abc") "abcde"
+    , expected: Just "de"
+    }
+  assertEqual
+    { actual: SCU.stripPrefix (Pattern "xyz") "abcde"
+    , expected: Nothing
+    }
+  assertEqual
+    { actual: SCU.stripPrefix (Pattern "abcd") "ab"
+    , expected: Nothing
+    }
+  assertEqual
+    { actual: SCU.stripPrefix (Pattern "abc") "abc"
+    , expected: Just ""
+    }
+  assertEqual
+    { actual: SCU.stripPrefix (Pattern "") "abc"
+    , expected: Just "abc"
+    }
+  assertEqual
+    { actual: SCU.stripPrefix (Pattern "") ""
+    , expected: Just ""
+    }
+
+  log "stripSuffix"
+  assertEqual
+    { actual: SCU.stripSuffix (Pattern "cde") "abcde"
+    , expected: Just "ab"
+    }
+  assertEqual
+    { actual: SCU.stripSuffix (Pattern "xyz") "abcde"
+    , expected: Nothing
+    }
+  assertEqual
+    { actual: SCU.stripSuffix (Pattern "abcd") "cd"
+    , expected: Nothing
+    }
+  assertEqual
+    { actual: SCU.stripSuffix (Pattern "abc") "abc"
+    , expected: Just ""
+    }
+  assertEqual
+    { actual: SCU.stripSuffix (Pattern "") "abc"
+    , expected: Just "abc"
+    }
+  assertEqual
+    { actual: SCU.stripSuffix (Pattern "") ""
+    , expected: Just ""
+    }
+
   log "charAt"
   assertEqual
     { actual: SCU.charAt 0 ""