address a bug where Vector.sub (keys, ~1) was used, solving subscript error, but revealing another error in a subsequent test case

src/string-set.sml

@@ -538,6 +538,7 @@ struct
      * -  otherwise, just remove the key and child at this idx from parent
      * *)
     if Vector.length keys > 1 then
+      if idx > 0 then
         let
           val newKeys = Vector.tabulate (Vector.length keys - 1, fn keyIdx =>
             Vector.sub (keys, if keyIdx >= idx then keyIdx - 1 else keyIdx))
@@ -547,7 +548,24 @@ struct
               Vector.sub
                 (children, if childIdx >= idx then childIdx - 1 else childIdx))
 
-        val newNode = parentConstructor {keys = newKeys, children = newChildren}
+          val newNode =
+            parentConstructor {keys = newKeys, children = newChildren}
+        in
+          CHANGED newNode
+        end
+      else
+        (* if idx = 0, then have to slice first element off from vector *)
+        let
+          val keySlice = VectorSlice.slice (keys, 1, SOME
+            (Vector.length keys - 1))
+          val newKeys = VectorSlice.vector keySlice
+
+          val childrenSlice = VectorSlice.slice (children, 1, SOME
+            (Vector.length children - 1))
+          val newChildren = VectorSlice.vector childrenSlice
+
+          val newNode =
+            parentConstructor {keys = newKeys, children = newChildren}
         in
           CHANGED newNode
         end

tests/string-set-tests.sml

@@ -118,18 +118,19 @@ struct
       val trie3 = StringSet.remove ("abcd", trie)
       val _ = assertTrue (trie = trie3, "removing key (abcd) which doesn't exist in trie returns same trie")
 
-      (* ERROR: Removing "abcde" causes a subscript error. Need to fix. *)
       val _ = assertTrue (StringSet.exists ("abcde", trie), "abcde exists before remove in remove5")
       val trie = StringSet.remove ("abcde", trie)
       val _ = assertFalse (StringSet.exists ("abcde", trie), "abcde does not exist after remove in remove5")
 
       val _ = assertTrue (StringSet.exists ("x", trie), "x exists before remove in remove5")
       val trie = StringSet.remove ("x", trie)
-      val _ = assertTrue (StringSet.exists ("x", trie), "x does not exist after remove in remove5")
+      val _ = assertFalse (StringSet.exists ("x", trie), "x does not exist after remove in remove5")
 
+      (* error: "abc" should exist at this point, but it seems not to. 
+       * find out why and fix. *)
       val _ = assertTrue (StringSet.exists ("abc", trie), "abc exists before remove in remove5")
       val trie = StringSet.remove ("abc", trie)
-      val _ = assertTrue (StringSet.exists ("abc", trie), "abc does not exist after remove in remove5")
+      val _ = assertFalse (StringSet.exists ("abc", trie), "abc does not exist after remove in remove5")
 
       val _ = assertTrue (StringSet.exists ("ab", trie), "trie still contains ab after removals in remove5")
       val _ = assertTrue (StringSet.exists ("ade", trie), "trie still contains ade after removals in remove5")