progress with coding insert function

src/string-set.sml

@@ -58,10 +58,9 @@ struct
       else SEARCH_KEY_CONTAINS_TRIE_KEY
     else 
       (* implicit: keyPos > String.size searchKey *) 
-      if keyPos <= String.size trieKey then
-        TRIE_KEY_CONTAINS_SEARCH_KEY
-      else
-        NO_SEARCH_MATCH
+      if keyPos <= String.size trieKey 
+      then TRIE_KEY_CONTAINS_SEARCH_KEY
+      else NO_SEARCH_MATCH
 
   fun isFoundNode node =
     case node of
@@ -157,20 +156,59 @@ struct
       else INSERT_KEY_CONTAINS_TRIE_KEY
     else 
       (* implicit: keyPos > String.size insertKey *) 
-      if keyPos <= String.size trieKey then
-        TRIE_KEY_CONTAINS_INSERT_KEY
+      if keyPos <= String.size trieKey 
+      then TRIE_KEY_CONTAINS_INSERT_KEY
+      else NO_INSERT_MATCH
+
+  datatype insert_bin_search_result =
+    INSERT_NEW_CHILD of int
+  | FOUND_INSERT_POS of int
+  | APPEND_NEW_CHILD
+
+  fun linearSearch (findChr, keyPos, idx, children) =
+    if idx = Vector.length children then
+      APPEND_NEW_CHILD
     else
-        NO_INSERT_MATCH
+      let
+        val curStr = Vector.sub (children, idx)
+        val curChr = String.sub (curStr, keyPos)
+      in
+        if curChr > findChr then INSERT_NEW_CHILD idx
+        else linearSearch (findChr, keyPos, idx + 1, children)
+      end
+
+  fun helpInsertBinSearch (findChr, keyPos, children, low, high) =
+    let
+      val mid = low + ((high - low) div 2)
+    in
+      if high >= low then
+        let
+          val midStr = Vector.sub (children, mid)
+          val midChr = String.sub (midStr, keyPos)
+        in
+          if midChr = findChr then
+            FOUND_INSERT_POS mid
+          else if midChr < findChr then
+            helpInsertBinSearch (findChr, keyPos, children, mid + 1, high)
+          else
+            helpInsertBinSearch (findChr, keyPos, children, low, mid - 1)
+        end
+      else
+        linearSearch (findChr, keyPos, mid, children)
+    end
+
+  fun insertBinSearch (findChr, keyPos, children) =
+    helpInsertBinSearch
+      (findChr, keyPos, children, 0, Vector.length children - 1)
 
   (* 
    * todo: 
-   * Code another function for binary search; this time for insert.
-   * The difference between findBinSearch and insertBinSearch
-   * should be that, while findBinSearch returns NONE if the value is not found,
-   * insertBinSearch should return the appropriate index to insert at
-   * if the value is not found.
-   * The main insert function can then create new key/children vectors,
-   * containing the insertKey and FOUND at the appropriate index.
+   * Complete code for CHILDREN and FOUND_WITH_CHILDREN cases in insert function.
+   * The start will be similar to the exists function, 
+   * with binary search possibly followed by a string matching comparison.
+   * No string matching comparison is needed if the current char is not found
+   * in the binary search stage, however; 
+   * instead, we will insert a new element into the array when that happens.
    *)
 
   fun insert (insKey, keyPos, trie) =