add function to split left side of tree, given an index

src/rrb_rope.sml

@@ -21,31 +21,6 @@ struct
         Word.<< (w, bits)
       end
 
-  fun getLast tree =
-    case tree of
-      BRANCH n => getLast (Vector.sub (n, Vector.length n - 1))
-    | LEAF i => Vector.sub (i, Vector.length i - 1)
-
-  fun helpGet (key: Word.word, level, tree) =
-    case tree of
-      BRANCH nodes =>
-        let
-          val w = Word.>> (key, level)
-          val w = Word.andb (w, mask)
-          val node = Vector.sub (nodes, Word.toInt w)
-        in
-          helpGet (key, level - bits, node)
-        end
-    | LEAF items =>
-        let val idx = Word.andb (key, mask)
-        in Vector.sub (items, Word.toInt idx)
-        end
-
-  fun get (key, {shift, root, count}: t) =
-    let val key = Word.fromInt key
-    in if key >= tailoff count then getLast root else helpGet (key, shift, root)
-    end
-
   datatype append_result = UPDATE | APPEND
 
   fun helpAppend (item, tree) =
@@ -95,11 +70,56 @@ struct
           {count = count + 1, root = root, shift = shift}
         end
 
-  fun mk (count, acc) =
+  fun getLast tree =
-    if count = 97 then acc
+    case tree of
-    else let val acc = append (count, acc) in mk (count + 1, acc) end
+      BRANCH n => getLast (Vector.sub (n, Vector.length n - 1))
+    | LEAF i => Vector.sub (i, Vector.length i - 1)
 
-  val root = mk (0, empty)
+  fun helpGet (key: Word.word, level, tree) =
-
+    case tree of
-  val result = get (99, root)
+      BRANCH nodes =>
+        let
+          val w = Word.>> (key, level)
+          val w = Word.andb (w, mask)
+          val node = Vector.sub (nodes, Word.toInt w)
+        in
+          helpGet (key, level - bits, node)
+        end
+    | LEAF items =>
+        let val idx = Word.andb (key, mask)
+        in Vector.sub (items, Word.toInt idx)
+        end
+
+  fun get (key, {shift, root, count}: t) =
+    let val key = Word.fromInt key
+    in if key >= tailoff count then getLast root else helpGet (key, shift, root)
+    end
+
+  fun splitKeepingLeft (idx, level, tree) =
+    case tree of
+      BRANCH nodes =>
+        let
+          val w = Word.>> (idx, level)
+          val w = Word.andb (w, mask)
+          val nodeIdx = Word.toInt w
+
+          val node = Vector.sub (nodes, nodeIdx)
+          val newNode = splitKeepingLeft (idx, level - bits, node)
+          val newNode = Vector.fromList [newNode]
+          val newNode = VectorSlice.full newNode
+
+          val newNodes = VectorSlice.slice (nodes, 0, SOME nodeIdx)
+          val newNodes = VectorSlice.concat [newNodes, newNode]
+        in
+          BRANCH newNodes
+        end
+    | LEAF items =>
+        let
+          val w = Word.andb (idx, mask)
+          val idx = Word.toInt w
+          val items = VectorSlice.slice (items, 0, SOME idx)
+          val items = VectorSlice.vector items
+        in
+          LEAF items
+        end
 end