From 185ef3fc12b28a52c7f8b03bd11a7ef1d3c6da48 Mon Sep 17 00:00:00 2001
From: Humza Shahid <humzasaur@gmail.com>
Date: Sun, 18 Jan 2026 06:58:41 +0000
Subject: [PATCH] done implementing 'PersistentVector.merge', which means that
 'PersistentVector.delete' is now complete. (Todo: test
 'PersistentVector.delete'.)

---
 fcore/persistent-vector.sml | 27 +++++++++++++++++++++++++--
 todo.md                     |  1 -
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/fcore/persistent-vector.sml b/fcore/persistent-vector.sml
index e7eb3f0..6a33c76 100644
--- a/fcore/persistent-vector.sml
+++ b/fcore/persistent-vector.sml
@@ -671,8 +671,7 @@ struct
             val difference = newChildSize - oldChildSize
 
             val nodes = Vector.update (nodes, 0, child)
-            val sizes = 
-              Vector.map (fn el => el + difference)
+            val sizes = Vector.map (fn el => el + difference) sizes
           in
             BRANCH (nodes, sizes)
           end
@@ -680,6 +679,30 @@ struct
           raise Fail "PersistentVector.mergeWhenRightDepthIsGreater: \
           \reached LEAF before (curDepth = leftDepth)"
 
+  fun mergeWhenLeftDepthIsGreater (left, right, rightDepth, curDepth) =
+    if rightDepth = curDepth then
+      mergeSameDepth (left, right)
+    else
+      case left of
+        BRANCH (nodes, sizes) =>
+          let
+            val lastIdx = Vector.length sizes - 1
+            val child = mergeWhenLeftDepthIsGreater 
+              (Vector.sub (nodes, lastIdx), right, rightDepth, curDepth + 1)
+
+            val oldChildSize = Vector.sub (sizes, lastIdx)
+            val newChildSize = getFinishIdx child
+            val difference = newChildSize - oldChildSize
+
+            val nodes = Vector.update (nodes, lastIdx, child)
+            val sizes = Vector.map (fn el => el + difference) sizes
+          in
+            BRANCH (nodes, sizes)
+          end
+      | LEAF _ =>
+          raise Fail "PersistentVector.mergeWhenLeftDepthIsGreater: \
+          \reached LEAF before (curDepth = rightDepth)"
+
   fun merge (left, right) = 
     let
       val leftDepth = countDepth left
diff --git a/todo.md b/todo.md
index e497af4..fb27a52 100644
--- a/todo.md
+++ b/todo.md
@@ -1,4 +1,3 @@
 # To-do list
-- Implement function to merge left and right subtrees for `PersistentVector.delete` function
 - Test `PersistentVector.delete` function
 - Implement 'yj' motion and add tests for it