fix calculation of number to decrement by in 'PersistentVector.delete', after additional test showed that calculation resulted in incorrect metadata. (Todo: just implement function to merge left and right subtrees.)

fcore/persistent-vector.sml

@@ -548,9 +548,17 @@ struct
   fun merge (left, right) = raise Fail "unimplemennted"
 
   fun delete (start, length, tree) =
+    if isEmpty tree then
+      empty
+    else
       let
         val finish = start + length
         val matchBeforeStart = prevMatch (start, tree,  1)
+        val matchBeforeStart =
+          if matchBeforeStart >= start then
+            ~1
+          else
+            matchBeforeStart
         val matchAfterFinish = nextMatch (finish, tree, 1)
       in
         if matchBeforeStart = ~1 andalso matchAfterFinish = ~1 then
@@ -573,7 +581,8 @@ struct
             else
               let
                 val startIdx = getStartIdx right
-              val difference = matchAfterFinish  - startIdx
+                val shouldBeStartIdx = matchAfterFinish - length
+                val difference = startIdx - shouldBeStartIdx
               in
                 if difference = 0 then
                   right
@@ -594,9 +603,10 @@ struct
               let
                 val leftSize = getFinishIdx left
                 val rightStartRelative = getStartIdx right
-
                 val rightStartAbsolute = leftSize + rightStartRelative
-              val difference = matchAfterFinish - rightStartAbsolute
+
+                val shouldBeStartIdx = matchAfterFinish - length
+                val difference = rightStartAbsolute - shouldBeStartIdx
               in
                 if difference = 0 then
                   merge (left, right)

test/persistent-vector-tests.sml

@@ -342,6 +342,39 @@ struct
            in
              Expect.isTrue (outputList = expectedOutput)
            end)
+    , test
+        "decrements subsequent elements correctly \
+        \when deletion range is before first element to middle element"
+        (fn _ =>
+           let
+             (* arrange *)
+             val inputList =
+               [ {start = 1, finish = 1}
+               , {start = 2, finish = 2}
+               , {start = 3, finish = 3}
+               , {start = 4, finish = 4}
+               , {start = 50, finish = 50}
+               , {start = 60, finish = 60}
+               , {start = 70, finish = 70}
+               , {start = 80, finish = 80}
+               ]
+             val pv = PersistentVector.fromList inputList
+
+             (* act *)
+             val pv = PersistentVector.delete (0, 3, pv)
+
+             (* assert *)
+             val outputList = PersistentVector.toList pv
+             val expectedOutput =
+               [ {start = 1, finish = 1}
+               , {start = 47, finish = 47}
+               , {start = 57, finish = 57}
+               , {start = 67, finish = 67}
+               , {start = 77, finish = 77}
+               ]
+           in
+             Expect.isTrue (outputList = expectedOutput)
+           end)
     ]
 
   val tests = [appendTests, toListTests, splitLeftTests, deleteTests]

todo.md

@@ -1,3 +1,4 @@
 # To-do list
-- Implement `PersistentVector.delete` and test it
+- Implement function to merge left and right subtrees for `PersistentVector.delete` function
+- Test `PersistentVector.delete` function
 - Implement 'yj' motion and add tests for it