structure PersistentVector =
struct
  (* Clojure-style persistent vector, for building search list.
   * There is an "int table" too, which stores the last index 
   * at the node with the same index.
   * We can use the size table for binary search.
   * *)
  datatype t =
    BRANCH of t vector * int vector
  | LEAF of {start: int, finish: int} vector * int vector

  val maxSize = 32
  val halfSize = 16

  fun isEmpty t =
    case t of
      LEAF (_, sizes) => Vector.length sizes = 0
    | _ => false

  val empty = LEAF (#[], #[])

  datatype append_result = APPEND of t | UPDATE of t

  fun isInRange (checkIdx, t) =
    case t of
      BRANCH (nodes, sizes) =>
        let
          val searchIdx = BinSearch.equalOrMore (checkIdx, sizes)
        in
          if searchIdx = ~1 then
            false
          else if searchIdx = 0 then
            isInRange (checkIdx, Vector.sub (nodes, searchIdx))
          else
            let val nextCheckIdx = checkIdx - Vector.sub (sizes, searchIdx - 1)
            in isInRange (nextCheckIdx, Vector.sub (nodes, searchIdx))
            end
        end
    | LEAF (values, sizes) =>
        let
          val searchIdx = BinSearch.equalOrMore (checkIdx, sizes)
        in
          if searchIdx = ~1 then
            false
          else
            let val {start, finish} = Vector.sub (values, searchIdx)
            in checkIdx >= start andalso checkIdx <= finish
            end
        end

  fun getFinishIdx t =
    case t of
      BRANCH (_, sizes) => Vector.sub (sizes, Vector.length sizes - 1)
    | LEAF (_, sizes) => Vector.sub (sizes, Vector.length sizes - 1)

  fun helpAppend (start, finish, tree) =
    case tree of
      BRANCH (nodes, sizes) =>
        let
          val lastNode = Vector.sub (nodes, Vector.length nodes - 1)
          val prevSize =
            if Vector.length sizes > 1 then
              Vector.sub (sizes, Vector.length sizes - 2)
            else
              0
        in
          case helpAppend (start - prevSize, finish - prevSize, lastNode) of
            UPDATE newLast =>
              let
                val lastPos = Vector.length nodes - 1
                val newNode = Vector.update (nodes, lastPos, newLast)
                val newSizes = Vector.update (sizes, lastPos, finish)
                val newNode = BRANCH (newNode, newSizes)
              in
                UPDATE newNode
              end
          | APPEND newVec =>
              if Vector.length nodes = maxSize then
                let
                  (* adjust "finish" so that it does not consider
                   * offset for "lower" vector *)
                  val finish =
                    finish - Vector.sub (sizes, Vector.length sizes - 1)
                  val newNode = BRANCH
                    (#[newVec], #[finish])
                in
                  APPEND newNode
                end
              else
                let
                  val newNodes = Vector.concat [nodes, #[newVec]]
                  val newSizes = Vector.concat [sizes, #[finish]]
                  val newNodes = BRANCH (newNodes, newSizes)
                in
                  UPDATE newNodes
                end
        end
    | LEAF (values, sizes) =>
        if Vector.length values + 1 > maxSize then
          (* when we split a leaf into two vectors, 
           * we want to adjust the start and finish parameters
           * so that they don't contain the offset relevant to the
           * "lower" vector, which was split from *)
          let
            val prevFinish = Vector.sub (sizes, Vector.length sizes - 1)
            val start = start - prevFinish
            val finish = finish - prevFinish
            val newNode = LEAF
              ( #[{start = start, finish = finish}]
              , #[finish]
              )
          in
            APPEND newNode
          end
        else
          let
            val newNode = Vector.concat
              [values, #[{start = start, finish = finish}]]
            val newSizes = Vector.concat [sizes, #[finish]]
            val newNode = LEAF (newNode, newSizes)
          in
            UPDATE newNode
          end

  fun append (start, finish, tree) =
    case helpAppend (start, finish, tree) of
      UPDATE t => t
    | APPEND newNode =>
        let
          val maxSize = getFinishIdx tree
        in
          BRANCH
            (#[tree, newNode], #[maxSize, finish])
        end

  fun getStart tree =
    case tree of
      LEAF (values, _) => Vector.sub (values, 0)
    | BRANCH (nodes, _) => getStart (Vector.sub (nodes, 0))

  fun helpNextMatch (cursorIdx, tree, acc) =
    case tree of
      LEAF (values, sizes) =>
        let
          val idx = BinSearch.equalOrMore (cursorIdx, sizes)
        in
          if idx = ~1 then
            {start = ~1, finish = ~1}
          else
            let val {start, finish} = Vector.sub (values, idx)
            in {start = start + acc, finish = finish + acc}
            end
        end
    | BRANCH (nodes, sizes) =>
        let
          val idx = BinSearch.equalOrMore (cursorIdx, sizes)
        in
          if idx = ~1 then
            {start = ~1, finish = ~1}
          else
            let
              val prevSize = if idx = 0 then 0 else Vector.sub (sizes, idx - 1)
              val acc = acc + prevSize
              val cursorIdx = cursorIdx - prevSize
            in
              helpNextMatch (cursorIdx, Vector.sub (nodes, idx), acc)
            end
        end

  fun startNextMatch (cursorIdx, tree) =
    case tree of
      LEAF (values, sizes) =>
        if Vector.length sizes = 0 then
          {start = ~1, finish = ~1}
        else
          let
            val idx = BinSearch.equalOrMore (cursorIdx, sizes)
            val idx = if idx = ~1 then 0 else idx
          in
            Vector.sub (values, idx)
          end
    | BRANCH (nodes, sizes) =>
        let
          val idx = BinSearch.equalOrMore (cursorIdx, sizes)
        in
          if idx = ~1 then
            {start = ~1, finish = ~1}
          else
            let
              val prevSize = if idx = 0 then 0 else Vector.sub (sizes, idx - 1)
              val cursorIdx = cursorIdx - prevSize
            in
              helpNextMatch (cursorIdx, Vector.sub (nodes, idx), prevSize)
            end
        end

  fun loopNextMatch (prevStart, prevFinish, tree, count) =
    if count = 0 then
      prevStart
    else
      let
        val {start, finish} = startNextMatch (prevFinish + 1, tree)
      in
        if start = ~1 then
          let val {start, finish} = getStart tree
          in loopNextMatch (start, finish, tree, count - 1)
          end
        else
          loopNextMatch (start, finish, tree, count - 1)
      end

  fun nextMatch (cursorIdx, tree, count) =
    if isEmpty tree then
      ~1
    else
      let
        val {start, finish} = startNextMatch (cursorIdx, tree)
      in
        if start = ~1 then
          let val {start, finish} = getStart tree
          in loopNextMatch (start, finish, tree, count - 1)
          end
        else
          let in
            if cursorIdx >= start andalso cursorIdx <= finish then
              loopNextMatch (start, finish, tree, count)
            else
              loopNextMatch (start, finish, tree, count - 1)
          end
      end

  fun getLast (tree, acc) =
    case tree of
      LEAF (values, _) =>
        let val {start, finish} = Vector.sub (values, Vector.length values - 1)
        in {start = start + acc, finish = finish + acc}
        end
    | BRANCH (nodes, sizes) =>
        let
          val acc =
            if Vector.length sizes > 1 then
              acc + Vector.sub (sizes, Vector.length sizes - 1)
            else
              acc
        in
          getLast (Vector.sub (nodes, Vector.length nodes - 1), acc)
        end

  (* slightly tricky.
  * The `sizes` vector contains the last/finish position of the item
  * at the corresponding index in the `nodes` or `values` vector
  * However, what we when searching for the previous match
  * is different: we want the node that has a start prior
  * to the cursorIdx.
  * This information cannot be retrieved with 100% accuracy 
  * using the `sizes` vector.
  * To get what we want, we recurse downwards using the `sizes` vector.
  * If we found the node we want, we return it. 
  * Otherwise, we return a state meaning "no node at this position"
  * and we use the call stack to descend down the node at the previous index.
  * There might not be a previous index because the current index is 0.
  * In this case, either the call stack will handle it,
  * or the caller to `helpPrevMatch` will. *)
  fun helpPrevMatch (cursorIdx, tree, acc) =
    case tree of
      LEAF (values, sizes) =>
        let
          val idx = BinSearch.equalOrMore (cursorIdx, sizes)
        in
          if idx < 0 then
            {start = ~1, finish = ~1}
          else if idx = 0 then
            let
              val result = Vector.sub (values, 0)
            in
              if #start result < cursorIdx then
                {start = #start result + acc, finish = #finish result + acc}
              else
                {start = ~1, finish = ~1}
            end
          else
            let
              val current = Vector.sub (values, idx)
              val {start, finish} =
                if cursorIdx > #start current then current
                else Vector.sub (values, idx - 1)
            in
              {start = start + acc, finish = finish + acc}
            end
        end
    | BRANCH (nodes, sizes) =>
        let
          val idx = BinSearch.equalOrMore (cursorIdx, sizes)
        in
          if idx < 0 then
            {start = ~1, finish = ~1}
          else if idx = 0 then
            helpPrevMatch (cursorIdx, Vector.sub (nodes, idx), acc)
          else
            let
              val node = Vector.sub (nodes, idx)
              val prevSize = Vector.sub (sizes, idx - 1)
              val result =
                helpPrevMatch (cursorIdx - prevSize, node, acc + prevSize)
            in
              if #start result = ~1 then
                let
                  val prevPrevSize =
                    if idx - 2 < 0 then 0 else Vector.sub (sizes, idx - 2)
                in
                  getLast (Vector.sub (nodes, idx - 1), acc + prevPrevSize)
                end
              else
                result
            end
        end

  fun loopPrevMatch (prevStart, prevFinish, tree, count) =
    if count = 0 then
      prevStart
    else
      let
        val {start, finish} = helpPrevMatch (prevFinish - 1, tree, 0)
      in
        if start = ~1 then
          let val {start, finish} = getLast (tree, ~1)
          in loopPrevMatch (start, finish, tree, count - 1)
          end
        else
          loopPrevMatch (start, finish, tree, count - 1)
      end

  fun prevMatch (cursorIdx, tree, count) =
    if isEmpty tree then
      ~1
    else
      let
        val {start, finish} = helpPrevMatch (cursorIdx, tree, 0)
      in
        if start = ~1 then
          let val {start, finish} = getLast (tree, ~1)
          in loopPrevMatch (start, finish, tree, count - 1)
          end
        else if cursorIdx >= start andalso cursorIdx <= finish then
          loopPrevMatch (start, finish, tree, count)
        else
          loopPrevMatch (start, finish, tree, count - 1)
      end

  fun getMaxSize tree =
    case tree of
      LEAF (_, sizes) => Vector.sub (sizes, Vector.length sizes - 1)
    | BRANCH (_, sizes) => Vector.sub (sizes, Vector.length sizes - 1)

  fun splitLeft (offset, tree) =
    case tree of
      BRANCH (nodes, sizes) =>
        if offset <= Vector.sub (sizes, 0) then
          splitLeft (offset, Vector.sub (nodes, 0))
        else if offset >= Vector.sub (sizes, Vector.length sizes - 1) then
          let 
            val prevSize =
              if Vector.length sizes > 1 then
                Vector.sub (sizes, Vector.length sizes - 1)
              else
                0
            val result =
              splitLeft 
                (offset - prevSize, Vector.sub (nodes, Vector.length nodes - 1))
          in
            if isEmpty result then
              let
                val len = SOME (Vector.length sizes - 1)
                val sizeSlice = VectorSlice.slice (sizes, 0, len)
                val sizes = VectorSlice.vector sizeSlice
                val nodeSlice = VectorSlice.slice (nodes, 0, len)
                val nodes = VectorSlice.vector nodeSlice
              in
                BRANCH (nodes, sizes)
              end
            else
              let
                val newChildSize = getMaxSize result + prevSize
                val sizes = Vector.update (sizes, Vector.length sizes - 1, newChildSize)
                val nodes = Vector.update (nodes, Vector.length nodes - 1, result)
              in
                BRANCH (nodes, sizes)
              end
          end
        else
          let
            val idx = BinSearch.equalOrMore (offset, sizes)
            val prevSize = 
              if idx > 1 then
                Vector.sub (sizes, idx - 1)
              else
                0
            val result = splitLeft (offset - prevSize, Vector.sub (nodes, idx))
          in
            if isEmpty result then
              let
                val len = SOME idx
                val sizeSlice = VectorSlice.slice (sizes, 0, len)
                val sizes = VectorSlice.vector sizeSlice
                val nodeSlice = VectorSlice.slice (nodes, 0, len)
                val nodes = VectorSlice.vector nodeSlice
              in
                BRANCH (nodes, sizes)
              end
            else
              let
                val len = idx + 1
                val sizes = Vector.tabulate (len,
                  fn i =>
                    if i = idx then
                      getMaxSize result + prevSize
                    else
                      Vector.sub (sizes, i)
                )
                val nodes = Vector.tabulate (lane,
                  fn i =>
                    if i = idx then
                      result
                    else
                      Vector.sub (nodes, i)
                )
              in
                BRANCH (nodes, sizes)
              end
          end
    | LEAF (items, sizes) =>
        if Vector.length sizes > 0 then
          if offset > Vector.sub (sizes, Vector.length sizes - 1) then
            tree
          else if offset <= Vector.sub (sizes, 0) then
            LEAF (#[], #[])
         else
           let
             val len = BinSearch.equalOrMore (offset, sizes)
             val sizes = VectorSlice.slice (sizes, 0, SOME len)
             val sizes = VectorSlice.vector sizes
             val items = VectorSlice.slice (items, 0, SOME len)
             val items = VectorSlice.vector items
           in
             LEAF (items, sizes)
           end
        else
          tree

    (* Unlike 'splitLeft' which leaves the size metadata alone
     * (except for splitting it),
     * we want splitRight to decrement the size metadata 
     * by the largest entry in the size table that was split.
     * This is so that we can maintain relative indexing metadata. *)
    fun splitRight (offset, tree) =
      case tree of
        BRANCH (nodes, sizes) =>
        if offset <= Vector.sub (sizes, 0) then
          let
            val firstSizeBefore = Vector.sub (sizes, 0)
            val result = splitRight (offset, Vector.sub (nodes, 0))
          in
            if isEmpty result then
              let
                val len = Vector.length sizes - 1

                val sizeSlice = VectorSlice.slice (sizes, 1, SOME len)
                val sizes = VectorSlice.map (fn el => el - firstSizeBefore) sizeSlice

                val nodeSlice = VectorSlice.slice (nodes, 1, SOME len)
                val nodes = VectorSlice.vector nodeSlice
              in
                BRANCH (nodes, sizes)
              end
            else
              let
                val firstSizeAfter = getMaxSize result
                val sizeDiff = firstSizeBefore - firstSizeAfter

                val sizes = Vector.mapi (fn (idx, el) =>
                if idx = 0 then
                  firstSizeAfter
                else
                  el - sizeDiff
                ) sizes

                val nodes = Vector.update (nodes, 0, result)
              in
                BRANCH (nodes, sizes)
              end
        else if offset >= Vector.sub (sizes, Vector.length sizes - 1) then
          let
            val prevSize =
              if Vector.length sizes > 1 then
                Vector.sub (sizes, Vector.length sizes - 2)
              else
                0
            val node = Vector.sub (nodes, Vector.length nodes - 1)
            val result = splitRight (offset - prevSize, node))
          in
            if isEmpty result then
              result
            else
              let
                val len = Vector.length sizes - idx
                val sizeSlice = VectorSlice.slice (sizes, idx, SOME len)
                val sizes = VectorSlice.map (fn el => el - prevSize) sizeSlice

                val nodeSlice = VectorSlice.slice (nodes, idx, SOME len)
                val nodes = VectorSlice.vector nodeSlice
              in
                BRANCH (nodes, sizes)
              end
          end
        else
      | LEAF (items, sizes) =>
            if offset > Vector.sub (sizes, Vector.length sizes - 1) then
              LEAF (#[], #[])
            else if offset <= Vector.sub (sizes, 0) then
              tree
            else
              let
                val idx = BinSearch.equalOrMore (offset, sizes)
                val len = Vector.length sizes - idx
                val len = SOME len

                val prevSize =
                  if idx < 1 then
                    0
                  else
                    Vector.sub (sizes, idx - 1)
                val sizes = VectorSlice.slice (sizes, idx, len)
                val sizes = VectorSlice.map (fn i => i - prevSize) sizes

                val items = VectorSlice.slice (items, idx, len)
                val items = 
                  VectorSlice.map 
                  (fn {start, finish} => 
                    {start = start - prevSize, finish = finish - prevSize})
                    items
              in
                LEAF (items, sizes)
              end
end