/-- Get line index (0-based) containing the byte offset.
    Newline byte itself belongs to the previous line (same as getLineRange semantics). -/
def getLineIndexAtOffset (t : ViE.PieceTree) (offset : Nat) (pt : ViE.PieceTable) : Nat := Id.run do
  let target := min offset (length t)
  let mut node := t
  let mut currLines := 0
  let mut rem := target
  while true do
    match node with
    | ViE.PieceTree.empty =>
        return currLines
    | ViE.PieceTree.leaf pieces _ _ =>
        let mut i := 0
        while i < pieces.size && rem > 0 do
          let p := pieces[i]!
          let pLen := p.length.toNat
          if rem >= pLen then
            currLines := currLines + p.lineBreaks.toNat
            rem := rem - pLen
          else
            let buf := PieceTableHelper.getBuffer pt p.source
            let (lines, _) := ViE.Unicode.countNewlinesAndChars buf p.start.toNat rem
            currLines := currLines + lines
            rem := 0
          i := i + 1
        return currLines
    | ViE.PieceTree.internal children _ _ imeta =>
        if children.isEmpty then
          return currLines
        let childIdx := findChildForOffset imeta rem.toUInt64
        currLines := currLines + imeta.cumulativeLines[childIdx]!.toNat
        let childStart := imeta.cumulativeBytes[childIdx]!.toNat
        rem := rem - childStart
        node := children[childIdx]!
  return currLines

  ViE.start config args