structure BinSearch =
struct
  fun helpExists (findNum, vec, low, high) =
    if high >= low then
      let
        val mid = low + ((high - low) div 2)
        val curNum = Vector.sub (vec, mid)
      in
        if curNum = findNum then true
        else if curNum < findNum then helpExists (findNum, vec, mid + 1, high)
        else helpExists (findNum, vec, low, mid - 1)
      end
    else
      false

  fun exists (findNum, vec) =
    helpExists (findNum, vec, 0, Vector.length vec - 1)

  fun helpFind (findNum, vec, low, high) =
    if high >= low then
      let
        val mid = low + ((high - low) div 2)
        val curNum = Vector.sub (vec, mid)
      in
        if curNum = findNum then mid
        else if curNum < findNum then helpFind (findNum, vec, mid + 1, high)
        else helpFind (findNum, vec, low, mid - 1)
      end
    else
      ~1

  fun find (findNum, vec) =
    helpFind (findNum, vec, 0, Vector.length vec - 1)
end