lua Help for an algorithm

[Lover1500]

I need a help in writhing script. Read code. 

a = 1
check = 8
divided = check / 2
minus = check - a
if a <= divided then print(minus) end

So what i want is to create an algorithm by using original check number 8.

As example,

Give a random number. Check it if its lower than half of 8.if it is,then show me that

8 - given number. 

So if a number is 1 then show me 7.

If 2, show 6.

If 4,show 4.

 

If 5, then i wont use 8 as check number anymore. Because 5 is greater than half of 8(4). Now i am going use 16(2×8) as check number. 

So if 5 then show 11.

If 7, show 9.

if 8, show 8.

If 9, now its greater than half of 16(8). 

 

So i will use 24(3×8) instead of 16.

Yap the progress is that. Ive tried in many ways till to what i can think.with conbination of loop and table.  But i couldnt. 

How can i create that algorithm.

 

Im really appreciate to hear your help news. Thanks in advance all sir!

 

[CmP]

The following solution should work for positive numbers:

-- Taken from https://stackoverflow.com/a/3407254
function roundUp(num, multiple)
  if multiple == 0 then
    return num
  end

  local remainder = num % multiple
  if remainder == 0 then
    return num
  end

  return num + multiple - remainder
end

function calculateResult(num)
  local checkMinBound = num * 2
  local check = roundUp(checkMinBound, 8)
  local result = check - num
  print(result)
  return result
end

-- Test cases
calculateResult(1) -- 7
calculateResult(2) -- 6
calculateResult(4) -- 4
calculateResult(5) -- 11
calculateResult(7) -- 9
calculateResult(8) -- 8
calculateResult(9) -- 15
calculateResult(11) -- 13
calculateResult(15) -- 17

solution.lua

[Lover1500]

10 hours ago, CmP said:

The following solution should work for positive numbers:

-- Taken from https://stackoverflow.com/a/3407254
function roundUp(num, multiple)
  if multiple == 0 then
    return num
  end

  local remainder = num % multiple
  if remainder == 0 then
    return num
  end

  return num + multiple - remainder
end

function calculateResult(num)
  local checkMinBound = num * 2
  local check = roundUp(checkMinBound, 8)
  local result = check - num
  print(result)
  return result
end

-- Test cases
calculateResult(1) -- 7
calculateResult(2) -- 6
calculateResult(4) -- 4
calculateResult(5) -- 11
calculateResult(7) -- 9
calculateResult(8) -- 8
calculateResult(9) -- 15
calculateResult(11) -- 13
calculateResult(15) -- 17

Pretty perfect code sir! I need more learning in coding. I'll try. Really thank you !its worth with hundreds likes.