Kata

Failure on the factory floor!! One box of 'deluxe' ball-bearings has been mixed in with all the boxes of 'normal' ball-bearings! We need your help to identify the right box!

Information

What you know about the bearings:

• 'deluxe' ball-bearings weigh exactly 11 grams
• 'normal' ball-bearings weigh exactly 10 grams
• Besides weight, both kinds of ball-bearings are identical
• There are (effectively) infinite bearings in each box
• Each box contains exclusively one type of bearing (either regular, or 'deluxe')

To help you identify the right box, you also have access to a Super Scale™ which will tell you the exact weight of anything you give it. Unfortunately, getting it ready for each measurement takes a long time, so you only have time to use it once!

Task

Write a function which accepts two arguments:

• bearings: A list of the bearing types contained in each 'box'. (length between 1 and 200 inclusive)

• weigh: a function which accepts any number of arguments, returning the total weight of all. Can only be used once!

Your function should identify and return the single 'deluxe' bearing sample from bearings.

Example

def identify_bb(bearings, weigh):
    a, b, c = bearings
    if weigh(a, b) == 20:
        # bearings 'a' and 'b' must both be 10, so 'c' must be deluxe
        return c 
    if weigh(a) == 10: # Error: weigh has already been used!
        return b
    return a

public static Bearing IdentifyBb(Bearing[] bearings, Func<IEnumerable<Bearing>, long> weigh)
{
  Bearing a = bearings[0],
          b = bearings[1],
          c = bearings[2];
  if (weigh(a, b) == 20)
    // bearings 'a' and 'b' must both be 10, so 'c' must be deluxe
    return c;
  if (weigh(a) == 10) // Error: weigh has already been used!
    return b;
  return a;
}

let identify_bearing (bearings: bearing list) (weigh: bearing list -> int): bearing = 
  let a = List.at bearings 0
  and b = List.at bearings 1
  and c = List.at bearings 2
  in if weigh [a; b] = 20 then
    (* bearings 'a' and 'b' must both be 10, so 'c' must be deluxe *)
    c
  else if weigh [a] = 10 then (* Failure: weigh has already been used! *)
    b
  else
    a

identifyBB :: [Bearing] -> ([Bearing] -> IO Int) -> IO Bearing
identifyBB bearings@[a,b,c] weigh = do
  weightAB <- weigh [a,b]
  if weightAB == 20
    then return c
    else do
      weightA <- weight [a] -- Failure: weigh has already been used!
      if weightA == 10
        then return b
        else return a

Note: modules sys and inspect have been disabled.

Try some other riddle kata:

• Hat Game
• Extreme Hat Game
• Hat Game 2