RogerBW's Blog

I’ve been doing the Weekly Challenges. The latest involved daisywheels and a list breakdown. (Note that this ends today.)

Task 1: Minimum Time

You are given a typewriter with lowercase English letters a to z arranged in a circle.

Typing a character takes 1 sec. You can move pointer one character clockwise or anti-clockwise.

The pointer initially points at a.

Write a script to return minimum time it takes to print the given string.

This seems to fall into two main ideas, getting the inter-character transfer time and getting the total. So I'll break that into two functions. in Perl:

sub transfer($a, $b) {

Take the numerical code of each character (doesn't have to be ASCII as long as the letters are in order)

  my $ac = ord($a);
  my $bc = ord($b);

Work out the minimum difference between them, going in either direction.

  min(($ac - $bc + 26) % 26, ($bc - $ac + 26) % 26);
}

(One could cache that, but at this scale it doesn't seem important.)

sub minimumtime($a) {

The total starts off as the number of characters.

  my $tot = length($a);

and the wheel is initially at 'a'.

  my $prev = 'a';
  foreach my $c (split '', $a) {

Then for each character add the transfer time, and set the wheel position for the next character.

    $tot += transfer($prev, $c);
    $prev = $c;
  }
  $tot;
}

Task 2: Balls and Boxes

There are $n balls of mixed colors: red, blue or green. They are all distributed in 10 boxes labelled 0-9.

You are given a string describing the location of balls.

Write a script to find the number of boxes containing all three colors. Return 0 if none found.

This smelt to me like a problem for sets, so that's what I did. In Crystal (and almost identically in Ruby):

def ballsandboxes(a)

Set up the hash of sets.

  boxes = Hash(Int32, Set(Char)).new

Initialise with a dummy colour which won't get used. (We're not checking for correctness of input. If we were, I'd do it in Rust and use winnow.)

  colour = '@'

Loop over the input characters.

  a.chars.each_with_index do |c, i|

Even-numbered characters specify the colour.

    if i % 2 == 0
      colour = c
    else

Odd-numbered characters specify where the colour goes. (In a new set, or extending an existing set.)

      boxid = c.to_i
      if boxes.has_key?(boxid)
        boxes[boxid].add(colour)
      else
        boxes[boxid] = Set.new([colour])
      end
    end
  end

Then, having made somewhat heavy weather of rearranging the input, all I have to do is filter the sets for ones that are big enough, and return the count of them.

  boxes.values.select{|x| x.size >= 3}.size

Full code on github.