Advent of Code. Year 2015. Day 10

Published: (2 min read)

--- Day 10: Elves Look, Elves Say ---

In today’s puzzle, the Elves are playing look-and-say. They take turns making sequences by reading aloud the previous sequence and using that reading as the next sequence. For example, 211 is read as “one two, two ones”, which becomes 1221 (1 2, 2 1s).

Table of Contents

Open Table of Contents

Part 1

In the first part, we need to apply this process to our puzzle input 40 times and get the length of the result. Firstly, we need to create a function that accepts the previous sequence and returning next sequence.

private static string ProcessInput(string input)
{
  var counter = -1;
  var processing = '\0';
  var sb = new StringBuilder();
  foreach (var num in input)
  {
    if (counter == -1)
    {
      processing = num;
      counter = 1;
      continue;
    }

    if (num == processing)
      counter++;
    else
    {
      sb.Append(counter.ToString());
      sb.Append(processing);
      counter = 1;
      processing = num;
    }
  }
  sb.Append(counter.ToString());
  sb.Append(processing);
  return sb.ToString();
}

Then, we just call this function 40 times in a loop to get the answer.

public string Part1(IEnumerable<string> lines)
{
  var input = GetInput(lines);
  for (var i = 0; i < 40; i++)
  {
    input = ProcessInput(input);
  }
  return input.Length.ToString();
}

Part 2

In the second part, we need to call that function 50 times instead of 40. The solution will run a little bit slower but other than that it has no difference from the first part.

public string Part2(IEnumerable<string> lines)
{
  var input = GetInput(lines);
  for (var i = 0; i < 50; i++)
  {
    input = ProcessInput(input);
  }
  return input.Length.ToString();
}

Links:

Other posts