Advent of Code. Year 2015. Day 3

Published: (3 min read)

--- Day 3: Perfectly Spherical Houses in a Vacuum ---

In this puzzle, we have Santa delivering presents to an infinite two-dimensional grid of houses. He begins by delivering a present to the house at his starting point [0, 0] and then an elf at the North Pole calls him via radio and tells him where to move next. But the elf’s directions are a little of, and Santa ends up visiting some houses more than once. We have to find how many houses receive at least one present.

Table of Contents

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Part 1

For the first part, we created an anonymous object (1) for Santa’s location and HashSet of points (2). After that, we go through all of the directions and compute Santa’s new location based on the direction and the previous location and add it to the HashSet of points (3).

public string Part1(IEnumerable<string> input)
{
  var directions = input.First();
  var location = new {X = 0, Y = 0}; // 1
  var points = new HashSet<object>{location}; // 2
  foreach (var direction in directions)
  {
    if (direction == '>')
      location = new {X = location.X + 1, location.Y};
    if (direction == '<')
      location = new {X = location.X - 1, location.Y};
    if (direction == '^')
      location = new {location.X, Y = location.Y + 1};
    if (direction == 'v')
      location = new {location.X, Y = location.Y - 1};
    points.Add(location); // 3
  }
  return points.Count.ToString();
}

Part 2

In the second part, we have Santa and Robo-Santa to deliver presents. We created two objects that represent the location of the Santa’s (1) and boolean variable for Santa’s turn (2) then, based on this variable we are computing the new location and adding it to the HashSet of points.

public string Part2(IEnumerable<string> input)
{
  var directions = input.First();
  var santa = new {X = 0, Y = 0}; // 1
  var roboSanta = new {X = 0, Y = 0}; // 1
  var santaTurn = true; // 2
  var points = new HashSet<object>{santa};
  foreach (var direction in directions)
  {
    var location = santaTurn ? santa : roboSanta;
    if (direction == '>')
      location = new {X = location.X + 1, location.Y};
    if (direction == '<')
      location = new {X = location.X - 1, location.Y};
    if (direction == '^')
      location = new {location.X, Y = location.Y + 1};
    if (direction == 'v')
      location = new {location.X, Y = location.Y - 1};
    if (santaTurn)
      santa = location;
    else
      roboSanta = location;
    santaTurn = !santaTurn;
    points.Add(location);
  }
  return points.Count.ToString();
}

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