--- title: 🚀 spaceship operator, rope bridge (snake) - Advent of Code 2022 - Day 9 with Ruby slug: spaceship-operator-rope-bridge-snake-advent-of-code-2022-day-9-with-ruby published_at: 2022-12-09 22:47:44 +0000 updated_at: 2026-03-04 20:15:10 +0000 summary: description: spaceship operator, rope bridge (snake) - Advent of Code 2022 - Day 9 with Ruby Challenge: https://adventofcode.com/2022/day/9 Solution: https://gist.github.com/cjavdev/db881b693bf33a754401328d73e410ba #ruby #adventofcode tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, rails, javascript, ruby on rails, active record, ruby on rails tutorial] views: 354 author: CJ Avilla url: https://www.cjav.dev/videos/spaceship-operator-rope-bridge-snake-advent-of-code-2022-day-9-with-ruby youtube_url: https://www.youtube.com/watch?v=kKOBOlk6tqI youtube_id: kKOBOlk6tqI embed_url: https://www.youtube.com/embed/kKOBOlk6tqI thumbnail_url: https://i.ytimg.com/vi/kKOBOlk6tqI/hqdefault.jpg type: video --- # 🚀 spaceship operator, rope bridge (snake) - Advent of Code 2022 - Day 9 with Ruby *Published: December 09, 2022* *Views: 354* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=kKOBOlk6tqI) [![🚀 spaceship operator, rope bridge (snake) - Advent of Code 2022 - Day 9 with Ruby](https://i.ytimg.com/vi/kKOBOlk6tqI/hqdefault.jpg)](https://www.youtube.com/watch?v=kKOBOlk6tqI) ## Description spaceship operator, rope bridge (snake) - Advent of Code 2022 - Day 9 with Ruby Challenge: https://adventofcode.com/2022/day/9 Solution: https://gist.github.com/cjavdev/db881b693bf33a754401328d73e410ba #ruby #adventofcode ## Transcript thank you what's up welcome back in this episode you'll see how to solve day nine of the Advent of code for 2022 in Ruby and today's problem is called rope Bridge it reminds me a lot of snake in today's exercise what we get is a series of moves so this is our input we're going to be told okay we're gonna move right four up four left three down one right four Etc and we need to move the head of some rope and have the tail of the Rope follow it as if it was a knot that was behind the rope and then when we get to the second part of today's challenge it's going to be more than just a head and tail but we're going to start off with just moving the head and a single knot right behind the head and one thing that's interesting about the way that we're moving is that the head is always going to move left right up or down but the tail could potentially be diagonal and if we are in a situation where the head is diagonally in a different row a different column than the tail then the tail is going to make a diagonal step to catch up to where the head is all right let's grab our example input we'll create a new file here and we're going to drop this as always at the end of our file because a lot of this movement can go awry again I really want to just include our spec and read some tests so we're going to say rspec.describe and we're going to say like it parses input as expected and so here we're just going to say before all do and we're going to say that moves is going to be data.readlines and now we're going to make some method that takes in our moves and that's going to give us back basically like what we got down here so we want these to be broken up as letters and integers in their own arrays so we're just going to say all right so this is what we should get back from some parse moves methods so we'll run Ruby day nine and we get back a an error that says we don't have this parse moves method so let's go Implement that so we're going to say parse moves takes in some input say input dot map Chomp dot map and then we want to split on space and then we're going to map and yeah we want the integer version so let's see if this works okay oh wait we don't need to convert the first one to integer we just want to convert the second one to integer okay cool we've got a passing test so that should parse our moves out into the expected format now let's make it so that we can test some Logic for making a single move where we're going to move the head in Us in some Direction so like it moves the head so we'll make some method called move that's going to take in I don't know some array we'll start at 0 0 to make it easy and then it'll take some Direction so let's say we want to go up one and we're going to expect that that's going to give us back a new array where it has moved up one row so we'll run this test and there's no method move okay so we're going to take in some piece or something and then we want to say we're going to get a direction and we're going to get a distance and I don't think this is actually what we want because we need to use up down left right instead of north south east west so I'm actually going to do something different here so we'll say like when the direction is up we want to say head at 1 plus equals 1. right and then we'll if we're going down East is left and West is right and so then we're going to say yeah if we go down we want to go down and then if we're going left yep that's correct okay so now what we should get back is our new location for head and oh so here we need this okay all right so now we have two passing tests we're able to at least like move the head around now what I want to do is potentially pass in some more details to this move method so that we can also track where the tail is going okay so we have a test here for up but let's also make one for down left and right and instead of just one let's make this five just so that we can get some more interesting numbers here so that's going to go 5 and this should be like negative five right and then in this case if we're going to the left we should have negative five if we're going to the right we should have five and we're not moving rows or columns yet okay so at this point we're not actually working correctly and I'm glad we modified our test because this distance now needs to be executed several times we're going to say distance that times do and then we want to like modify the head and ultimately return the head the reason we're doing it distance times here instead of incrementing by the distance is that every single time we move the head we also want to move the tail so down inside of here like while we're making steps as we're stepping through with the head we also want to like move the tail so let's make a new method called follow here and we're going to pass in the head and we'll also pass in some tail so now at this point it kind of makes sense to pass in maybe be the head and the tail both as arguments to move and so here we'll just add We'll add a new array to all of these positions so that is going to represent our tail and instead of just returning the head I think we want to return the head and the tail okay as an array so we're going to get back the same value for the head and then we should get back zero and one less for the tail okay so it's gonna the tail should follow except it should be sort of like one behind in all of the same directions okay so if we run this test now we don't have a method follow so let's make a new method follow head and tail and the follow method should be something this is this is where it gets a little bit interesting because if we're going in the same direction so in the in the easy case here for instance if we're going to the right the head moves to the right and then the tail just moves to the right if we're going down right the head moves down and then the tail follows right behind it so we can Implement those first so we'll say something like if the head at zero minus tail at zero if the absolute value of those two things is greater than one then we want to move we want to move the tail so then we want to move the Tails zero to something like well we want to follow we want to follow the head and so this is where we can start talking about the spaceship operator so here we want to move the like head spaceship operator tail before we go too far let's crack open IRB and look at the spaceship operator so we have a let's say that a is equal to 5 and B is equal to one thousand so the spaceship operator or the compared comparator comparison operator is this thing it looks like this it's like less than equal to the greater than and you can pass this is a method that we're calling on a passing in B and we will get back a value that is either 1 0 or negative one and this is called three-way comparison it gives you sort of like if you pass in A and B then you'll either get a result that tells you a is less than b a is equal to b or a is greater than b this is based on the law of trichotomy so every number should be either negative positive or zero and so in Ruby what we can do is we can pass in these arguments and so in this case a is less than b and then when we call the comparator operator we're going to get back negative one because we would sort of the way that I like to think about this is like we would need to move B backwards to get closer to a and similarly if we see B comparator operator spaceship operator thing one we would need to make a bigger we would need to increase a to get it closer to B so this operator here is going to tell us which direction the head or the tail needs to move so because we're only moving one we know that we want to go either we want to go up by one or we want to go down by one so this is going to be either one or negative one in the case that the difference or the Delta here is greater than one okay we want to do the same thing for yeah for head at one okay and [Music] let's see if our okay so our tests are passing now for it moves the head I guess like and tail in One Direction and let's let's try to let's add another test here that works for diagonals so let's say that the head is going to start at if the head is at 1 1 and the Tail's at zero zero and we move the head up then the tail needs to move diagonally to join it and be like right behind it so let's say that we're just going to move these all one step and so in this case we're going to get one two and here we should get one one because the tail should follow exactly into the place where one where it was before okay so if we're going to go down let's say that we actually want to start with zero or yeah one negative one and we want to move down so that should make this negative 1 negative two and we want our tail to follow in its place to negative one negative one all right if we're moving to the left then we can be at or negative one one and when we move left we should end up at negative 2 1 and or the tail should end up at negative one one now if we're moving to the right and we start at 1 1 move to the right 2 1 right it's going to move to the right yeah okay I think that's right let's see okay so now what we need to do is we need to update our follow method so that it first checks if the head and tail are on different rows and columns so if head at zero does not equal tail at zero and head at one does not equal tail at one that means they're on different columns that means that we need to do like sort of a catch-up in the diagonal direction we also need to check to make sure that there are at least two apart in some in one case or another so then we also we have to do the same sort of absolute value check here so we're going to do that or head at one yep okay so we have this absolute value check all right let's just put these all onto different lines here and so if that's the case then we want to do both for the tail we want to move it in both both directions right so we need to go diagonal up into the left or up into the right so let's see if our test passes boom okay we've got a we've got a passing test so now what we should be able to do is just like call move several times and move our head and tail around so let's say it works for the example case so we're going to say something like at moose dot each do move and for each move we want to call move or we're going to get back head and tail from yeah we want to call this m or I guess we want to split out M into like direction and distance and we want to move the yeah head tail direction and distance and we need to create a head at zero zero and we need to create a tail at zero zero and at the end of this we right okay so the problem wants us to keep track of where the tail has visited so as the tail is moving around the board we need to keep track of where it's where it's visited and so in order to do that I think we also need some other New Concept called visited which can be a set of all the places that the tale has seen so we need to require the set gem and we're going to use this set class as part of move so we need to pass in visited and into the move method and so now we have visited here and every time we follow we need to put in tail.doop that's like where the Tail's been okay so now we need to update all of our move methods set new and we can just reuse the same variable here because we're not gonna make any expectations about it all right let's see how that works okay distance times for nil class okay so all right we need to parse our moves parse moves of moves and then we should execute this again boom okay we have a passing test and now we want to just p visid.link and we should get back 13. okay so 13 is the answer for part one or I mean we can actually write an expectation about this like expect that this is a DOT length to equal 13 and that will that will be that'll be grand that'll be grand okay fantastic now what we want to do is run it against our input our puzzle input so there's this giant puzzle input here so we're gonna let's go to day nine and make an input and we'll paste that in and now what we're going to do is we only want to do this R spec stuff if our V Dot empty otherwise we're going to read in the moves so moves is going to be parse moves of file.readlines ERG V Dot zero or first and now we can say it moves that each do direction and distance and we want to make our moves and then we're going to put visited.length and we need some visited set we need a head and a tail okay 6023. 6023 was the puzzle answer fantastic okay all right so what we've got now is we are keeping track of all the places the tail has visited we have our head and tail we're calling parse moves to get back all the different directions and distances then we're calling move passing and visited our head tail direction and distance and we get back the head and tail each time and then we are printing out where the tail has been so that is the solution for part one part two is the way that we're gonna increase the complexity is that instead of just one tail we have nine knots in the tail so now as the head is moving around it's being followed by the first closest knot the second closest not third fourth Etc until there is some ninth knot and there is a second sort of example here that gives you this bigger initial state so that as H is moving around or as the head is moving around you can sort of see the the tail follow it with a little bit more detail and this is where it starts to really feel like snake right where like the head of the snake is sort of swimming around in this grid so at this for this example it visited 36 positions so the the problem is that right now the way that we've set this up is we only have two knots and we just have the head and the tail and the only difference is that as we're moving the head around and when we're ready to follow the head with the tail what I I think we want to do is not only follow the head with the tail but we want to follow each knot with the next knot so we want to iterate over all of the different pieces of the rope in pairs and make sure that we first we're going to move the head and then we're going to like go through all of the pairs of the chain or of the rope and then make sure that we're moving those so in order to make this refactoring the first thing that I want to do is go through our tests and instead of passing in an array for the head and an array for the tail I'm going to combine those into one thing and we'll just call it rope so we'll say okay we're going to pass in this rope here and this rope there and that we're still getting back an array of arrays that's exactly what we want so we'll still continue to like receive back a copy of the rope from the move method it's just that we're like making this a little bit better by passing in the full rope so that we can iterate over all the different pieces so here in our example we need to pass head and tail together and then down here we would need to pass head and tail together in fact I'm just going to call this rope down here and we'll make a new thing here called rope which is going to be an array that is too long and it starts with two zeros okay so now our code should continue to work I believe maybe no okay so moves our move method here now just needs to take in rope and we still need head and that I think that should well okay so now tail is an array so we actually want this visited tracking to be something like rope.last.doop and let's see how we're doing here okay right okay so now tail is an array so instead of passing entail here we need to go through like rope dot each do not and we need to follow well actually in fact it's like each cons of two so each consecutive two knots in the Rope we want to pass in like the head and tail of the like of that sort of subsection and then we want to return the Rope at the end let's see all right so we're getting the right answer for our input and our tests are still passing so what what the this refactoring did in fact like I don't think we need this tail here like I think we can get rid of that but because we're not using it anywhere but for now we'll keep that so that we can correctly extract and then modify the head and what we can do now is instead of having our rope be too long we want our rope to be like nine long or ten long I think it's 10. because it'll have the head plus the nine that are going to follow and then this is going to be rope okay so let's see day 9 input okay twenty five thousand thirty three twenty five thousand thirty three was the puzzle answer so the the small like the slight modification was just switching so that instead of just following one head and tail now we're iterating over all of the pairs and following those so this was a pretty fun little project this comparator operator if you haven't seen it or used it with Ruby before go check that out it's actually in lots of different languages so that can be handy if you need to know the sort order of something is common or if you need to know if something is less than greater than or zero it can be useful in the little trick of like which direction are you going I thought about maybe instead of modifying the head directly and modifying the tail as we were kind of like working through it I thought about trying to make it immutable so that you kind of like had these fixed objects that were being passed around but I decided against it just a little easier to modify the underlying things so yeah this was kind of a fun one hopefully that was useful and yeah thanks so much for watching and thanks for your time and attention we'll see you in the next one cheers the --- [Back to all videos](https://www.cjav.dev/videos) --- ## Hey Agent, thanks for visiting! 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