---
title: grid traversal with pattern matching - Day 16 - Advent of Code 2023
slug: grid-traversal-with-pattern-matching-day-16-advent-of-code-2023
published_at: 2023-12-16 19:00:02 +0000
updated_at: 2026-03-04 20:12:54 +0000
summary: 
description: In this episode of the Advent of Code 2023 solutions series, we tackle day 16, The Floor Is Lava, using Ruby. We start by examining the puzzle input, representing a contraption through which a beam of light travels. The beam interacts with different components like mirrors, splitters, empty spaces, etc. Our goal is to follow the path of the beam and count the number of \"energized\" tiles it travels over.  First, we set up the input grid and beams data structures to track the position and direction of beam segments. We use a while loop to traverse the grid, using pattern matching to check the tile type ahead and turn, split beams, or continue on accordingly. We skip over seen tiles and handle edge cases.  For part 1, we print the number of energized tiles for the given input. For part 2, we maximize energy by checking all possible starting points along the edges. This results in a brute-force approach that takes several minutes to run. We make an optimization to use just 1 set instead of 2 to improve performance.  Overall, this solution leverages pattern matching, recursion, and data structures like arrays and sets. The video explains the code step-by-step and suggests further improvements as well. It's aimed at intermediate Ruby developers looking to strengthen their skills.  Advent of Code: https://adventofcode.com/ My Solutions: https://gist.github.com/cjavdev/d15a2a4ffed6c840c2fb28a093e9f927/ Playlist https://www.youtube.com/playlist?list=PLS6F722u-R6KYlGyUv65EFpGKl2Esmurr  #adventofcode  #ruby
tags: [cjav_dev, Learn to code, Beginner ruby, Advent of code, Advent of code 2023, Advent of code ruby, Aoc ruby, Aoc 2023, Vim, Advent of code vim, Advent of code explainer, Advent of code challenge, Code challenge, Advent of code tutorial, Web development tutorial, pattern matching, ruby pattern matching]
views: 207
author: CJ Avilla
url: https://www.cjav.dev/videos/grid-traversal-with-pattern-matching-day-16-advent-of-code-2023
youtube_url: https://www.youtube.com/watch?v=7yAgKw2PvT0
youtube_id: 7yAgKw2PvT0
embed_url: https://www.youtube.com/embed/7yAgKw2PvT0
thumbnail_url: https://i.ytimg.com/vi/7yAgKw2PvT0/hqdefault.jpg
type: video
---

# grid traversal with pattern matching - Day 16 - Advent of Code 2023

*Published: December 16, 2023*
*Views: 207*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=7yAgKw2PvT0)

[![grid traversal with pattern matching - Day 16 - Advent of Code 2023](https://i.ytimg.com/vi/7yAgKw2PvT0/hqdefault.jpg)](https://www.youtube.com/watch?v=7yAgKw2PvT0)

## Description

In this episode of the Advent of Code 2023 solutions series, we tackle day 16, The Floor Is Lava, using Ruby. We start by examining the puzzle input, representing a contraption through which a beam of light travels. The beam interacts with different components like mirrors, splitters, empty spaces, etc. Our goal is to follow the path of the beam and count the number of "energized" tiles it travels over.

First, we set up the input grid and beams data structures to track the position and direction of beam segments. We use a while loop to traverse the grid, using pattern matching to check the tile type ahead and turn, split beams, or continue on accordingly. We skip over seen tiles and handle edge cases.

For part 1, we print the number of energized tiles for the given input. For part 2, we maximize energy by checking all possible starting points along the edges. This results in a brute-force approach that takes several minutes to run. We make an optimization to use just 1 set instead of 2 to improve performance.

Overall, this solution leverages pattern matching, recursion, and data structures like arrays and sets. The video explains the code step-by-step and suggests further improvements as well. It's aimed at intermediate Ruby developers looking to strengthen their skills.

Advent of Code: https://adventofcode.com/
My Solutions: https://gist.github.com/cjavdev/d15a2a4ffed6c840c2fb28a093e9f927/
Playlist https://www.youtube.com/playlist?list=PLS6F722u-R6KYlGyUv65EFpGKl2Esmurr

#adventofcode  #ruby

## Transcript

what's up welcome back in this episode you'll learn how to solve day 16 for the Advent of code in 2023 first take a look at this this is such a cool animation on the main page here you can see the fire is just like falling down maybe that's our lava let's jump into day 16 which is called the floor will be Lava the floor is lava watch out okay so right now we have worked on getting this beam of light focused we went through and we changed a bunch of lenses in a box and now it's pouring into this space and we have a map of how the light is going to filter through and bounce against mirrors and Splitters we're taking a look at the contraption and that is our puzzle input is the layout of the contraption we have several instructions about how the light beam will pass through the contraption if the beam encounters an empty space it's going to keep traveling in the same direction it's already traveling if it encounters a mirror one of the slashes it's going to change direction will'll be reflected at 90° if we're coming in from here we're headed towards the east and we bounce off of this mirror we're going to go down right so we're coming in this way we're going to bounce off there and come down if we're coming at the mirror from this other side over here we're going to bounce and go up right if we encounter the pointy end of a splitter so a splitter is a pipe or a dash if we encounter the pointy end so if we look at maybe this this pipe operator here if we counter the pointy end like maybe we're coming down and we hit the top of it we just keep going or if we're coming up and we hit the top the bottom of it we just keep going same with the horizontal splitter if we're coming from the left or the right we just keep going but otherwise if we hit the flat side of the splitter so if if we're looking at this pipe again and we're coming from this side then two beams of light are going to come out one through the top and one through the bottom as the beams travel through the contraption they're going to kind of like leave a path of which tiles were energized and so this gives us kind of an example based on that input of the direction that the beam is traveling so first it goes here and then it hits this splitter so it's going to go up but that's off the grid and it's also going to go down down down down hits this splitter so it'll go to the left and the right when it enters this splitter it does nothing because it went in the pointy end of the splitter and then it's going to hit this mirror and go up and then it's going to hit this mirror and go to the right and then it will hit this mirror and go down and split and then it's going to go to the left and down and off the board our goal is to basically just follow the instructions here we're going to have a beam that starts on this top left corner is pointing to the right and is traveling through this Contraptions because a a beam can be split into two we'll need to go through the contraption with lots of different paths so I want to keep an array of the fronts of the beams and if we encounter a splitter we'll just add a new head onto the beam and then we'll go in like the two directions away from the beam the beam of light needs to know its XY coordinate and what direction is it traveling and it's possible that a single tile can have beams that are traveling two different directions on top of it for instance this specific uh tile right here has a beam that can is traveling to the right and also a beam that's traveling down that can cross at the same time that's totally fine so we need to keep track of where we are and what direction we're headed ultimately we count up all the tiles where the beam passed through so if we grab this puzzle input usually we drop it in just like this right we'll say input is equal to this thing um and then input but if we print out input dosit on new lines. map chars you'll see something interesting it's actually a little hard to tell based on on this look here so if we go through let's just call this grid and we'll say grid. each do row puts row. jooin okay if you look at the grid and the way that it printed out it does not look like a square right the input looked squarey and Vim is telling us that something about these slashes is different right and so if if you look at the output we actually don't even see many of those backslashes we only see one of them right here and that's because Ruby uses backslashes to escape characters so inside of this string these backslashes are escaping the period in a way so if we open up IRB here and just explore backslashes for a second so if we have a string and we try to say let's create a string with the backslash then this doesn't actually give us anything a is invalid right it's because we're still in the middle of a quote we're like in in the middle of defining a quote so if we try this again a is equal to backslash and then we close the backslash we end up with like backslash and then a new line and the quote here is escaped so the whole point of this is so that we can do stuff like this we can say a is they said um hey there right so that you can print out like the quotes around it now when we evaluate this previous line it's showing the inspected version but if we do puts or put s for a now like the string that was printed out has quotes around hey there so the backs slash here is a special kind of character that's used for escaping other characters and if for some reason we needed to include a backslash in our output the way that we would do that is by putting two backs slashes and with if we look at this string it looks like there's two backslashes but if we put a there's actually only one backslash and in fact if we look at like a.length it's only one character a. or the asy value for a is 92 so if we look at this asky table and we look at 92 we see the single backslash when we're defining a her do here this is a string it's a type of multi-line string in Ruby and this is expecting that we go through and add a second backslash to all of these however we don't want to have to do that on our input right if we run this okay so now it looks like a grid right but our actual puzzle input is going to be massive we don't want to manually go through and add backslashes rather than adding our input like this we want to read it from a file so let's just add a new file. read and we'll say/ example and that's going to be our input so we're going to open up examp Le and we'll paste in this all because we have some backslashes so now we can do uh Ruby main. RB and we see the backslashes as expected so now we have a grid that we can work with now that we have this grid of characters we want to figure out how we can travel through the grid so we need to start with a list of beams and our list of beams is going to start at basically 0 0 and it will have a direction of it's going to be heading to yeah we'll just say to the right okay and what we want to do is while the beams are EMP are like have something in it we're going to work our way through the head of the beams until we fall off the edge of the grid what we want to do is grab the first so X and Y and direction is going to be beams. shift and then we want to say okay if so we also want like a DX and a Dy and that is going to be based on the direction that we're already going so if we're headed in if we're going to the right then that means we want to not change the row we want to only change the column and we want to increase the column by one if we're going to the left we're going to do the same thing but we want to decrease the column if we're going up we want to decrease the row and if we're going down we want to increase the row so dxdy is going to be a mapping between the direction of our beam and this pair of of directions that's going to give us a new like a new grid coordinate that is like the next step forward if we were to follow this the Grid in this direction and that's going to be Grid at X DX and Grid at y Dy and and now what we can do is use pattern matching based on what's at the front so we want to say case for the front it's going to be one of these characters right a DOT a slash a pipe or whatever if the thing that's in front of us is a if it's a DOT then we keep going if it's a pipe then we maybe split if it's in a Dash we need to split if it's in a slash we need to turn if it's in the back slash we need to turn and then we might have some other cases here while we're going forward if we encounter this dot what does it mean to keep going forward it means that we want to add into our list of beam heads like this new beam here so we have DX Dy and the current Direction so we're going to keep going in the same Direction now if we encounter a beam we might want to split but we also might not want to split so here we can we can build our pattern matching not only on the type of tile that's in front of us but we can also build it on the direction that we're currently going so if we're going forward and we encounter a dot it doesn't matter what the direction is we're just going to keep following the same direction but if we encounter a pipe and we're either going to the right or we're going to the left then we want to split but if we're going up or down we don't want to do anything we want to just keep going all right in what does it mean to split let's see in this case if we're going to split at this position we want to add into the beam our current position and we want to add up and down so when we're splitting on the beam we're going to going to go up and down okay and then this we're going to do the same thing here but instead of going up and down we're going to go left and right because if we're coming yeah if we encounter the flat part of this then we're going to go both left and right from that same position okay so that gives us our splits in our turns oh and we only want to do this if we're going up or down if we're going left or right we're just going to continue on so we'll we'll also need an else case here if we're in any other case we want to keep going which means that we might be able ble to pull off this one and put this in like the default case but we'll see in a second okay so if we encounter this slash we want to turn and it depends on what direction we're coming from which way we're actually going to turn and so we need some mapping between the type of turn so I guess in this case it's going to be slash and that's going to point at something okay so if we're going to the right we we want to go up and if we're going left we want to go down and if we're going up we want to go right and if we're going down we want to go left and we might as well do the other one also so if we're going to the right we'll go down left up up left wait left up we're going up we go to the left yeah that's right and then if we're coming down we go to the right okay so now we're going to say beams we only add in one single beam in this case and instead of Direction it's going to be like turn of Slash uh in that direction does that make sense so we're indexing into turns at slash and that matches what we just pattern matched on and then we're going to map from that direction to the New Direction okay same with this one so we're going to to go from turn to the other direction okay otherwise we just put this in I don't know if this is actually going to work let's let's see here while we're traveling through all these different beams we're probably going to encounter something where we fall off the edge of the grid here so we want to also add some thing that says if x + DX is less than zero um so this is if we're off the grid then break or next and this beams. shift is going to pull the front element off of the beam so this is going to modify the underlying array and eventually we'll get to an empty beam we can keep track of a list of energized tiles meaning like we've traveled through that tile and at the end we'll print out P energized. length yeah let's just see what that gives us okay okay zero that's because we're not so energized plus equals beams and because we're using a set that should be okay all right so we got back 20 and that's so that's interesting so let's look at what those actually look like on the grid so we'll do grid. each do row and then let's actually do each with index to row row. each with index do cell and and we want to say something like if energize includes XY then print um the pound sign otherwise print a DOT and then at the end we'll print a new line um okay but we have a problem here because our energized like when we add the beams they also include the direction so here we we need we need to have the list of energized without the direction and it's almost like it's listening to me okay all right so now if we run this okay it looks like it split here it came down it split and then it traveled here and then here and then it went like right through this so something is going on here why aren't we H let's take a look again okay so we're starting at 0 0 we're going to the right it splits and comes down and then here it splits and then goes across it goes up and then it just goes like straight off the edge over here so it's almost like it's not encountering this backslash and coming down so we've got to figure out why it's not hitting this backslash oh because if it's yeah this needs to have an underscore for it to match there okay so now we are hanging when we run this so it's stuck in a it's stuck in a loop so now we need some way to know to break out because if the light is just bouncing around inside of a loop then we'll we'll never come out of that Loop this isn't going to really work because we don't include the direction in energized so we need to have another set that has the the tiles that we visited along with the direction so we're going to have some list of scene tiles here and we're going to say something like if we've seen it then then we'll skip over it but we want to do this a little bit carefully because we don't want to just eliminate all of them so again right now it's hanging and what we want to do at the bottom here is maybe we keep track of a subset of beams so we'll call it like new beams is some array and we'll only add new beams to the list of beams that we should keep exploring if we haven't already seen it so we're going to go through new beams. each do beam beams shift beam if we haven't seen the beam before then at the end we're going to say scene plus equals beams and we want to add all of the beams that were valid and we want to add our energized ones that are filtered out because we need these seen ones to include the direction and the the energize to keep us keep track of like where we've been so far all right this is looking great this pattern matches what we're looking for if you look at this now we're off by one and in this example we would have seen 46 energized tiles we actually saw 45 energized tiles and you'll notice that this top left one is not energized in our solution but it is in the example and part of that issue is that right now our first beam is entering assuming that we're starting on that tile but we actually need to start off the grid pointing at that tile so we can put this at ne1 we can start our yeah we can start our list of beams at negative 1 that way when we run we're properly energizing that cell there okay that should work for the example input let's grab our input and grab our puzzle input here copy this giant thing paste it in I'm glad we don't have to manually go through and update all of those backslashes okay and let's comment out this printing we don't actually need to see it anymore and then we'll leave in the one at the end but okay so now instead of example we want this to come from input okay run it again all right we've got some cool input Happening Here and 6361 is our answer and let's see if that matches yeah 6361 that's the puzzle answer for part one awesome okay let's take a look at part two part two the difference is instead of just starting from that top left corner we can start from any part of any of the edges and we need to figure out what the maximum energized tiles are for any configuration so rather than just going through the the contraption once we need to go through the contraption and get the best energy from the contraption if we were to go through it from any angle we want to maximize that maximize that energy so let's take a look here I think what I want to do is just refactor this whole thing into a new method called Energy and it's going to take in the beam and the grid and it will do all of this work and it will give us back the energy so now if we just said something like P the energy of um Z1 for the grid and we run it we should get back something ah so now we need to create beams start with this starting beam great okay all right 6361 great so now we've got something so we need to go okay so this is for the zero row want to go to at the first row and the second row and the third row and so on until we have explored all the different rows so let's comment out the hours and look at the example so we want to go through zero up to grid size and this will give us all the rows and that's when we want to go right and this will say uh this is going to be X and negative one means we're starting right off the left side of the grid we're going to the right and then we also want to start off of the right side of the grid so this is going to be grid do yeah grid. first. length and that should be the because we're starting on the right side of the grid we want to be going left and we want to do this same thing but for the columns so grid. first. size and then this will be y I don't know okay so if we're starting at oh this is actually going to be -1 y that means that we're looking from the top of the grid and we're going to be going down and then grid. size or grid. length or whatever and this case we're going to be going up and why I think think that's right I don't know I'm pretty sure all right and then at the end we're going to get some Maximum maximum energy maximum power all right so what I think we might do is just say like Max is equal to uh yeah zero and then we'll say Max is this max. Max and same thing here Max uh is to this oh we need to we still need to return that though okay so what did we end up with here that's actually wrong why is that wrong oh this one doesn't have a Max okay there we go so started at 46 we got up to 51 and that looks good so the actual input is slower so 6361 okay getting a little higher getting a little higher and there are 110 rows and 110 columns in the example input and these are Trucking through at one or two per second here in Ruby so we can just let it run for minute is so that would be yeah that would give us like 110 * 2 yeah actually we'll just do like times four because that should give us 440 we need to go through and solve 440 times so if we just do some 440 divided by 60 let's assume that it takes a second per time to go through we end up with seven minutes which is survivable right I feel like the part of computer science is just figuring out how long things are going to take and then figuring out what what you can stomach we are letting this run and now that we see 6701 a bunch of times this seems like it's probably the max so we can come over here and look at 6701 all right boom that is the maximum for part two I I had a couple ideas for how to optimize it I think if you wanted to trim off some of the execution you could cache and figure out okay as I'm solving this if I encounter some tile where I know the cached maximum energy from that position out to the edge then I only have to I can just return that instead of counting it up but yeah I felt like brute force was a little easier okay so we did end and we got 6701 let's take a look here and see if there's anything that we could improve I don't think we need this case because it's covered by the bottom base case or the default case and yeah so if we're going to the right or left we want to go up and down if we're going up and down we want to go right or left and then for these turn cases I think we can combine these two cases and we could say if it's a turn at all then we pass in front as our argument here so if it's a slash or other direction slash then we key into our turn thing and go the other way there's probably aart way to do this beams situation too but this seems to work okay for skipping over those those scene ones at the very end and then keeping track of what's energized versus what's not we could probably do this with one single set like maybe there's a a way to filter out which ones you've seen and exclude the direction so potentially you could comment this out and then say scene dot length and then we can remove move the concept of energized here and use scene directly so now if we run this against the example input so that it's fast okay so now we get 51 in that's pretty quick and our okay so just to for the sake of double-checking running it against the main input started at two minutes in 1 second so let's run it again and see if it's any faster against the main the actual input now that we've made that change where we removed one of the sets and we only really need to do this step where we're mapping over and pulling out X and Y once per energy check right like we're going to keep track of the scene we need the scene anyways we might as well just keep track of all of the scene and then yeah convert it back to a set and take a look we're taking a chunk out of it not half the memory but we're taking a good chunk out of the the memory usage by removing this other set here so that should go a little bit faster we're also removing like some set operations another question that I have is how else could we trim this down so if you have thoughts or ideas yeah i' love to hear so this one now it's 1 minute and 3 seconds so we basically cut it in half the time by removing that one set so that's pretty cool yeah this is this is pretty neat I'm really loving pattern magic I'm using it a ton in in this year's Advent a code really really enjoying it hey if you learn something new I would really love if you hit subscribe it's free you just push subscribe that's it and yeah it would mean a lot to me hey thanks so much for watching I really appreciate your time and attention we'll see you in the next one

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