--- title: Hash.new, tally, one?, tap, path finding - Advent of Code 2021 - Day 12 with Ruby slug: hash-new-tally-one-tap-path-finding-advent-of-code-2021-day-12-with-ruby published_at: 2021-12-18 14:00:24 +0000 updated_at: 2026-03-04 20:13:43 +0000 summary: description: Hash.new, tally, one?, tap, path finding - Advent of Code 2021 - Day 12 with Ruby 06:23 Hash.new tricks 08:34 find_paths 17:00 Part 2 18:00 tally! 20:45 tap tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, rails, advent of code, aoc, advent of code 2021, hash.new, tally, tap, path finding] views: 267 author: CJ Avilla url: https://www.cjav.dev/videos/hash-new-tally-one-tap-path-finding-advent-of-code-2021-day-12-with-ruby youtube_url: https://www.youtube.com/watch?v=_spUSatNdbs youtube_id: _spUSatNdbs embed_url: https://www.youtube.com/embed/_spUSatNdbs thumbnail_url: https://i.ytimg.com/vi/_spUSatNdbs/hqdefault.jpg type: video --- # Hash.new, tally, one?, tap, path finding - Advent of Code 2021 - Day 12 with Ruby *Published: December 18, 2021* *Views: 267* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=_spUSatNdbs) [![Hash.new, tally, one?, tap, path finding - Advent of Code 2021 - Day 12 with Ruby](https://i.ytimg.com/vi/_spUSatNdbs/hqdefault.jpg)](https://www.youtube.com/watch?v=_spUSatNdbs) ## Description Hash.new, tally, one?, tap, path finding - Advent of Code 2021 - Day 12 with Ruby 06:23 Hash.new tricks 08:34 find_paths 17:00 Part 2 18:00 tally! 20:45 tap ## Transcript hey what's up welcome back this is day 12 of the 2021 advent of code we're solving these with ruby um and this one is all about finding all of the paths through a sort of a graph when you have a source node and a destination node and then there's a couple of different little tricks uh mixed in so this is the way that we receive the input is sort of the the edges or this like okay so in graph terminology a um a vertice or a node represents sort of like a dot on a map or a dot on a graph um you can sort of think of that as like the um like a city in a map and then the edge is going to be the road between the cities so that is kind of like the connection between two nodes so here we have like start and a this is we're receiving this as kind of like the two nodes and their relation so here on like the drawing of the graph this kind of like input here would help us build this connection between start and a and it turns out that if we have like all of this same input it would look like a graph like this and our goal is to find the number of distinct paths that start at start and end at end so you can go from like start to a to end or start to beat to end or start to a to c and then the the one trick is that you can you can only visit the small caves or the caves that are lower case one time at most one time and you can visit these big caves as many times as you want and so um the trick here instead of just having like a standard path like finding algorithm where you can just like generate all the paths through the through the graph we have to be able to kind of like go back and revisit certain nodes um so we'll go through the process of implementing this and hopefully um yeah hopefully we learned some stuff all right so we're going to go in here and add day 12 and in here i'm going to add graph.rb um i don't know what we should call it but that seems fine day 12 spec.rb require relative day 12 graph.rb i like looked up a bunch of snippets um realizing that i was typing like the same stuff all the time for vim and snippets i use snip mate which is a little bit on the older side and i'm super excited to start using um github copilot and there's a video on the channel getting that all set up but i didn't want to use it for these videos because it basically writes like the entire thing for you and i do kind of feel like it's a little bit of cheating all right so the first thing we want to do is build build a representation of the graph that will help us work our way through it and so there's a couple different ways you can do that there's several different approaches to building representations and when i went through this i actually took a couple different approaches like one was building each node as its own class like in an object-oriented way and then that node was related to other neighbors but i think that so i saw some solutions that just used an adjacency list i think that might be better so let's try to do it that way so we're going to have some method that builds um an adjacency list from the input so this is going to be something like graph is equal to build graph and our input will just kind of be the lines that we're getting from from input um so this is kind of like uh i guess we'll just yeah take these in as strings and then we want to split on that we're going to split on that sort of dash and we should expect that we get back a graph that looks like this um it's going to be a hash and there's there's a couple different ways you can set this up so you can have it be like string to string or string to list of strings for us let's just keep it easy so that we know that we only have a unique set of neighbors we'll use ruby's set class so we can say something like start points at so in this case start is related to some uh set of other nodes so in our case it's related to a it is related to b and then um yeah i think that's a good start there and then we have to put in a so a is related to start because it's going to be both ways it's going to be like bi-directional because it has that property where you can go back through um back through the upper case letters so a is also related to b also related to c and also related to end so we've got an end in there at the bottom um is that right so star oh it doesn't it's not related to itself okay that's good all right so then we have b b is related to start b is related to a b is related to d and b is related to end then we have c and c is just related to a and then we have d which i think is just related to b then we have end which is related to a and b i think that might be what we want all right so let's say build graph this is going to be a method that takes in our input lines uh maybe we'll just call these like lines or something i don't know edges um not sure and then we'll say edges dot split on um actually yeah i guess we have to map over it edges.each do edge and we'll say edge.split on dash i don't know if it's really technically the edge people are probably going to call me on that whatever and then that's going to give us like point a and point b and a and b are just like the two nodes right and so we want to build our adjacency list and so this is here's a trick that can be really helpful when you're initializing a hash you can initialize that hash so that um when new keys are added to the added to the hash uh you can set like a default value so if you pass a block to new that accepts in the hash and the new key so now we can say hash at key is equal to set dot new this is really helpful if you're building like a counter cache type thing or if you want to collect up like arrays of arrays or hashes of arrays or things like that okay so now we can say graph at a and we want to add b into the set of neighbors for a and then we want to do the same thing but the opposite so b we're going to shove in a so then we'll return our graph at the end and that should be our adjacency list cool so we have a passing test the next step here is we want to like we'll just build the the thing that goes through and finds all of the pads through the list and i think the easiest way to do that is to like start with something super simple and say like it finds paths through a basic graph or something and we'll just start with like um yeah we'll start with building a a more basic graph than this and we'll just have it be like start points at a a points at b and b points at end and then we'll expect that find paths through graph to equal something so this is going to be all the all the paths to start let's just try to solve the simple case of just going like from start to a to b to end so we'll have like start a b end and the output that it wants is actually just like the number of paths so in this case we just want to count up the paths but it is helpful to just see what all of those unique paths are and that it's like yeah it's helpful for debugging so we'll start here and yeah so we've gotta implement uh find paths through the graph and in practice we might take in like a start and an end and whatever but for now we'll just say that like we have to we have some like paths to visit and this will start with our start node and then that's what we're gonna ultimately no no so then we're gonna um this will be like the thing the cueing mechanism that we're going to use to like track each of the neighbors that we want to visit and so while paths to visit empty while it's not empty what we want to do is we want to pop off the path that was most recently added so we'll have like a current path is paths to visit dot pop and then what we want to do is like just check that path and if that path ends with an end um then it's like a valid path and we want to include it for like our results in fact like let's keep track of those so valid paths is some empty list and that's what we'll ultimately return down here is valid paths and valid paths are like all the paths that have an end or that end with end and so here i guess we want to say something like if current path dot last is equal to end then we want to we kind of like want to return that or like add it to valid paths current path dot or a current current path yeah so let me think about this uh if we found an end we don't want to keep going um so let's just say next here so we'll look at the next thing so that's kind of it's almost like a base case or whatever like we're not doing this recursively but i would consider that sort of like the common case now the next case if we didn't find the end we want to iterate through the neighbors of the current like of the last node that was added to the current path so we want to say graph at currentpath.last that each do neighbor so that's going to give us like the um this will help us iterate through the neighbors in that adjacency list so this is actually iterating like through the set right and each neighbor is going to be a string value and what we want to do is we want to add to our list of paths to visit paths to visit is going to be the current path plus plus the neighbor so this is going to we're kind of just like appending the neighbor to the current path um yeah so that will actually like when we do this concatenation here that's gonna create new paths so like every time we we visit a neighbor it's gonna create a new path and sort of like fork off um and then yeah the okay so then the other thing is that we only want to add or we only want to fork if we've never actually visited this neighbor so we could i think yeah we could keep track of like visited but i think we can also just sort of hack this by saying like we already have the current path right here we're looking at the current path and what we're trying to avoid doing is adding duplicates of this neighbor into the current path so we can say like if currentpath.includes neighbor or if it doesn't include the neighbor i mean then we want to add it to the paths to visit let's see let's see what this gives uh that seems too easy all right so now let's make it let's make it more complex and let's make it so that we can go through um [Music] yeah so that we can go through a or b so start points at b and then we should get like a couple more interesting paths here so start b end um and also we didn't really like talk about the case where you can go be you can repeat visiting the um the uppercase letters so we should really have like another one in here that is like um well actually instead of start b let's add um a to end because that will make it so that we can go through start b end or start a b a and i think and also start a end um yeah so that should be our list but we got what did we get um we got two things but we expected three okay so this is where we have to add the case where you can visit the big um the large neighbors or like the uh the bigger caves more than once and so what we can do is in order to allow supporting visiting that larger cave more than once we just want to like skip this guard clause if the neighbor is uppercase so we want to say something like actually is that a thing uh let's see so pry um a a dot is upper no uh check if letter is upper ruby i was going to just say like upper equals equals oh yeah okay is that the right thing oh that's neat uh regular expression okay well whatever that's too fancy that doesn't we don't need to be that fancy we can just use this we can just say like if neighbor is the same as neighbor dot up case um so if the or like if the neighbor is a capital letter or the neighbor is not in the current path then add it to the current path and that should help us get what we're looking for um okay so i think these might be the same thing they're just ordered differently and so instead of using equals we want to say contain exactly and we want to remove that run it again cool so now we've got all of our tests are passing and i think we can just write some driver code now so going back to the graph here at the bottom um we can say uh let's see so we want to read in the file file.readlines rv.first dot map chomp um and i guess that's kind of lines or that should be our edges build graph and then we want to say find paths for graph and we want to let's print out dot length and let's add our some example input here and i think they give us yeah so here's some example input there should be 19 paths through it and let's see uninitialize constant set okay so require require set 19. okay so that's working as expected and then this one should have 226 so let's go look and make sure that we work for that one 226. fantastic all right and then my answer was five two two two five two two eight i guess we gotta add our input here and it looks like this okay um so we'll run on our input 5228 okay cool so we can pick one single small cave to visit twice and um so that is a new constraint for for part two this is kind of like part two so i guess like um this was where we were checking if whether or not we can visit that path so i think what we want to do is basically the same thing as this but we want to we want to be able to visit a small cave twice one single small cave twice and so let's add a helper method like duplicate or like dip duplicate small and that's going to take in a path and tell us whether or not we've visited small things twice we'll select out those that are where one is the same as one dot down case so that should give us all the small letters and then we want to call is there like a duplicates method or no i guess how do we want to do this um tally so tally is a really cool method let's take a look at how it works so if you have some list one two one three four whatever four five something something and you say dot tally that gives you a hash where like the value is the number of times that object appears in the in the list so if we use tally and then say dot values and then dot one two okay so i think um so this was another this was another enumerable method that we saw when we were checking out the docs and so i wanted to use it uh so one so let's see if we can let's see if we can use that tally. values.1 where there's like one um two so duplicate smalls and if they're uh yeah so if we've if we've already duplicated the number of smalls then we don't want to add it so i guess if there's no duplicate smalls for the um for the current path or yeah okay so let's see if that works so let's run it on some example input and we get uh oh actually let's run it on this this original input and i think that's what the the like reference number is showing us okay so if we run it on that input slightly larger example above now has 103 paths through it so oh no now the 36 possible paths okay so did we get we didn't get 36 we got 63. so duplicate small the other thing is that you're not allowed okay there was another stipulation here however the caves named small and end can only be visited exactly once each if if the path already has end then we don't want to add end again so let's do it this way next if current path includes um end and neighbor is end and the same thing with start and let's see what that gets us okay that gets us to 36 so that was the example answer and then the slightly larger example above has 100 or 103 and the larger example now has wait slightly larger and the even larger oh here's the slightly larger okay and this one we're thinking has a 103 okay as 103. great i'm just going to run it on an input because i think we're pretty close and oh wow it's taking a long time okay okay come on computer 131 thousand 228 131 228 awesome uh so this was uh generating all of the paths through a graph when you have a single start and a single end a source and destination sometimes they're called so this was a fun one there's a couple of things that i wanted to refactor though so let's just play around but if you're if you're satisfied you can you can bounce out but um what did i want to show oh right there was there's an interesting thing we can do here called tap so we could say like hash.new.tap do graph and tap basically just like creates a wrapper around this so that we don't actually have to define graph on its own um this can sometimes be like referred to as uh i don't know like there is this t there's like the t command in unix that you can like pipe input and output to that can be really helpful so tap is a cool feature let's make sure that our day 12 specs still run and actually you know what now that we have support for multiple um yeah now that we have support for multiple directions that one isn't going to run anymore but this one still runs so that's cool um so tap tap is something that you might want to look into it's kind of like an interesting tool for um yeah tapping into parts of a process one of the reasons why people don't like to use it is that it ends up increasing like the indented like how indented all your stuff is and um yeah so that's something to keep in mind we looked at tally we looked at the one method um those are pretty cool also like when you're when you're going through uh finding paths it can be helpful to just work directly with the paths themselves sometimes um rather than just like indexes or kind of like uh references so yeah i don't know hopefully that's helpful thanks so much for watching and we'll see the next one [Music] --- [Back to all videos](https://www.cjav.dev/videos) --- ## Hey Agent, thanks for visiting! 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