--- title: Caching and Pattern matching - Day 12 - Advent of Code slug: caching-and-pattern-matching-day-12-advent-of-code published_at: 2023-12-12 23:00:18 +0000 updated_at: 2026-03-04 20:15:20 +0000 summary: description: In this episode, we walk through solving day 12 of Advent of Code 2023, called \"Hot Springs\". The challenge involves parsing records of hot springs, some of which are damaged (#) or operational (.), along with checksums indicating groups of damaged springs. First, we set up some test cases and write a parse method to process the input. Then we implement a count method using recursion and pattern matching to count all valid arrangements of springs that match the checksums. Key cases include: - Encountering a \"?\" which represents variable springs - Incrementing \"group size\" when we hit pound signs to track damaged groups - Checking that group sizes match checksums when we reach periods - Caching results to speed things up For part 2, we expand the strings by duplicating parts and joining them to make the problem more complex. More optimizations like collapsing multiple dots and returning early based on string lengths are added. In the end, we arrive at a working solution that passes the test cases and puzzle input! 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] views: 315 author: CJ Avilla url: https://www.cjav.dev/videos/caching-and-pattern-matching-day-12-advent-of-code youtube_url: https://www.youtube.com/watch?v=ZIWk05CqC4s youtube_id: ZIWk05CqC4s embed_url: https://www.youtube.com/embed/ZIWk05CqC4s thumbnail_url: https://i.ytimg.com/vi/ZIWk05CqC4s/hqdefault.jpg type: video --- # Caching and Pattern matching - Day 12 - Advent of Code *Published: December 12, 2023* *Views: 315* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=ZIWk05CqC4s) [![Caching and Pattern matching - Day 12 - Advent of Code](https://i.ytimg.com/vi/ZIWk05CqC4s/hqdefault.jpg)](https://www.youtube.com/watch?v=ZIWk05CqC4s) ## Description In this episode, we walk through solving day 12 of Advent of Code 2023, called "Hot Springs". The challenge involves parsing records of hot springs, some of which are damaged (#) or operational (.), along with checksums indicating groups of damaged springs. First, we set up some test cases and write a parse method to process the input. Then we implement a count method using recursion and pattern matching to count all valid arrangements of springs that match the checksums. Key cases include: - Encountering a "?" which represents variable springs - Incrementing "group size" when we hit pound signs to track damaged groups - Checking that group sizes match checksums when we reach periods - Caching results to speed things up For part 2, we expand the strings by duplicating parts and joining them to make the problem more complex. More optimizations like collapsing multiple dots and returning early based on string lengths are added. In the end, we arrive at a working solution that passes the test cases and puzzle input! 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 hey what's up welcome back in this episode you'll see how to solve day 12 or at least we'll struggle together through day 12 of the Advent of code for 2023 this one's called hot springs and it's a doozy it is one of those ones that requires quite a bit of dynamic programming and yeah once we get to part two it gets pretty messy so let's jump right into it the idea is that we get a bunch of records and each row represents a record where we're keeping track of which sort of Hot Springs are damaged versus operational so the damaged ones are all of the pound signs the operational ones are all of the dots but in some of our records we have this question mark which could be operational or it might not be operational and then on the far right we have a check sum type thingy which um tells us the groups of numbers that are damaged in this case we see there are one damaged one damaged and three damaged so we should expect to see a pound sign a pound sign and then three pound signs and each group of damaged Hot Springs need to be separated by an operational hot spring based on this check sum the check sum in this case 1316 would be something like this one is damaged these three are damaged this one is damaged and then these six are damaged so part one is we've got to figure out uh how do we want to iterate through and decode the number of ways that we could achieve this same check sum with these variables so like where the question mark is the variable we want to either put a dot there or a pound sign there and have it be valid based on this check some and so we need to know like how many different Arrangements of operational and broken Springs will work with our our list here on the right the way that I solved this at first was to create all the combinations and permutations of pounds and dots that would fit into uh three spaces and then four spaces and then uh or just to replace all of the question marks so explode out all the different options and then filter down the one to the ones that are valid um but that doesn't scale definitely doesn't scale to part two and so in part two I had to rewrite the entire thing and so we're going to go through uh that direction first so let's open up main. RB and one of the other things that I found was when you're working on a project like this that is just sort of like wackamole meaning like every time you make a change to your code there's a chance that it's breaking some other code somewhere else I call that whacka code because you like whack down one of them and another one pops up somewhere else what I like to do in that case is write some tests let's pop in require rpec auto run there is a tool within rspec that lets you put in um this command at the top and now we can just describe I don't know count um it works on basic cases we don't need to have the best prettiest test Suite here we just need to have some that we can work with I'm thinking maybe we should have our test cases be the input here and then for each one we can say cases is some list of cases here and then for these we can make these what we're going to pass into some parser thing that ends up giving us back what we need to pass down ultimately to count so that we don't have to do too much wrangling uh but we will also need to have some expectation piece here so for each case let's pass in some number and okay so like for part one here this will tell us in the example that there is one possible arrangement for the first row and that possible Arrangement is we have a pound sign a DOT a pound sign and then three pound signs and so here we have one Arrangement 41 1 4 10 so 1 41 1 4 10 and then what we want to do is say cases. each do input and we want to say something like expect that count or maybe we want to like parse um input into a record [Music] to um equal the expected so this parsed input will then be passed maybe to our count method and maybe our count method is going to take in the string and and then all of these different sizes of the groups and we expect it to come back and give us what we want so the way that we can run this is just say Ruby main. RB and because we now have this rspec auto run we will see the output here so now we're seeing undefined method parse so let's add a parse method parse uh input and we could even write parses as expected and we want it to parse this thing expect pars of that uh to equal um we want this to be an array of numbers I think that might actually just be it no we want it to be yeah we want it to give us back of this tupple so let's see okay parse failed we expected this and we got back that okay so we want to um we want to take the line that's like our argument and we want to split on space and then on the left side we're going to have some record and then uh group or like sizes and then we'll say sizes is sizes. spit. map toi we'll return record and sizes let's see if this works okay so we have a passing test for the parse now undefined method count okay let's make a count method and this is going to Tak in the record and the sizes now what we need to do is figure out a strategy for breaking down into sub problems okay so anytime we encounter a question mark that is going to be a fork where we will have to count what are the possible answers that had a dot versus what what are the possible answers that had a pound sign so that is going to be one of our Forks if we encounter a pound sign we need to like group count count up the groups and figure out okay if we just saw a DOT then this is the first pound sign and if we see three pound signs and we knew that the number was three then we're good if we see more than three pound signs then it's like an invalid combination and so then we need to bail so in this case if we replace this question mark with a DOT then it's still valid if we replace it with a pound sign it's all still valid so then we need to go to the next step and on the next step we need to check like okay if we had a pound sign there and the question mark is a DOT is it valid yes because this was a one is it valid if it's a pound sign instead of a DOT no because this is a one not a two so we're going to have a bunch of these different cases that are based on the record and like characters of the record and we're also going to need to keep track of like where we are while we're Gathering up the group so the first step that I want to do is take the record and break up the characters into the maybe the current character and then the rest so this is going to give us the current first character that we're looking at and the rest of all the characters as an array and what we want what we need to do is say a bunch we have a we have a bunch of different cases that we need to cover right the case when Uh current is in question mark we're going to have something if it's a DOT we're going to do something if it's a pound sign we're going to do something else end okay so if it's a question mark we want to recursively call count with both so now we're going to call count and we want to call count with both the period and the pound sign um instead of the question mark okay so we need to call count and for our record what we're going to do is we're going to use the rest of the characters. jooin that's going to give us like the tail and then at the head what we want to do is pound sign plus the rest okay and we're going to still pass pass our sizes forward and we want to take the result of that and we want to add that to what if we added the dot on to it also so in both cases we're returning uh the rest of the string so as soon as we encounter this question mark we're going to recursively call count on the rest of this substring so like we're just swapping out this question mark for a pound sign or a DOT and then recursively calling okay so that's one of the cases is now if it's a DOT then we need to know whether or not we are in a group or not let's actually let's do the pound sign next as the next state so if we are on a pound sign then we also need to recursively call so we're going to return count but this this time we can just say rest. jooin and sizes so if we encounter a pound sign that means we're like inside of a group and we need to keep going if we're at a DOT we want to return okay so there's a couple cases here so if we encounter a DOT either we're at like the end of a group right so we're either at the end of a group or maybe we're at the end of the entire list so maybe at the end of a group maybe at the end of a record one thing that's missing is how we are checking again against the sizes so sizes here will start out as an array like 1 1 3 okay but as we recursively call on smaller and smaller strings and substrings if we encounter a valid hashtag or pound sign that is a single pound sign followed by a DOT then we can remove the first size from this list and the goal is to like empty out this list with valid elements so in addition to sizes I'm going to pass down the like current group or like group size and it'll start out at zero so if we encounter a question mark the group size will be the same and yeah that'll be the same for DOT too so if we encounter a pound sign then the group size is going to increase by one because we just checked off a group size now we don't want to remove from the list of sizes until we get to a period right because the periods represents the end of a group of pound signs so we have a pound sign here and we have maybe let's say like these four were pound signs and then we have a DOT that ends that group before and so then we're calling we're going to call count on this substring that has one pound sign and an array of one one for our sizes so we know that we're done with this group when we encounter the period that's right after this pound sign and similarly we know we're done with this group when we encountered this period after this pound sign and if the group size when we're finishing the group matches the size that's at the the head of the list of sizes then we know it's valid and we can continue on if it's different then we need to return zero because that means we have an invalid sort of uh size here so what we want to do is say if we're if group size is greater than zero which means we're inside of a group right because this group size that's being passed forward is increasing as we encounter pound signs so if we're inside of a group um and okay so if we're at the end of a group then what we want to return forward is zero ideally but it's possible yeah so hold on a second so if we're at the end of the group if the group size is the same as sizes. first then we can go forward because that means that we're still in a valid State and we can go forward with sizes from one to the end right this is the rest of the sizes because if the group size that we just closed off matches the size that's at the front of the list then we only need to pass forward the rest of the sizes and we're only going to pass forward like the remaining part of the list and we're resetting the group size back to zero so that if we encounter another period and group size is zero then we're like fine to keep going okay if group size was not the same as the first then we need to return zero because this means that we have an invalid case where either the group size was like the group of pounds was smaller than the number or it was bigger than the number and so if we don't hit it right on the head we want to return zero which is like our terminal case to get out of this thing okay maybe we're at the end of a record so if the group size is not zero then what we want to do I think is return count of yeah we want to just keep going because if it's a DOT if we encounter dots we should just keep going down the list all right let's see if any of our tests are passing here let's comment out a bunch we'll start with one test and in fact what might be even better is let's start with literally just a single pound sign and a one or we expect that this is going to give us the count of there should be one valid instance of this in here so let's run our tests okay we got a failure no matching pattern for nil so that means that record that record was empty so if we get to a state where so we we're also we also need like our another base case here for returning from this so if um if the record is the empty string what does that mean yeah so if we get to an empty string and sizes is empty then we want to return one otherwise we want to return zero we might need some more base cases there but we'll start there okay so we expected one and we got zero another thing that can be helpful when doing this sort of debugging is to put a Lambda as the last argument to rspec that will print out input and expected just so that we can see or just even just input so that we know which example we're looking at okay pound one all right that didn't do what we wanted so let's do this let's print out record sizes and group size up here and run this again okay oh okay so another thing that can be helpful because we're only we're using the period to know like when we're at the end of a group um what I did was while parsing I'm going to append a period to all records that way yeah we'll say record equals process record or something I don't know let's just do dot or record dot at the end just so that every single record will have a DOT okay expected okay it so now our parsing our parse method doesn't get what it's expecting so this should get a a DOT okay now we have two passing tests so that's fantastic let's level this up one more it should work with a DOT before it and Dot after it those should all work okay and then we would expect there to be zero examples if we put a two here but there's only one pound sign and only one way to make a pound sign okay there's zero examples in that case let's see if it'll work against one of these example rows oh my gosh it's working it's working still working um okay wow I'm surprised that that worked okay all right we might need to come back and handle this or maybe this is handled by our base cases let's throw some let's throw some example input at this so let's let's parse our input into these lines so we'll say data is is input. each line and now we'll say data. each do line and we want to yeah yeah that seems good let's do that okay we need to remove some printing too loud okay 14141 holy moly okay so this that's like working so we actually want to add this up so we'll inject zero and then this is going to make us have a sum and a line and we'll add sum plus count that'll give us some result we'll say puts part one is the result all right part one is 21 and that matches the example number for the total Arrangements in in the part one example so our puzzle answer was 8180 so let's grab our puzzle input here and go to the end and then instead of reading from input. each line we want data data. readlines all right 8180 8180 is all right that was our puzzle answer for part one awesome okay amazing that's that was fun okay all right so let's take a look at part two as you look at the field of Springs it turns out the the records were folded up folded up what are you doing folding your records up the the result is that we actually have five copies of itself so uh the way that we're going to expand this and make this problem way way harder is that uh we're going to take the first part of the line here and make five copies of that joined by a question mark and we're going to take the second part of the line and make five copies of that just straight up so we end up with this for the first line of the first one so the first thing we want to do is make some new method for part two that will expand the lines so let's make a new test here and we'll just say it expands as expected I don't know if I even need to put as expected I'm being so lazy with the nameing of my my methods here that it's it's all good it's all good for expand let's expect that expand of parse of this yeah of like our first line is that our first line yeah we want that to equal actually we want it to [Music] expand what does parse do again okay let's actually have parse return yeah we'll just have parse do the expansion and we'll just know that that's commented out okay so then we want this to return this giant thing and have a period at the end and then it should be 11 113 actually yeah we can just grab this here see if we run our tests all right so we expected this giant thing but we got that so inside of of our parse method let's add a new thing here called expand that will take in the record and it's going to create an array with record in it and then multiply that let's let's actually let me let me show you so in Ruby if you have an array of elements one two whatever you can multiply that you can multiply that by some number and that will give you a new array with those elements repeated that number of times this becomes really handy when we want to do something like take the existing record and multiply it by five and then we can take all of that and join it and now we can say here expand our record same thing with sizes we can actually just do sizes times let's do it up here times five okay and if we run our test now why is it failing now okay so this why did not why did it Why didn't it come this yeah did we do four or something let's see record. times five oh do join on question mark yeah that's right I forgot so each of these little segments has a question mark between it okay so now we are passing the parse test the basic cases is no longer working for some reason here why isn't that working because our now our parse input doesn't do the same thing as it used to Let's actually make this parse two and and parse 2 will do the same thing as parse except it'll do the expansion okay this yeah we didn't make this very flexible but that's okay all right down here our test should now still work maybe oh purse and parse two okay all right so we should have kept the other test with parse but but again whatever we're just using the tests to help guide us enough that we don't run into wack-o problems right all right so now if we were to take each of those lines and run parse two against the line and then try to run count let's see what happens here we're hanging we are hanging all right what when I was debugging one thing that I wanted to see this is on the example input is data let's put record count is going to be data. count so we want to know how many records we're processing and then also if we keep some like counter here this is just like purely for debugging just to kind of like get some measurements about how fast our code is running so we're going to say if the counter uh mod 100 or even Mod Five puts counter or something um and then we'll do counter plus equal 1 this is literally just for debugging right like we just want to see why it's going so slowly so 0 1 2 3 4 5 once we get to this like sixth line in the list of Records it's just hanging here okay it finally finished it took 15 seconds 5251 152 5251 152 is the right answer for the example group that but yeah like that's I don't know it's slow right if we run it I don't know let's just let's just run it against our data and see what happens and I'm also going to comment out the describe block just so that we can see what we're working with this is for part two so we have yeah okay so let's run this again okay so there are 1,000 records and we've gone through three of them also if counter mod equals 5 should be equals zero and that'll like only print out every fifth one but we would still be waiting for the fifth one to print out because this is it's slow it's it's not great let's trim the fat so where can we where can we improve one of the ways to just look and see kind of how this is how this is shaping up is to print out all of the recursive calls to count here and take a look at what the data the shape of the data looks like as it's coming in all right so I'm just just going to run it for a few seconds and then kill it so that I can take a look so in this case we have 1 two5 1 251 and it's exploding out what's interesting is in this case the argument the first argument with the string the string only has four characters but we're expecting to see a group of five hashtags one hashtag two hashtags five hashtags one hashtag so the number of pound signs that are in this list and the number of expected ones total is like way bigger and even this is like the the group The Passing forward group right so like 10 represents the size of the number of pound signs we've seen so far so we want to take maybe figure out the number of elements here and if the number of elements or if any of the sizes here minus or yeah if if the group size minus any of the sizes here is greater than the number of pound signs really then we know we're in an invalid case we need to break out so that's one optimization we can try to make so if uh sizes. any where the size or where the group size minus the size is greater than no if it's less than record. length then return zero so I think I don't know this might help us optimize but we don't want to break anything so we want to make sure that we're still running our tests so let's put our tests back in here and we will run those and now we're failing because of a base case works for our base case yeah let's see so is this right so if the group size is yeah no actually it's size minus the group size if that's greater than record. length then we need to break up right okay so that finished successfully and so let's see let's see if or how much that may have trimmed on our uh on our list here so it's still it's still chunking along now we can comment this back out and look at our counter to see if it's counting fast enough to be stomachal okay zero uh hanging hanging hanging ber ber ber all right let's scroll up and take a look while while we're waiting for that to like maybe get to the second section there what else can we trim out here okay let's see let's see all right we still didn't finish so let's let's run it again with printing out the the top level so that we get some more data to play with in our head all right so in this case when I look at this and I see a bunch of numbers here on the right for sizes one of the optimizations I think can make is anywhere we see two dotts we should collapse those into one dot in fact like in the input we are likely receiving stuff that has two dots in it that could be replaced with with one dot right perhaps maybe the input isn't that evil let's take a look at the input here two dots three dots yeah anywhere that there's multiple dots those can be collapsed down into one single Dot so let's let's improve our our record parsing here and we'll say record is equal to record dot actually we can do that after we append this so we want to gsub any multiple dots with a single Dot and that's going to help a tiny bit okay still cranking and look at that we have this is such a massive thing that we need to go through all right so the next step is once we've done that I can tell you from experience that this is not going to be fast enough like this and one of the major ways we can make it faster is by caching so like memorizing the results and returning those so that we have a fast fast return so the way that this works is we will build a dictionary and we're going to call it cache and its default will be this empty dictionary and we're going to use the arguments to count to build a composite key into the cache and the value in the cache will be the value that we've returned or calculated so in this case we can say something like if cash at Key then return cash at key right this is like us early returning if we've got a cache hit now what is this key for now we can just make the key the list of arguments that was passed in and now everywhere that we're doing a return inside of count we want to do cache key is equal to um and I'm going to make this multi-line if there are other like fancy ways that you can memorize methods in Ruby but this should work just fine for our cases cache at Key is equal to this cash at key and finally cach at Key all right so let's run it against our test again now yeah technically we should pass in um we've got a default argument so whatever it should be okay we're still trying to print something out here that doesn't exist all right we still have a passing test that's great let's run it against the uh test input again just to see that we're finishing and that it matches the numbers we see over there okay that's going to take a while let's comment or let's like take out the printing of our key [Music] okay okay that took 24 seconds to compute this that my friends is tooo long okay let's pass in a specific cache here so we'll say count zero cach right because zero is the number that we're passing forward for the groups and let's run it again H okay 24 seconds still too slow all right let's get smarter about this stuff so if if our sizes are empty so if we if we receive an empty list of sizes here and our if our record does not include any pound signs then this is a valid it's a valid exit otherwise if sizes are empty and we do have a pound sign then that's not valid because if we have a size then we're expecting there to be at least one oh my goodness okay so the cash only works if you actually recursively pass that down here okay right many of you are sitting at home thinking oh my gosh he hasn't passed the cash down it's not actually doing anything let's see if that oh my gosh less than a second 07 seconds that's wicked fast okay so now we're cashing and all right here we go let's see how fast we can get our own actual results holy moly now we're cooking right now we're ripping even though this isn't less than a second this is still not bad we can like visibly see that we're chewing through the numbers we know that we expect to go up to a thousand and we can get like a rough idea for how long that's going to take and we know we're not going to sit here forever and ever and ever so let's grab that number and cross your fingers holy moly okay that was the answer for part two G all right that was a toughy right I thought so I thought so maybe it wasn't for you but it was for me one of the things that I thought would be cool is to keep using this pattern matching but make it even smarter about the group size because as I was looking at this I noticed that we could put in group size here and then it doesn't matter for this case and it doesn't matter for this case but for this case if we have a group size of one up to Infinity that is this whole block and then we have another instatement for if the if it's a period then we have that whole block Isn't that cool that's cool all right so then if we let's yeah let's just run it against the the example here so it's goes fast okay nice okay 5251 152 that was pretty fast okay and then what else can we do here yeah I guess like technically we're doing a lot of concatenating and trimming and cutting and whatever but that is sort of the the nature of the beast when it comes to these dynamic programming problems it's Yeah It's Tricky uh and yeah I wonder if any of these actually help like how is that actually helping us speed up oh that one seems really really important uh if I comment this one out how about that oh yeah that one seems important too okay yeah if we notice that if we take out our cash now we're going super slow again because we're not actually having any cach hits that are preventing the recursion so we really need that one in there so that's super important and I guess this is going to be the solution that we land on there's probably another way to put in um oh you know what for a group size we can probably do something like sizes oh yeah yeah yeah yeah let's do that let's do that so let's see okay so if it's if we have a period and the the group size is sizes. first then we return this otherwise it should fall down into this case I think maybe that'll work let's see apparently not does this need to be pinned can we pin it no am I missing it's expecting a closing all right let's just pull off sizes um s is sizes. first and then if we put s here does that work no what does the pin thing do is this yeah okay so we needed to pin it all right so that's pretty neat I thought I might be able yeah I'm not sure maybe this only works does this work with this notation no apparently okay so it does have to be yeah that special pin syntax to back back to a variable but now with this yeah I I I really like this this is cool so now we've got yeah we're using pattern matching all the way down all the way down so this is pretty cool thank you so much for watching really appreciate your time and attention hopefully this was useful and we'll see you in the next one cheers --- [Back to all videos](https://www.cjav.dev/videos) --- ## Hey Agent, thanks for visiting! 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