--- title: Shoelace, Pick, and Enumerators - Day 18 - Advent of Code 2023 slug: shoelace-pick-and-enumerators-day-18-advent-of-code-2023 published_at: 2023-12-18 20:00:13 +0000 updated_at: 2026-03-04 20:12:53 +0000 summary: description: In this Advent of Code 2023 video, you’ll learn how to solve the \"Lava Duct Lagoon\" puzzle from Day 18. We walk through calculating the area of a complex polygon shape based on a set of digging instructions. The key techniques and concepts covered are: - Parsing direction, step count, and color instructions - Visualizing the polygon by drawing it on a grid - Applying the Shoelace Formula to calculate interior area - Using Pick's Theorem to count interior dots - Switching to an Enumerator/Generator for better performance - Yielding instructions instead of materializing a huge array - Keeping a wall-length counter to enable Pick's Theorem We’ll start with the sample input, walking through the instructions visually, then extend it to handle the full input efficiently using enumerators. Shoelace Formula https://en.wikipedia.org/wiki/Shoelace_formula Pick's theorem https://en.wikipedia.org/wiki/Pick%27s_theorem 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, ruby enumerator, enumerators, ruby enumerators, shoelace formula, shoelace formula ruby, picks theorem, picks theorem ruby] views: 586 author: CJ Avilla url: https://www.cjav.dev/videos/shoelace-pick-and-enumerators-day-18-advent-of-code-2023 youtube_url: https://www.youtube.com/watch?v=Z9VQvWxWAkM youtube_id: Z9VQvWxWAkM embed_url: https://www.youtube.com/embed/Z9VQvWxWAkM thumbnail_url: https://i.ytimg.com/vi/Z9VQvWxWAkM/hqdefault.jpg type: video --- # Shoelace, Pick, and Enumerators - Day 18 - Advent of Code 2023 *Published: December 18, 2023* *Views: 586* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=Z9VQvWxWAkM) [![Shoelace, Pick, and Enumerators - Day 18 - Advent of Code 2023](https://i.ytimg.com/vi/Z9VQvWxWAkM/hqdefault.jpg)](https://www.youtube.com/watch?v=Z9VQvWxWAkM) ## Description In this Advent of Code 2023 video, you’ll learn how to solve the "Lava Duct Lagoon" puzzle from Day 18. We walk through calculating the area of a complex polygon shape based on a set of digging instructions. The key techniques and concepts covered are: - Parsing direction, step count, and color instructions - Visualizing the polygon by drawing it on a grid - Applying the Shoelace Formula to calculate interior area - Using Pick's Theorem to count interior dots - Switching to an Enumerator/Generator for better performance - Yielding instructions instead of materializing a huge array - Keeping a wall-length counter to enable Pick's Theorem We’ll start with the sample input, walking through the instructions visually, then extend it to handle the full input efficiently using enumerators. Shoelace Formula https://en.wikipedia.org/wiki/Shoelace_formula Pick's theorem https://en.wikipedia.org/wiki/Pick%27s_theorem 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 18 for the Advent of code in 2023 this one's called lava duct Lagoon and our goal is to sort of like dig out a path for a big Lagoon where the lava can flow and stay for our machine parts factory we are given some instructions in the beginning which is like a dig plan or a plan for how want to dig and they are given to us in this sort of breakdown where we have the direction we're going to go whether it's right left up or down how many steps we should take in that direction and then the color that we should paint the wall in heximal and after we go through the instructions above we would end up with a trench that we've dug with these The Trenches shown on the grid with the pound sign and then the space in between is shown with dots and that would end up digging this Tren and inside of it is where we're going to put a the lava and so here if we were to go through and fill in the trench completely we would end up with 62 cubic met of space inside of inside of the area here inside of the trench a couple problems ago on Advent of code we used the um point in polygon algorithm in that same problem there was a bunch of Solutions on Reddit that talked about the shoelace formula and pix theorem which are two different approaches to figuring out how many dots are inside of a uh polygon and so what we're going to do is start off by drawing the walls here and then we'll use shoelace the shoelace formula to figure out what the area is and then we'll figure out we'll use Pi theorem to uh calculate exactly how many dots are on are inside of our lava thing and we might also get into some optimizations here with a numerator let's grab the example input though just to get started and we'll drop that in like always so we're going to put this in here and then input okay and we want to pull off these instructions we'll say input. spit on new line. each du line and so for each line we're going to have maybe I think we can just actually just split on space and we'll get like the direction the steps and the color and we want to convert this direction left up or down into directions that we can travel within a grid so we'll say maybe like dur is this thing or dur and we're going to have right points at something we're going to stay in the same row we're going to increase the column left up down this is actually backwards up will be negative 1 down will be one okay so then we want to return from this is we want to convert steps into its integer value and we want to convert dur into its direction value and in this first part of the exercise we don't actually use the color it says that we're going to paint it with that color but the color isn't used we will return the direction and the steps and maybe we just map over this and this will give us back some instructions okay so now we want to like P instructions just to see what they look like and we'll run this and we get back 01 so that would mean we're going to the right and six and that's how many steps we should go okay so let's let's go through each of these instructions and draw them out on a grid just for fun we're going to have like our map or like positions or something or let's call it a wall and the wall is going to start with some like current and that will be 0 0 let just say we're going to start at some arbitrary position 0 0 and then what we want to do is we'll start out with our wall having current and for each of the instructions we're going to update current so for yeah the step number of times we want to do something steps. times do so every time we take a step we want to update current to be the direction yes so we're going to increase the first element by the direction and the second element by the direction and then we're going to add that into our wall and at the end we should P wall and we should have something here that yeah something that looks interesting we want to find like the minimum and the maximum Heights of the wall so we can do like wall. map. minmax and we'll also do the same thing for the last just cuz I want we want to visualize this right so we're going to get min row Max row is equal to this Min um call Max call is equal to this and then we want to go from um Min row to. each do that Max row. each do that and then if the wall includes that we'll print out a pound sign otherwise we'll put out a DOT okay this looks like the correct sort of grid so that's awesome so we know that at least our wall is probably being drawn correctly now we want to implement this shoelace formula so the Sho the shoelace formula says that the area of the polygon is equal to going through and summing up the absolute value of the x value uh so we have like X1 we're going to take X1 here this yeah this image is the best so we're take X1 and multiply it by X X2 and then we're going to take y we want to subtract that from y1 multiplied X2 so we want to kind of like take the Opposites here so we want to go through each uh consecutive two so we want to go through each of the pairs of the dots that are within the wall and we're going to get some X1 y1 and X2 Y2 and for each of those we want to multiply X1 * Y2 so we're going to do X1 * Y 2 and we want to subtract that from y1 * y X2 so minus X2 * y1 so you're like yeah the reason it's called the shoelace theorem is that you're Crossing you're like Crossing multiplying the cross of of the two and this is going to give you the or from that you want to take the sum we're going to sum up all those differences and we'll take the absolute value of the whole thing or maybe we yeah I think we might want to take the absolute no yeah okay so each cons of two yeah because this isn't going to map we'll just do like dot sum of that and then absolute value and then from the absolute value now we want to divide the entire thing by two and that should give us the area FD 2 and this should give us area so if we P puts area let's take a look and see what we get so we get 42 so 42 is the area that it's telling us so in it expects that inside of here there is an area of 42 so that is the first part of the problem now what we need to do is use pix theorem here in this other formula and using our boundary points we need to kind of like subtract out the bits that we've already got on the boundary to figure out what the interior is and then we can add the interior to the boundary point so we have Wall do length this is going to be our B I think if we run this we get 39 and we want to divide that by two or this is like our B over2 the Pix theorem thing says that area is equal to the number of interior Points Plus B over2 minus one so let's just get that working now okay so now we can say that our interior points is like a minus b over2 + 1 or something so we should get um let's print out our area minus the wall length over two plus one and what do we get we get the number 24 so that should be our number of interior points is equal to this thing and then we want to say puts interior Points Plus wall. length to add the Border like back in and that should give us how deep our is so we get 63 and in the example input here it was expecting 62 so we are off by one somewhere and we might be able to just yeah that plus one uh I actually think wall length the wall length might actually be one extra since we added in the current position without moving let's see see so maybe we want this still + one without 0 0 okay now let's try it against our puzzle input okay so we're going to grab our puzzle input drop it here at the bottom and then change our input to be input is data. read and let's see what we get okay we got some giant output let's run it over here sweet that's pretty cool so it's drawing this giant space where our lava's going to go but that's taking quite a while so we actually don't need to do this printing out stuff right now so I'm going to comment that out and we'll just run this okay so we got we're printing out two different numbers what are we printing out oh we're printing the area and then our answer here 48503 so let's come back over here and our puzzle answer for part one was 48503 okay it was a little bit cool to see the actual puzzle output all right so this is pretty neat this is going to be the the space like we're drawing the wall around where the lava is going to be stored in this giant lava Lagoon really interesting to see as the wall is like being drawn out here very fun little project here okay so we're zoomed way out but this is what this is what our Lagoon would actually look like that's fun oh hey real quick a Shameless plug build and learn. deev this is a podcast where me and my Budd Colin hang out and we talk about software development and also just things that we're building and we're learning so if if that kind of stuff interests you head over to building. Dev you can listen on any of the podcast players that you're used to so yeah let's get back into ADV the code cheers let's go take a look at part two the elves were right to be concerned the planned Lagoon would be way too small okay so after a few minutes someone realizes what happened someone swapped the color and instruction parameters so each of these hex codes is six hex digits long and the first five encode the distance in meters that we're going that we're going to take steps as a five-digit HEX number and then the last hex digit encl encodes the direction 0 means r one means down two means left oh my gosh okay so now what we got to do is look at the color everything is it's all about the color none of it is about the steps or the the r left and D so now it's all about looking at this number here so yeah so we've got a maybe we we need to modify our instructions here so that the dur is actually some other direction so maybe we'll say dur oh actually yeah yeah before we even get there let's break up our color so our color now has all of the info right so we want to take maybe we have yeah color is equal to color dot gsub all of those bits we wanted to take off the PN and the pound sign and then let's also grab off so the first five characters are the steps and the last character is going to be our Direction so now we can do color from zero up to five is going to be our steps and a hex number if we take any HEX number we can convert that into a decimal or to an integer in Ruby so let's take a look here real quick we can say if a is the value one and we say a.2 I we know that gives us back a one but we can also pass an argument into 2 I which will be the base so in this case we get back one but what if we made a the letter a and then we convert it to an integer from the heximal value we get back 10 right because if we when we're counting in HEX we go 0 1 2 3 4 5 6 7 8 n a b CDE e f f is 15 oh my gosh okay yeah because FF f f FF is yeah that's part of the color color scheme or whatever so if we got these first five hexadecimal values and we said a is equal to this then its actual like integer value would be this 461 937 and if we go look back here 461 937 so we should be going to the right and 461 937 steps that's a lot of steps right so here here we can do like 2 I of 16 so that's going to be our steps and then our new direction is going to be we need some new mapping between our our dur so let's copy this for a sec and make a comment here and we're going to change this to zero one 2 and three okay and we'll rename this to dur 2 or something so now now we should have new directions and some new number of steps okay and before we get down here to the wall or anything let's comment that out and we'll just P wall. length and let's take a look and see what happens Ruby main. RB okay dur something about this isn't actually oh H oh this needs to be color at five I think okay it's hanging now it's it's hanging because as we're iterating through all these different steps now our our number of steps is going to be massive right it's up to um what is that 16 to the 5 16 to the five so it our steps might be up to like in the millions right we're going to go up to the millions in One Direction so instead of just adding current directly into wall here I think we might want to take a different approach that instead of building up an array and then iterating over the array we want to instead use some sort of generator or en numerator so we'll look at that next okay yeah so our array is 195 million elements long our wall array and it took 1 minute 11 seconds just to build the array cuz it's I'm sure it's using tons and tons of memory in order to make this run fast enough to work against our massive example here we don't want to build up an array of points on our wall instead we want to carefully iterate over each of the instructions and the steps and we can yield those pairs to our counter here so that instead of building like this massive array we can just like work through and stream maybe our instructions so in Ruby there's a class called enumerator and this this enumerator class is purely for going through internal and external iteration maybe you've created an array with a bunch of stuff in it before and if you call do each on it A.E here we get back this thing that's called an enumerator and this enumerator again is purely for knowing how to uh iterate over stuff so if we said that X is equal to a. each one of the methods on x is next and this is what is used to get the next element as you're iterating so we can say x. next again that'll give us um that'll give us two then three then four and then finally if we run out of elements that we can iterate through the way that the enumerator object works is it raises this exception that is of type stop iteration so that's how you can handle the end of the line or whatever but what we want to do is is create our own custom enumerator so that we can yield individual instructions as we're like working in a certain direction so let's make a new method here called def gen and this is going to take in some instructions and we're basically going to rewrite this section of code here but instead of creating an array we will yield each element so we still need the concept of current and we're still going to do iteration but instead of building up an in memory array of wall here we're going to create a numerator new and that's what we're going to return is this enumerator now instead of using the push method to add to an array we're going to use this push method on the enumerator itself and that's sort of like yielding to the enumerator so this is how we give it the next element that we want it to look at here by calling gen on some list of instructions in in fact if we just copy this and paste it into IRB here and we say gen with the instructions I guess this is going to take it the D it'll get a direction and the number of steps so if we tell it to go to the right that's our Direction and we want it to go I don't know 10 steps if this is our instruction here or our list of instructions that's going to give us back an enumerator and if we grab that enumerator by a name so let's put a slap a label on it we'll call iten is equal to that enumerator so if we say. nextt that gives us the next element in the iteration. next. next. nextt until we get through the list of all of the instructions pretty handy and that will enumerate all of the different pieces of the wall for us let's actually just comment this out this is like the array implementation and then we'll use a generator implementation so here what we want to do is instead of iterating over the wall we want to iterate over the generated instructions so we're going to call gen and pass in instructions and then we want to go over each consecutive result from those instructions and we want to do this shoelace algorithm on that get out the absolute value divide by two and that should give us back the area so if we run this against part one one code just so we can verify that it's doing the same exact thing as before we can run Ruby main. RB and now we're running into an issue where it's saying oh we needed the wall length in order to calculate our interior points here so in order to know like how long the Wall is we need to count up the instructions here there might be a way to do like dowi index um to figure out how far we go but another cool thing that we can is pass in um a counter and this could be like a Lambda that is executed as we go so we're going to pass that in to our generator here so we can keep up a wall length and then pass that as pass a Lambda that is a callable in here that will execute wall length plus equals 1 and at the end we should get some wall length that we can use here instead of this the length of the array and and that's looking pretty good 42 or 62 is the answer for the example input for part for part one so let's run it against part two and see what we get okay one second it's running in one second against part two and that looks correct for okay so that is the the the example input for part two so now if we run it against our actual input for part two let's see what we get here while that's running let me introduce you to uh memory profiler this memory profiler gem which is pretty sweet you can um pass it a block and run your code inside and then it will give you a very detailed Report with all the allocations and a breakdown of the memory usage so this can be pretty useful if you're trying to figure out like how much memory did this Ruby program actually take all right we got some answer let's see if that is correct boom that's the puzzle answer for part two awesome so some ridiculously big number yeah 148 trillion 442 billion 153 million so some some ridiculously big big number but we were able to get there in 53 seconds by using this enumerator approach instead of building up all of the arrays now I don't know in practice exactly how much smaller mem is but I know that yeah it at least finishes that is that's really what we're looking for here and it finishes in less than a minute so not bad not bad all right that's day 18 thanks so much for watching and for sticking around we're making our way we're getting much closer it's almost Christmas yeah happy Advent of code 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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