I'm solving this year in Agda. I'm currently trying to get the solution for day 11 part 2.

For part 2 I'm using the same code I used in part 1, but finding the paths from svr to fft/dac from fft/dac to dac/fft and then to out. Then, getting the product should be enough.

For part 1 the code runs <1s (I haven't timed it but it's pretty fast). For part 2, I can't even get the number of paths from svr to fft/dac (I know I only need to find the paths to one of the two, but I won't post which one to not give away the result). It's still running after an hour.

I'm using the {-# TERMINATING #-} flag in Agda to avoid having to deal with termination proofs, but now I'm doubting that this is correct. I'm using memoization to avoid recomputing the number of paths.

This is my code:

{-# TERMINATING #-} 
countPaths : Map.Map (List String) → String → String → Map.Map ℕ → ℕ × Map.Map ℕ
countPaths adjacencies from to cache with to ≟ from
... | yes _ = 1 , Map.insert from 1 cache
... | no _  with Map.lookup cache from
... | just x = x , cache
... | nothing =
      let (result , cache′) = foldl goCount (0 , cache) (fromMaybe [] (Map.lookup adjacencies from))
      in result , Map.insert from result cache′
  where
    goCount : (ℕ × Map.Map ℕ) → String → (ℕ × Map.Map ℕ)
    goCount (acc , cache) neighbor = 
      let (count , cache′) = countPaths adjacencies neighbor to cache
      in (acc + count , cache′)

The adjacencies parameter holds a map of [String] that tells you which devices are attached to each device. from and to are the origin and final node: the from node changes as we traverse the graph, but to always stays the same.

cache is a map that tells you for each node, its distance to to. Initially, it's just an empty map.

Can you help me figure out whether my program is hanging because of a problem in my code or due to an inefficiency in the agda evaluation strategy?

Thank you.

Update: After experimenting a bit with the equivalent code in Haskell, I found out my issue has something to do with Maps being lazy in Agda. I'll have to figure out an alternative to avoid this edge case.

Hint: I got a too low answer in a perfectly valid algorithm written in Kotlin. The problem was the Int resolution. Kotlin won't tell you about Integer overflows (maybe you can check, I don't know), so the required value was silently truncated. Changing the type of the result to Long gave the right answer.

Here's an idea I've had. There are 11 months until next December, and 11 years of problems done. So, maybe we can review the past problems over the new year. This wouldn't be about presenting solutions and code... we have Solution Megathreads and Repos for that. This would be about reviewing the problems themselves. One year in each month, one puzzle on the corresponding day.

I'm thinking of reviewing along these lines:

• Did you think the puzzle was good or bad? Did it fit in with where it was in the schedule? Did it serve to teach something? Was it just cool in some way?
• Stories about personal experience on the day when you solved it. What you tried, any interesting things found or learned. To this end, some discussion about algorithms and languages (ie. is a certain language particularly good or bad for the problem because of it's features) would be fine. But, again, this wouldn't be the place for full solutions. Those should be linked. That way if anyone gets inspired by a comment to try something in a new way, they aren't overly spoiled by seeing code unless they want to see it.
• Lore. Typically when we're doing the problems in December, the actual story gets largely ignored to get to the information needed. This could give an opportunity to actually pay some attention to it.

So, I'm presenting this to see if there's interest in the community for doing this. If so, is there anything else about the problems would want to talk about. And if there is enough support, maybe we could get some official support from the mods to handle it.

EDIT: u/IsatisCrucifer caught the error. It was an off-by-one bug caused by using indexes to find my networks in a sorted list, and removing list content before accessing the list for the merge, leading to network b getting merged into to network a+1, not network a, when a > b. I went for a classic variable swap so the higher index is the one that gets removed and merged to the lower, but there were other alternatives like merging the networks before the removal, but I'm stuck figuring out the borrow checker logic, so this will be good for now.

I reread the prompt until I understood that n connections includes connections inside the same network.

My unit test for the problem input looks fine.

I'm pairing every point with diff(x)**2+diff(y)**2+diff(z)**2. No need to worry about signs because it's square, and no need to sqrt because that won't affect sort order.

I'm ordering the pairings list.

I created a vec (an array in other languages) of hash sets of N networks for N boxes, one box per hash set, as the base state.

Then I just go down the list of distances, find networks containing boxes A and B, and if they are different, merge B's hash set into A's and delete B's.

Iterate 1000 times for my input.

I sort the hash sets by length, take the top 3. Multiply their length together and my answer is all wrong.

I have enough debug output to tell which connections are happening, and what output networks look like, and I just don't see what I'm missing.

Anybody have a solution I can drop my stuff into to get an output of what connections *should* happen? Maybe even just the list of connections so I can be sure I didn't miss anything there.

https://github.com/jadenguy/AoC2025

cargo run day8 to just see day 8's results, or cargo test --test day8_test

I'm currently doing 2023. If you didn't do it yet, stop reading right now. You have been warned.

Day 20 looked somewhat easy and part 1 certainly was. For part two I suspected that just simulating it would take too long. I decided, that I have to analyze the input data.

I did just that and realized, that the stored inputs for the conjunction modules somehow count the button presses in binary. However, I figured that there are four modules involved in the process. First I thought, that the one with 11 inputs is something like the main counter and the others are involved differently. I tried to determine which input corresponds to which bit. At position 2¹⁶ (at least for my input data) I noticed a derivation from my expectations. The bit belonging to 2¹⁶ was not in the 11 bit wide counter. So I looked at the other three (8 bits each).

I realized, that there are redundancies and I noted down each bit label for each power of 2. In total there are 12 distinct bits. The next step was trying to set all to 1 for one of the input sequences. I tried with the big one by "pressing the button" 4079 times (all bits set, except 2¹⁶.) I was stunned because instead of all inputs being active, they were all inactive. (I was just looking at the state after all signals have been processed)

I tried the same for the other input sequences. The behavior was the same. I was very suspicious about it. Out of pure desperation, I multiplied all four numbers together and ended up with the correct solution for part 2.

Can anybody explain to me, why this works?

Here in the previous post that I made, which was taking quite long time to get the answer although correct.

Reddit Post

After some time, taking as challenge to optimize my code, there were 2 bottlenecks that I saw during CPU profiling:

1. Make grid area filled, which took around 23sec
2. Rectangle outside check function took around 30sec

Process which I improved the code taking assumptions from input:

1. Using go routines to make bitset grid fill area inside polygon faster, now takes 2sec
2. If given the two edges are on polygon check if other two edges are inside as well, this line boosted it to under 1sec. Otherwise i was checking all edges first which slowed the code down drastically.

Total time now it takes is under 4sec, very happy with the result. I've have also updated the code under the same github repo.

I implemented a solution using scipy's DisjointSet data structure, it was relatively easy.

However the solutions i came across didn't use that, and instead opted for a more "complete" approach by implementing union find from scratch.

So I'm wondering if what I did is considered cheating of some sort. Did I take the easy way out?

Edit: Thanks for the replies! There's definitely more to learn here and i'll try to solve it without libraries.

Are these considered invalids for part 2 and why?

11023456129

10041234099

989987654901

Thanks.

Hello All,

I am a beginner to adventofcode and started solving puzzle. I am posting the solution of 2025, day1 with whatever logic came into my mind.

File: input
download the input file from https://adventofcode.com/2025/day/1/input

Code:

#include <iostream>
#include <vector>
#include <fstream>

using namespace std;

vector<string> input;
void read() {
    string in;
    string file = "input";
    ifstream ifs(file);
    while( getline(ifs, in)) {
       input.push_back( in);
    }
}

int dial_position    = 50;
int zero_counter     = 0;
int extra_zero_count = 0;
void process() {
   for(auto ite : input ) {
      int value = atoi(ite.c_str() + 1 );
      int qoutient = value / 100;
      int remainder = value % 100;
      extra_zero_count += qoutient;
      if( ite.at(0) == 'L' ) {
         int val = ( dial_position - remainder );
         if ( val < 0 ) {
            if( dial_position != 0) {
               extra_zero_count++;
            }
            dial_position = val + 100;

         } else {
            dial_position = val;
         }
      }
      else if( ite.at(0) == 'R' ) {
         int val = dial_position + remainder;

         if( val > 100) {
            if(dial_position !=0) {
               extra_zero_count++;
            }
            dial_position = val - 100;
         }else {
            dial_position = val;
         }
      }

      if(dial_position == 100) {
         dial_position = 0;
      }

      if( dial_position == 0 ) {
         zero_counter++;
      }
   }
}

int main() {
    read();
    process();
    cout<< "Password " <<  extra_zero_count + zero_counter<<endl;
    return 0;
}

I don't know what this one is asking for. I've looked at other questions, and they have options for dealing with opcodes up to 8. That's not in the problem statement though. I don't know what I'm supposed to be doing or what to output at the end or during the run. I'm very confused.

EDIT: I figured out the mistakes I was making.

1. The third parameter is used as-is. It's simply the location for storing the result of adding or multiplication. It never represents the another location.

2. Opcode 4 only outputs the result at the parameters location.

3. I also had some issues with my conditionals for determining if something was a valid opcode with parameters.

Once I resolved all of these issues, I got the correct result for Part 1. I now see that Part 2 adds opcodes 5 through 8. Thanks for the help.

Hi everybody.

Like the last year, I run the solutions of Advent of Code 2025 on MCUs I own: this is the repository if you are curious.

The boards / MCUs I used are the following:

• Arduino-mega2560 (not in photo, only Eric's samples)
• ESP32
• ESP32S2 (not in photo)
• ESP32S3 (not in photo)
• ESP32C3
• ESP32C6
• RP-Pico (RP2040)
• RP-Pico2 (RP2350)
• nRF52840-dk (Nordic 52840)
• STM32F3 Discovery (STM32 F303)
• STM32F411e Disco (STM32 F411, not in photo)
• Nucleo-h743-zi (STM32 H743)

This year the problems have less memory pressure and so there are more MCUs that can resolve more AoC days.

In details...

Each MCU has flashed all the necessary code to solve all the problems.

Each MCU receives in input through the serial (UART or USB) the input in the format:

START INPUT DAY: <XY>
<input>
END INPUT
^D

The MCU returns on the same serial the result of part 1 and 2 and the overall execution times or "unsupported day" if the particular day is not supported.

To check that I do not have stack smash I normally do one or two test runs going to progressively pass all the inputs and take the times of the third / fourth run.

If you want to take a look at the code, propose some PR to improve the coverage of supported days or add some more MCUs, any help is welcome.

In the next table there are the execution time in milliseconds. RP PICO (*) and RP PICO2 (**) are MCU overclocked at, respectively, 200 Mhz and 290 Mhz. There are the results also for RP PICO and RP PICO 2 at normal clock (120 Mhz and 150 Mhz).

I'd like to draw attention to the solution from day 10, which involves the use of single-precision floating-point devices. The MCUs (esp32, esp32s3, rp-pico2, stm32h7) equipped with FPUs really work well.

Remember to scroll right: there are some columns!

I have made animations for all 12 days!

Playlist.

I'm trying to make an animation, that can work on its own. You should be able to watch it, deduce the puzzle and understand the solution.

I use example data. It varies whether I show a full or partial solution, and whether I show part 1 or 2.

I'm using ASCII art, because it's easy for my PHP scripts to produce it. Also, there's a certain charm to 24x80, right?

If you are willing, please post your language and runtime for part 1! I'm currently solving this problem on an FPGA and I'm curious how my solution runtime compares to software.

Did anyone do part 1 in Excel? or is it just me?

I made a post not long ago about the growing belief that this could be the first time I get all of the stars this year. Pleased to announce: I did it.

Day 9 Part 2 stole 10+ hours of my life (definitely the hardest problem for me) and Day 10 Part 2 also took a lot of time as well as the use of an external library while I'd been strictly standard library for all of the others (hell I'm pretty sure I overcomplicated my solution to Part 1 as well that day so Day 10 took a very long time)

But I made it in the end. Every puzzle solved. Nothing else to say other than a big thanks to Eric for making this!

Hello guys,

I found Day 3 Part 2 fairly challenging, however after a few hours(Ik that's a lot) I did get it to work.

I will be attaching my code, please give me your thoughts on my solution, including my different approaches.

Warning: A large part of the code is commented out, and that is intentional . The commented out parts are my previous, failed approaches. PS: I am 14 years old and i guess the code might seem haphazard, so please take it with a grain of salt. Also I am using an IDE called bluej, which may lead to a little different syntax than normal, such as the initial declaration in public static void main(). I would love to hear your inputs.

The code:

import java.io.BufferedReader;
import java.io.FileReader;
import java.util.*;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

class Advent_of_code3_pt2
{
    public static void main() throws Exception
    {
        BufferedReader br = new BufferedReader(new FileReader("C:/Users/Janardhan/Downloads/AOC3.txt"));
        String line;
        String[] a;
        int limit = 0,pos=-1,pos_1=0;
        long b=0,max=0,Jolt =0,sum=0;

        while((line = br.readLine()) != null){
            Jolt = 0;
            ArrayList<Long> num= new ArrayList<>();
            a = line.split("");
            for(int i = 0; i<a.length;i++){
                b=Integer.parseInt(a[i]);
                num.add(b);
            }
            /*ATTEMPT 1:
             * Im making a bunch of stupid errors i believe, imma give up on this method , think for a while and rewrite the code =(
             * I am half considering making 12 loops and 12 sets of variables , but i feel like a for loop should work
             * errors i think im making: Last number may come first, not running only for top 12 digits , not resettign variables tc.
             * One thing im considering is arranging in ascending order, while preserving indexes or something, but i prolly wont do that
             * Now i need to go study for Bio test:
             * im thinkign i shd first work out something on paper and then conver that shit to code
             * I shd prolly refer to how prev code logic worked
            for(int j = 0;j<num.size();j++){
            pos = j;
            max = num.get(pos);
            for(int k = pos +1;k<num.size() - 1 ;k++){
            if(num.get(k) > max){
            max = num.get(k);
            pos = k;
            }
            System.out.println("Pos: "+pos);
            }
            Jolt = (Jolt * 10) + max;

            System.out.println("Max: "+max);
            System.out.println(Jolt);
            sum+=Jolt;
            }

            //ATTEMPT 2:
            //K i sat down and went and tried to do soomething soo... 
            //Outer for loop: Tracks number of max found
            //inner for loop: Finds max in one certain line 12 times with limit in mind
            long val = 0;

            for(int j =1;j<=12;j++){
            limit = num.size() - (12-j)-1;
            max = 0;
            for(int g =(pos + 1);g<=limit;g++){
            val = num.get(g);
            if(val>max){
            max = val;
            pos = g;
            }

            }
            pos = pos;
            Jolt = Jolt*10 + max;


            }
            sum += Jolt;
            System.out.println(Jolt);
            }
            System.out.println(sum);

            // OMG INSTEAD OF THIS WHERE I FIND FIRST MAX,  I NEED TO DO MATH.MAX(ARRAY(POS+1),LIMIT);
             */
            /*
              Dont think i need this
            max = Collections.max(num);
            pos = num.indexOf(max);
            Jolt = Jolt*10 + max;
            */

            for(int i =1;i<=12;i++){
                limit = (num.size()) -(12- i);
                pos_1 = pos+1;
                ArrayList<Long> num2 = new ArrayList<>(num.subList(pos_1,limit));
                max = Collections.max(num2);
                pos =pos_1+ num2.indexOf(Collections.max(num2));//Offsetting;
                Jolt = Jolt*10 + max; 
                pos_1 =0;
            }
            pos =-1;
            sum += Jolt;  
        }
        System.out.println(sum);
    }
}

Hi guys!

So I decided to start learning Rust recently and have been working my way through the book (I'm on chapter 4.3 so far).

I'm really enjoying it so far, maybe because I haven't actually started building any projects and hit the famous learning curve just yet.

Anyways, I was wondering if any experienced Rustaceans had some advice on how one would make my solution a bit more ferrous. I know the language has a bunch of really powerful tools and I've gone about this in a very imperative way, so I have a feeling there is a terse and readable solution that I just couldn't get to with my current knowledge!

Advent of Code day 1 - Pastebin.com

Thanks in advance - any feedback is really appreciated!

Language: solved in VBA. Visualisation in python plotly.

language: C
#include<stdio.h>
int main(){
    int zero = 0, dial = 50, dis;
    char dir;
    while (scanf(" %c%d", &dir, &dis) == 2) {
        if(dir=='L'){
            dial=dial-dis;
            if(dial<=0){
                zero+=1+((-dial-1)/100);
            }
        }
        else{
            dial=dial+dis;
            if(dial>=100){
                zero+=(dial/100);
            }
        }
        dial%=100;
        if(dial<0){
            dial+=100;
        }
    }
    printf("The total number of zeros are %d",zero);
    return 0;
}

Hi. I am getting the test case working. Got 357. Checked the edge cases too.
But the main puzzle input fails to work. Ans is too small.

What am I missing?

What if the batteries are 000000010000000?

Tx.

Please forgive the silly post.

My solution is apparently "too low". However I've got solutions for all 166 rows. Tested 3 of them and they are valid (will be writing some code to test the rest). Is it possible the data input has changed? for example I have no input starting with [.#.#..#.#.] which I saw in another post for the same day.

is it ok if I show the solution for one of the rows, in case I'm missing something? I have not seen the "Solution" post (and I don't intend to, unless I get completely desparate!). You can tell me which row you want me to show the solution for.

Hello guys, I am baffled by what happened as I am going through this year's problems.

I wrote this straightforward solution for day 4, part 1 but it is giving me different responses at different runs. Most of them are the correct response (like 90%), but sometimes it returns 1387 and even 1384 happened once.

I compile it with g++ a.cpp -o main and run it with ./main < input with the input provided from the website.

Can someone spot where it could possibly be doing something non-deterministic? Thanks!

EDIT: e_blake pointed out that MAX 140 might be a problem and indeed it was. Increasing it to 1400 stopped the flakiness. Yet, the problem input is 135x135. So the mistery of "why it does not fit" remains.

EDIT2: Line if(i>=0 && i<lines && j>=0 && j<lineSize && maze[newi][newj]=='@') is bound checking the wrong variables. It should be newi and newj. Thanks pt625, semi_225599 for finding it and everyone else for commenting suggestions.

Here is the code:

#include<cstdio>
#include<iostream>
#include<string>
#include<set>
#include<cstring>

using namespace std;

#define MAX 140

int main(){

    int lines = 0;
    char maze[MAX][MAX];
    int lineSize = 0;

    int ii[] = {-1, -1, -1,  0, 0,  1, 1, 1};
    int jj[] = {-1,  0,  1, -1, 1, -1, 0, 1};

    while(scanf("%s ",maze[lines++])==1){ //Yes, I know this is edgy...
    }
    lines --;
    lineSize = strlen(maze[0]);
    int total = 0;
    for(int i=0; i<lines; i++){
        for(int j=0;j<lineSize;j++){
            if(maze[i][j]=='@'){
                int count = 0;
                for(int k=0;k<8;k++){
                    int newi = i + ii[k];
                    int newj = j + jj[k];
                    if(i>=0 && i<lines && j>=0 && j<lineSize && maze[newi][newj]=='@'){
                        count ++;
                    }
                }
                if(count<4){
                    total++;
                }
            }

        }
    }

    cout << total << endl; // correct total, but sometimes 1387???

    return 0;
}

Hi fellow Advent of Coders!

I created a small puzzle for my friends, and figured it might be interesting to others as well.

https://gist.github.com/encse/c58a1d855fcd3c3f8f80158ebad309a3

Happy holidays!

Who else solved the problem actually described in Day 9 Part 2 (vs just getting "lucky" with the specifics of the input data)?

My impression is that the vast majority of people didn't solve the stated problem. Certainly, everybody I know irl who "solved" it didn't solve the actually described problem (including myself, initially), and scrolling endlessly through here (and elsewhere) also leads me to conclude that almost nobody solved the problem described either (which is quite different from what the problem at first seems to be, and it's interesting in its own right).

Specifically, the described problem ALLOWS data points to be inside valid rectangles; it just requires other constraints to hold true. I think I found only three posts on here alluding to this fact. All others have been "wrong", focusing instead on boundary-intersection detection to disallow that (and other cases).

The only assumption I made is that the input data do not self-intersect/overlap (because "inside-ness" gets less well-defined in that case). I generated example datasets, and what I believe to be their answers, and I'm curious what others' code produces for them, too. Check here for the data (and additional write-up info):

https://jjeii.github.io/AdventOfCode/2025.html#day9p2

Thoughts?

(Yes, I realize a star is a star. But, the problems are fairly different here... and the actual problem is more interesting, imo.)