Nim

import strutils, times, sequtils, sugar

# check if level transition in record is safe
proc isSafe*(sign:bool, d:int): bool =
  sign == (d>0) and d.abs in 1..3;

#check if record is valid
proc validate*(record:seq[int]): bool =
  let sign = record[0] > record[1];
  return (0..record.len-2).allIt(isSafe(sign, record[it] - record[it+1]))

# check if record is valid as-is
# or if removing any item makes the record valid
proc validate2*(record:seq[int]): bool =
  return record.validate or (0..<record.len).anyIt(record.dup(delete(it)).validate)

proc solve*(input:string): array[2,int] =
  let lines = input.readFile.strip.splitLines;
  let records = lines.mapIt(it.splitWhitespace.map(parseInt));
  result[0] = records.countIt(it.validate);
  result[1] = records.countIt(it.validate2);

I got stuck on part 2 trying to check everything inside a single loop, which kept getting more ugly. So then I switched to just deleting one item at a time and re-checking the record.

Reworked it after first finding the solution to compress the code a bit, though the range iterators don’t really help with readability.

I did learn about the sugar import, which I used to make the sequence duplication more compact: record.dup(delete(it).

Haskell

This was quite fun! I got a bit distracted trying to rewrite safe in point-free style, but I think this version is the most readable. There’s probably a more monadic way of writing lessOne as well, but I can’t immediately see it.

safe xs = any gradual [diffs, negate <$> diffs]
  where
    diffs = zipWith (-) (drop 1 xs) xs
    gradual = all (`elem` [1 .. 3])

lessOne [] = []
lessOne (x : xs) = xs : map (x :) (lessOne xs)

main = do
  input :: [[Int]] <- map (map read . words) . lines <$> readFile "input02"
  print . length $ filter safe input
  print . length $ filter (any safe . lessOne) input

Uiua

Uiua is still developing very quickly, and this code uses the experimental tuples function, hence the initial directive.

Try it Live!

# Experimental!
$ 7 6 4 2 1
$ 1 2 7 8 9
$ 9 7 6 2 1
$ 1 3 2 4 5
$ 8 6 4 4 1
$ 1 3 6 7 9
⊜(⊜⋕⊸≠@\s)⊸≠@\n # Partition at \n, then at space, parse ints.

IsSorted ← +⊃(≍⇌⍆.|≍⍆.)        # Compare with sorted array.
IsSmall  ← /××⊃(>0|<4)⌵↘¯1-↻1. # Copy offset by 1, check diffs.
IsSafe   ← ×⊃IsSmall IsSorted  # Safe if Small steps and Ordered.
IsSafer  ← ±/+≡IsSafe ⧅<-1⧻.   # Choose 4 from 5, check again.

&p/+≡IsSafe .            # Part1 : Is each row safe?
&p/+≡(±+⊃IsSafe IsSafer) # Part2 : Is it safe or safer?

#Rust

initially, for part two I was trying to ignore a bad pair not a bad value - read the question!

Only installed Rust on Sunday, day 1 was a mess, today was more controlled. Need to look at some of the rust solutions for std library methods I don’t know about.

very focussed on getting it to actually compile/work over making it short or nice!

pub fn task_1(input: &str) -> i32{
    let mut valid_count = 0;

    let reports = process_input(input);

    for report in reports{
        let valid = is_report_valid(report);

        if valid{
            valid_count += 1;
        }
    }

    println!("Valid count: {}", valid_count);
    valid_count
}

pub fn task_2(input: &str) -> i32{
    let mut valid_count = 0;

    let reports = process_input(input);

    for report in reports{
        let mut valid = is_report_valid(report.clone());

        if !valid
        {
            for position_to_delete in 0..report.len()
            {
                let mut updated_report = report.clone();
                updated_report.remove(position_to_delete);
                valid = is_report_valid(updated_report);

                if valid { break; }
            }
        }

        if valid{
            valid_count += 1;
        }
    }

    println!("Valid count: {}", valid_count);
    valid_count
}

fn is_report_valid(report:Vec<i32>) -> bool{
    let mut increasing = false;
    let mut decreasing = false;
    let mut valid = true;

    for position in 1..report.len(){
        if report[position-1] > report[position]
        {
            decreasing = true;
        }
        else if report[position-1] < report[position]
        {
            increasing = true;
        }
        else
        {
            valid = false;
            break;
        }

        if (report[position-1] - report[position]).abs() > 3
        {
            valid = false;
            break;
        }

        if increasing && decreasing
        {
            valid = false;
            break;
        }
    }

    return valid;
}

pub fn process_input(input: &str) -> Vec<Vec<i32>>{
    let mut reports: Vec<Vec<i32>> = Vec::new();
    for report_string in input.split("\n"){
        let mut report: Vec<i32> = Vec::new();
        for value in report_string.split_whitespace() {
            report.push(value.parse::<i32>().unwrap());
        }
        reports.push(report);
    }

    return reports;
}

JavaScript

Also wrote a solution in JavaScript to play around with list comprehension. Wrote some utility functions for expressiveness (and lazy evaluation).

const fs = require("fs");
const U = require("./util");

const isSafe = xs =>
    U.pairwise(xs).every(([a,b]) => a!==b && a-b > -4 && a-b < 4) &&
    new Set(U.pairwise(xs).map(([a,b]) => a < b)).size === 1;

const rows = fs
    .readFileSync(process.argv[2] || process.stdin.fd, "utf8")
    .split("\n")
    .filter(x => x != "")
    .map(x => x.split(/ +/).map(Number));

const p1 = U.countBy(rows, isSafe);
const p2 = U.countBy(rows, row =>
    isSafe(row) || U.someBy(U.indices(row),
        i => isSafe([...row.slice(0, i), ...row.slice(i+1)])));

console.log("02:", p1, p2);

https://github.com/sjmulder/aoc/blob/master/2024/js/day02.js

Haskell

Had some fun with arrows.

import Control.Arrow
import Control.Monad

main = getContents >>= print . (part1 &&& part2) . fmap (fmap read . words) . lines

part1 = length . filter isSafe
part2 = length . filter (any isSafe . removeOne)

isSafe = ap (zipWith (-)) tail >>> (all (between 1 3) &&& all (between (-3) (-1))) >>> uncurry (||)
 where
  between a b = (a <=) &&& (<= b) >>> uncurry (&&)

removeOne [] = []
removeOne (x : xs) = xs : fmap (x :) (removeOne xs)

This is my very naive rust solution, part 2 is mostly just an extra function, so they’re bother covered in this one.

AoC day 2

G’MIC solution

it day2
crop. 0,0,0,{h#-1-2}
split. -,{_'\n'}
foreach { replace_str. " ",";" ({t}) rm.. }

safe_0,safe_1=0
foreach {
	({h}) a[-2,-1] y
	num_of_attempts:=da_size(#-1)+1
	store temp

	repeat $num_of_attempts {

		$temp

		if $> eval da_remove(#-1,$>-1) fi

		eval "
			safe=1;
			i[#-1,1]>i[#-1,0]?(
				for(p=1,p<da_size(#-1),++p,
					if(!inrange(i[#-1,p]-i[#-1,p-1],1,3,1,1),safe=0;break(););
				);
			):(
				for(p=1,p<da_size(#-1),++p,
					if(!inrange(i[#-1,p-1]-i[#-1,p],1,3,1,1),safe=0;break(););
				);
			);
			safe;"

		rm

		if $>
			if ${} safe_1+=1 break fi
		else
			if ${} safe_0,safe_1+=1 break fi
		fi

	}

}

echo Day" "2:" "${safe_0}" :: "${safe_1}

Rust

Turned out alright, I am looking forward to seeing what 2d coordinate grid code I can cannibalize from last year’s solutions 😄

Github link

Lisp

(defun p1-process-line (line)
  (mapcar #'parse-integer (str:words line)))

(defun line-direction-p (line)
  "make sure the line always goes in the same direction"
  (loop for x in line
        for y in (cdr line)
        count (> x y) into dec
        count (< x y) into inc
        when (and (> dec 0 ) (> inc 0)) return nil
        when (= x y) return nil
        finally (return t)))

(defun line-in-range-p (line)
  "makes sure the delta is within 3"
  (loop for x in line
        for y in (cdr line)
        for delta = (abs (- x y))
        when (or (> delta 3) )
          return nil 
        finally (return t)))

(defun test-line-p (line)
  (and (line-in-range-p line) (line-direction-p line)))

(defun run-p1 (file) 
  (let ((data (read-file file #'p1-process-line)))
    (apply #'+ (mapcar (lambda (line) (if (test-line-p line) 1 0)) data))))

(defun test-line-p2 (line)
  (or (test-line-p (cdr line))
      (test-line-p (cdr (reverse line)))
  (loop for back on line
        collect (car back) into front
        when (test-line-p (concatenate 'list front (cddr back)))
          return t
        finally (return nil)
  )))

(defun run-p2 (file) 
  (let ((data (read-file file #'p1-process-line)))
    (loop for line in data
          count (test-line-p2 line))))

def is_safe(report: list[int]) -> bool:
    global removed
    acceptable_range = [_ for _ in range(-3,4) if _ != 0]
    diffs = []
    if any([report.count(x) > 2 for x in report]):
        return False
    for i, num in enumerate(report[:-1]):
        cur = num
        next = report[i+1]
        difference = cur - next
        diffs.append(difference)
        if difference not in acceptable_range:
            return False
        if len(diffs) > 1:
            if diffs[-1] * diffs[-2] <= 0:
                return False
    return True

with open('input') as reports:
    list_of_reports = reports.readlines()[:-1]


count = 0

failed_first_pass = []
failed_twice = []

for reportsub in list_of_reports:
    levels = [int(l) for l in reportsub.split()]
    original = levels.copy()
    if is_safe(levels):
        safe = True
        count += 1
    else:
        failed_first_pass.append(levels)

for report in failed_first_pass:
    print(report)
    working_copy = report.copy()
    for i in range(len(report)):
        safe = False
        working_copy.pop(i)
        print("checking", working_copy)
        if is_safe(working_copy):
            count += 1
            safe = True
            break
        else:
            working_copy = report.copy()

print(count)

Rust

The function is_sorted_by on Iterators turned out helpful for compactly finding if a report is safe. In part 2 I simply tried the same with each element removed, since all reports are very short.

fn parse(input: String) -> Vec<Vec<i32>> {
    input.lines()
        .map(|l| l.split_whitespace().map(|w| w.parse().unwrap()).collect())
        .collect()
}

fn is_safe(report: impl DoubleEndedIterator<Item=i32> + Clone) -> bool {
    let safety = |a: &i32, b: &i32| (1..=3).contains(&(b - a));
    report.clone().is_sorted_by(safety) || report.rev().is_sorted_by(safety)
}

fn part1(input: String) {
    let reports = parse(input);
    let safe = reports.iter().filter(|r| is_safe(r.iter().copied())).count();
    println!("{safe}");
}

fn is_safe2(report: &[i32]) -> bool {
    (0..report.len()).any(|i| {  // Try with each element removed
        is_safe(report.iter().enumerate().filter(|(j, _)| *j != i).map(|(_, n)| *n))
    })
}

fn part2(input: String) {
    let reports = parse(input);
    let safe = reports.iter().filter(|r| is_safe2(r)).count();
    println!("{safe}");
}

util::aoc_main!();

C

First went through the input in one pass, number by number, but unfortunately that wouldn’t fly for part 2.

#include "common.h"

static int
issafe(int *lvs, int n, int skip)
{
	int safe=1, asc=0,prev=0, ns=0,i;

	for (i=0; safe && i<n; i++) {
		if (i == skip)
			{ ns = 1; continue; }
		if (i-ns > 0)
			safe = safe && lvs[i] != prev &&
			    lvs[i] > prev-4 && lvs[i] < prev+4;
		if (i-ns == 1)
			asc = lvs[i] > prev;
		if (i-ns > 1)
			safe = safe && (lvs[i] > prev) == asc;

		prev = lvs[i];
	}

	return safe;
}

int
main(int argc, const char **argv)
{
	char buf[64], *rest, *tok;
	int p1=0,p2=0, lvs[16],n=0, i;

	if (argc > 1)
		DISCARD(freopen(argv[1], "r", stdin));

	while ((rest = fgets(buf, sizeof(buf), stdin))) {
		for (n=0; (tok = strsep(&rest, " ")); n++) {
			assert(n < (int)LEN(lvs));
			lvs[n] = (int)strtol(tok, NULL, 10);
		}

		for (i=-1; i<n; i++)
			if (issafe(lvs, n, i))
				{ p1 += i == -1; p2++; break; }
	}

	printf("02: %d %d\n", p1, p2);
}

https://github.com/sjmulder/aoc/blob/master/2024/c/day02.c

Haskell

runningDifference :: [Int] -> [Int]
runningDifference (a:[]) = []
runningDifference (a:b:cs) = a - b : (runningDifference (b:cs))

isSafe :: [Int] -> Bool
isSafe ds = (all (> 0) ds || all (< 0) ds) && (all (flip elem [1, 2, 3] . abs) ds) 

isSafe2 :: [Int] -> Bool
isSafe2 ds = any (isSafe2') (zip [0..length ds] (cycle [ds]))

isSafe2' (i, ls) = isSafe . runningDifference $ list
        where
                list = dropIndex i ls

dropIndex _ []     = []
dropIndex 0 (a:as) = dropIndex (-1) as
dropIndex i (a:as) = a : dropIndex (i - 1) as

main = do
        c <- getContents
        let reports = init . lines $ c
        let levels  = map (map read . words) reports :: [[Int]]
        let differences = map runningDifference levels
        let safety = map isSafe differences
        let safety2 = map isSafe2 levels

        putStrLn . show . length . filter (id) $ safety
        putStrLn . show . length . filter (id) $ safety2

        return ()

Took me way too long to figure out that I didn’t have to drop one of them differences but the initial Number

Of course I ended up with a off-by-one error for the second part, so things took a bit longer than they really should’ve.

But either way, behold, messy C#:

int[][] reports = new int[0][];

public void Input(IEnumerable<string> lines)
{
  reports = lines.Select(l => l.Split(' ').Select(p => int.Parse(p)).ToArray()).ToArray();
}

public void Part1()
{
  int safeCount = reports.Where(report => CheckReport(report)).Count();
  Console.WriteLine($"Safe: {safeCount}");
}
public void Part2()
{
  int safeCount = reports.Where(report => {
    if (CheckReport(report))
      return true;

    for (int i = 0; i < report.Length; ++i)
      if (CheckReport(report.Where((_, j) => j != i)))
        return true;

    return false;
  }).Count();

  Console.WriteLine($"Safe: {safeCount}");
}

bool CheckReport(IEnumerable<int> report)
{
  var diffs = report.SkipLast(1).Zip(report.Skip(1)).Select(v => v.Second - v.First);
  return diffs.All(v => Math.Abs(v) <= 3) && (diffs.All(v => v > 0) || diffs.All(v => v < 0));
}