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

Uiua

Took me a bit longer to get this one but still quite simple overall.
Spent quite some time on getting to know the try and assert operators better.

Run with example input here

# Get the indices matching the ascending/
# descending criteria
CheckAsc ← ≡°□⍚(⍣(⊸⍤.≍⍆.)⍣(⊸⍤.≍⇌⍆.)0)
# Get the indices matching the distance criteria
CheckDist ← ≡°□⍚(⍣(⊸⍤.≠1∈:0)0×⊓≥≤1,3⌵⧈-)
Split     ← ⊙(▽≠1)▽,,

PartOne ← (
  &rs ∞ &fo "input-2.txt"
  ⊜(□⊜⋕≠@ .)≠@\n.
  CheckAsc.
  ▽
  CheckDist
  ⧻⊚
)

PartTwo ← (
  &rs ∞ &fo "input-2.txt"
  ⊜(□⊜⋕≠@ .)≠@\n.
  CheckAsc.
  Split
  CheckDist.
  Split
  ⊙(⊂)
  ⧻
  :
  ⍚(≡(▽:°⊟)⍜¤⊞⊟:≠1⊞=.⇡⧻.)
  ≡(⧻⊚CheckDist▽CheckAsc.°□)
  +⧻◴⊚
)

&p "Day 2:"
&pf "Part 1: "
&p PartOne
&pf "Part 2: "
&p PartTwo

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

TypeScript

import { AdventOfCodeSolutionFunction } from "./solutions";


/**
 * this function evaluates the 
 * @param levels a list to check
 * @returns -1 if there is no errors, or the index of where there's an unsafe event
 */
export function EvaluateLineSafe(levels: Array<number>) {
    // this loop is the checking every number in the line
    let isIncreasing: boolean | null = null;
    for (let levelIndex = 1; levelIndex < levels.length; levelIndex++) {
        const prevLevel = levels[levelIndex - 1]; // previous
        const level = levels[levelIndex]; // current
        const diff = level - prevLevel; // difference
        const absDiff = Math.abs(diff); // absolute difference

        // check if increasing too much or not at all
        if (absDiff == 0 || absDiff > 3)
            return levelIndex; // go to the next report

        // set increasing if needed
        if (isIncreasing === null) {
            isIncreasing = diff > 0;
            continue; // compare the next numbers
        }

        //  check if increasing then decreasing 
        if (!(isIncreasing && diff > 0 || !isIncreasing && diff < 0))
            return levelIndex; // go to the next report
    }

    return -1;
}


export const solution_2: AdventOfCodeSolutionFunction = (input) => {
    const reports = input.split("\n");

    let safe = 0;
    let safe_damp = 0;

    // this loop is for every line
    main: for (let i = 0; i < reports.length; i++) {
        const report = reports[i].trim();
        if (!report)
            continue; // report is empty

        const levels = report.split(" ").map((v) => Number(v));

        const evaluation = EvaluateLineSafe(levels);
        if(evaluation == -1) {
            safe++;
            continue;
        }
        
        // search around where it failed
        for (let offset = evaluation - 2; offset <= evaluation + 2; offset++) {
            // delete an evaluation in accordance to the offset
            let newLevels = [...levels];
            newLevels.splice(offset, 1);
            const newEval = EvaluateLineSafe(newLevels);
            if(newEval == -1) {
                safe_damp++;
                continue main;
            }
        }
    }

    return `Part 1: ${safe} Part 2: ${safe + safe_damp}`;
}

God, I really wish my solutions weren’t so convoluted. Also, this is an O(N^3) solution…

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!();

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\n1 2 7 8 9\n9 7 6 2 1\n1 3 2 4 5\n8 6 4 4 1\n1 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

use crate::utils::read_lines;

pub fn solution1() {
    let reports = get_reports();
    let safe_reports = reports
        .filter(|report| report.windows(3).all(window_is_valid))
        .count();

    println!("Number of safe reports = {safe_reports}");
}

pub fn solution2() {
    let reports = get_reports();
    let safe_reports = reports
        .filter(|report| {
            (0..report.len()).any(|i| {
                [&report[0..i], &report[i + 1..]]
                    .concat()
                    .windows(3)
                    .all(window_is_valid)
            })
        })
        .count();

    println!("Number of safe reports = {safe_reports}");
}

fn window_is_valid(window: &[usize]) -> bool {
    matches!(window[0].abs_diff(window[1]), 1..=3)
        && matches!(window[1].abs_diff(window[2]), 1..=3)
        && ((window[0] > window[1] && window[1] > window[2])
            || (window[0] < window[1] && window[1] < window[2]))
}

fn get_reports() -> impl Iterator<Item = Vec<usize>> {
    read_lines("src/day2/input.txt").map(|line| {
        line.split_ascii_whitespace()
            .map(|level| {
                level
                    .parse()
                    .expect("Reactor level is always valid integer")
            })
            .collect()
    })
}

Definitely trickier than yesterday’s. I feel like the windows solution isn’t the best, but it was what came to mind and ended up working for me.