Factor

: get-input ( -- reports )
  "vocab:aoc-2024/02/input.txt" utf8 file-lines
  [ split-words [ string>number ] map ] map ;

: slanted? ( report -- ? )
  { [ [ > ] monotonic? ] [ [ < ] monotonic? ] } || ;

: gradual? ( report -- ? )
  [ - abs 1 3 between? ] monotonic? ;

: safe? ( report -- ? )
  { [ slanted? ] [ gradual? ] } && ;

: part1 ( -- n )
  get-input [ safe? ] count ;

: fuzzy-reports ( report -- reports )
  dup length <iota> [ remove-nth-of ] with map ;

: tolerable? ( report -- ? )
  { [ safe? ] [ fuzzy-reports [ safe? ] any? ] } || ;

: part2 ( -- n )
  get-input [ tolerable? ] count ;

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

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

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))))

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

J

There is probably a way to write this more point-free. You can definitely see here the friction involved in the way J wants to regard lists as arrays: short rows of the input matrix are zero padded, so you have to snip off the padding before you process each row, and that means you can’t lift some of the operations back up to the parent matrix because it will re-introduce the padding as it reshapes the result; this accounts for a lot of the "1 everywhere (you can interpret v"1 as “force the verb v to operate on rank 1 subarrays of the argument”).

data_file_name =: '2.data'
data =: > 0 ". each cutopen toJ fread data_file_name

NB. {. take, i. index of; this removes trailing zeros
remove_padding =: {.~ i.&0

NB. }. behead, }: curtail; this computes successive differences
diff =: }. - }:

NB. a b in_range y == a <: y <: b
in_range =: 4 : '(((0 { x) & <:) * (<: & (1 { x))) y'

NB. a row is safe if either all successive differences are in [1..3] or all in [_3.._1]
NB. +. or
ranges =: 2 2 $ 1 3 _3 _1
row_safe =: (+./"1) @: (*/"1) @: (ranges & (in_range"1 _)) @: diff @: remove_padding

result1 =: +/ safe"1 data

NB. x delete y is y without the xth element
delete =: 4 : '(x {. y) , ((>: x) }. y)'"0 _
modified_row =: 3 : 'y , (i.#y) delete y'

modified_row_safe =: 3 : '+./"1 row_safe"1 modified_row"1 y'
result2 =: +/ modified_row_safe data

Kotlin:

import kotlin.math.abs
import kotlin.math.sign

data class Report(val levels: List<Int>) {
    fun isSafe(withProblemDampener: Boolean): Boolean {
        var orderSign = 0.0f  // - 1 is descending; +1 is ascending

        levels.zipWithNext().forEachIndexed { index, level ->
            val difference = (level.second - level.first).toFloat()
            if (orderSign == 0.0f) orderSign = sign(difference)
            if (sign(difference) != orderSign || abs(difference) !in 1.0..3.0) {
                // With problem dampener: Drop either element in the pair or the first element from the original list and check if the result is now safe.
                return if (withProblemDampener) {
                    Report(levels.drop(1)).isSafe(false) || Report(levels.withoutElementAt(index)).isSafe(false) || Report(levels.withoutElementAt(index + 1)).isSafe(false)
                }  else false
            }
        }

        return true
    }
}

fun main() {
    fun part1(input: List<String>): Int = input.map { Report(it.split(" ").map { it.toInt() }).isSafe(false) }.count { it }

    fun part2(input: List<String>): Int = input.map { Report(it.split(" ").map { it.toInt() }).isSafe(true) }.count { it }

    // Or read a large test input from the `src/Day01_test.txt` file:
    val testInput = readInput("Day02_test")
    check(part1(testInput) == 2)
    check(part2(testInput) == 4)

    // Read the input from the `src/Day01.txt` file.
    val input = readInput("Day02")
    part1(input).println()
    part2(input).println()
}

The Report#isSafe method essentially solves both parts.

I’ve had a bit of a trip up in part 2:

I initially only checked, if the report was safe, if either elements in the pair were to be removed. But in the edge case, that the first pair has different monotonic behaviour than the rest, the issue would only be detected by the second pair with indices (2, 3), whilst removing the first element in the list would yield a safe report.

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)

Kotlin

A bit late to the party, but here you go.

import kotlin.math.abs

fun part1(input: String): Int {
    return solve(input, ::isSafe)
}

fun part2(input: String): Int {
    return solve(input, ::isDampSafe)
}

private fun solve(input: String, condition: (List<Int>) -> Boolean): Int {
    var safeCount = 0
    input.lines().forEach { line ->
        if (line.isNotBlank()) {
            val nums = line.split("\\s+".toRegex()).map { it.toInt() }
            safeCount += if (condition(nums)) 1 else 0
        }
    }
    return safeCount
}

private fun isSafe(list: List<Int>): Boolean {
    val safeDiffs = setOf(1, 2, 3)
    var incCount = 0
    var decCount = 0
    for (idx in 0..<list.lastIndex) {
        if (!safeDiffs.contains(abs(list[idx] - list[idx + 1]))) {
            return false
        }
        if (list[idx] <= list[idx + 1]) incCount++
        if (list[idx] >= list[idx + 1]) decCount++
    }
    return incCount == 0 || decCount == 0
}

private fun isDampSafe(list: List<Int>): Boolean {
    if (isSafe(list)) {
        return true
    } else {
        for (idx in 0..list.lastIndex) {
            val shortened = list.toMutableList()
            shortened.removeAt(idx)
            if (isSafe(shortened)) {
                return true
            }
        }
    }
    return false
}

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…

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)

python

import re
import aoc

def setup():
    return (aoc.get_lines(2), 0)

def safe(data):
    order = 0 if data[0] < data[1] else 1
    for i in range(0, len(data) - 1):
        h = data[i]
        t = data[i + 1]
        d = abs(h - t)
        if d not in [1, 2, 3] or (order == 0 and h >= t) or (
                order == 1 and h <= t):
            return False
    return True

def one():
    lines, acc = setup()
    for l in lines:
        if safe([int(x) for x in re.findall(r'\d+', l)]):
            acc += 1
    print(acc)

def two():
    lines, acc = setup()
    for l in lines:
        data = [int(x) for x in re.findall(r'\d+', l)]
        for i in range(len(data)):
            if safe(data[:i] + data[i + 1:]):
                acc += 1
                break
    print(acc)

one()
two()

R (R-Wasm)

input = file('input2024day2.txt',open='r')
lines = readLines(input)
library(stringr)
safe = 0
safe2 = 0
for (ln in lines){
  vals = as.numeric(unlist(str_split(ln,' ')))
  diffs = diff(vals)
  cond1 = min(diffs) > 0 || max(diffs) < 0
  cond2 = max(abs(diffs)) < 4
  if (cond1 && cond2){
    safe = safe + 1
  }
  else { #Problem Dampener
    dampen = FALSE
    for (omit in -1:-length(vals)){
      diffs = diff(vals[omit])
      cond1 = min(diffs) > 0 || max(diffs) < 0
      cond2 = max(abs(diffs)) < 4
      if (cond1 && cond2){
        dampen = TRUE
      }
    }
    if (dampen){
      safe2 = safe2 + 1}
  }
}
print (safe) #Part 1
print (safe + safe2) #Part 2

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).

Elixir

defmodule Day02 do
  defp part1(reports) do
    reports
    |> Enum.map(fn report ->
      levels =
        report
        |> String.split()
        |> Enum.map(&String.to_integer/1)

      cond do
        sequence_is_safe?(levels) ->
          :safe

        true ->
          :unsafe
      end
    end)
    |> Enum.count(fn x -> x == :safe end)
  end

  defp part2(reports) do
    reports
    |> Enum.map(fn report ->
      levels =
        report
        |> String.split()
        |> Enum.map(&String.to_integer/1)

      sequences =
        0..(length(levels) - 1)
        |> Enum.map(fn i ->
          List.delete_at(levels, i)
        end)

      cond do
        sequence_is_safe?(levels) ->
          :safe

        Enum.any?(sequences, &sequence_is_safe?/1) ->
          :safe

        true ->
          :unsafe
      end
    end)
    |> Enum.count(fn x -> x == :safe end)
  end

  defp all_gaps_within_max_diff?(numbers) do
    numbers
    |> Enum.chunk_every(2, 1, :discard)
    |> Enum.all?(fn [a, b] -> abs(b - a) <= 3 end)
  end

  defp is_strictly_increasing?(numbers) do
    numbers
    |> Enum.chunk_every(2, 1, :discard)
    |> Enum.all?(fn [a, b] -> a < b end)
  end

  defp is_strictly_decreasing?(numbers) do
    numbers
    |> Enum.chunk_every(2, 1, :discard)
    |> Enum.all?(fn [a, b] -> a > b end)
  end

  defp sequence_is_safe?(numbers) do
    (is_strictly_increasing?(numbers) or
       is_strictly_decreasing?(numbers)) and all_gaps_within_max_diff?(numbers)
  end

  def run(data) do
    reports = data |> String.split("\n", trim: true)
    p1 = part1(reports)
    p2 = part2(reports)
    IO.puts(p1)
    IO.puts(p2)
  end
end

data = File.read!("input.in")
Day02.run(data)