Factor

: get-input ( -- reports )
  "aoc-2024.02" "input.txt" vocab-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 ;

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)

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
}

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))))

C#

using MathNet.Numerics.LinearAlgebra;
public class Day02 : Solver
{
  private ImmutableList<Vector<Double>> data;

  public void Presolve(string input)
  {
    data = input.Trim().Split("\n")
        .Select(
            line => Vector<Double>.Build.DenseOfEnumerable(line.Split(' ').Select(double.Parse))
        ).ToImmutableList();
  }

  private bool IsReportSafe(Vector<Double> report) {
    Vector<Double> delta = report.SubVector(1, report.Count - 1)
        .Subtract(report.SubVector(0, report.Count - 1));
    return (delta.ForAll(x => x > 0) || delta.ForAll(x => x < 0))
        && Vector<Double>.Abs(delta).Max() <= 3;
  }

  private bool IsDampenedReportSafe(Vector<Double> report) {
    for (Double i = 0; i < report.Count; ++i) {
      var dampened = Vector<Double>.Build.DenseOfEnumerable(
            report.EnumerateIndexed()
                .Where(item => item.Item1 != i)
                .Select(item => item.Item2));
      if (IsReportSafe(dampened)) return true;
    }
    return false;
  }

  public string SolveFirst() => data.Where(IsReportSafe).Count().ToString();

  public string SolveSecond() => data.Where(IsDampenedReportSafe).Count().ToString();
}

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

#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;
}

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

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

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}

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).

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()

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!();

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.