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🦌 - 2024 DAY 2 SOLUTIONS -🦌

Day 2: Red-Nosed Reports

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FAQ

57 comments

  • 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. 

     rust
        
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!();

    • is_sorted_by is new to me, could be very useful.

    • The is_sorted_by is a really nice approach. I originally tried using that function thinking that |a, b| a > b or |a, b| a < b would cut it but it didn't end up working. I never thought to handle the check for the step being between 1 and 3 in the callback closure for that though.

  • 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#: ::: spoiler C# 

     csharp
        
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));
}

    :::

  • #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!

  • Nim

    Got correct answer for part 1 on first try, but website rejected it. Wasted some time debugging and trying different methods. Only to have the same answer accepted minutes later. =( 

     nim
        
proc isSafe(report: seq[int]): bool =
  let diffs = collect:
    for i, n in report.toOpenArray(1, report.high): n - report[i]
  (diffs.allIt(it > 0) or diffs.allIt(it < 0)) and diffs.allIt(it.abs in 1..3)

proc solve(input: string): AOCSolution[int, int] =
  let lines = input.splitLines()
  var reports: seq[seq[int]]
  for line in lines:
    reports.add line.split(' ').map(parseInt)

  for report in reports:
    if report.isSafe():
      inc result.part1
      inc result.part2
    else:
      for t in 0..report.high:
        var mReport = report
        mReport.delete t
        if mReport.isSafe():
          inc result.part2
          break

    Codeberg repo

  • Rust

     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.

  • 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"). 

     undefined
        
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.&amp;0

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

NB. a b in_range y == a &lt;: y &lt;: b
in_range =: 4 : '(((0 { x) &amp; &lt;:) * (&lt;: &amp; (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 &amp; (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

  • I forgot that this started yesterday, so I'm already behind. I quite like my solution for part one, but part two will have to wait edit: part 2 was a lot simpler than I thought after a night's sleep.

    Rust

     rust
        
use color_eyre::eyre;
use std::{fs, num, str::FromStr};

#[derive(Debug, PartialEq, Eq)]
struct Report(Vec<isize>);

impl FromStr for Report {
    type Err = num::ParseIntError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let v: Result<Vec<isize>, _> = s
            .split_whitespace()
            .map(|num| num.parse::<isize>())
            .collect();
        Ok(Report(v?))
    }
}

impl Report {
    fn is_safe(&self) -> bool {
        let ascending = self.0[1] > self.0[0];
        let (low, high) = if ascending { (1, 3) } else { (-3, -1) };
        self.0.windows(2).all(|w| {
            let a = w[0];
            let b = w[1];
            b >= a + low && b <= a + high
        })
    }

    fn is_dampsafe(&self) -> bool {
        if self.is_safe() {
            return true;
        }
        for i in 0..self.0.len() {
            let damped = {
                let mut v = self.0.clone();
                v.remove(i);
                Self(v)
            };
            if damped.is_safe() {
                return true;
            }
        }
        false
    }
}

fn main() -> eyre::Result<()> {
    color_eyre::install()?;

    let part1 = part1("d02/input.txt")?;
    let part2 = part2("d02/input.txt")?;
    println!("Part 1: {part1}\nPart 2: {part2}");
    Ok(())
}

fn part1(filepath: &str) -> eyre::Result<isize> {
    let mut num_safe = 0;
    for l in fs::read_to_string(filepath)?.lines() {
        if Report::from_str(l)?.is_safe() {
            num_safe += 1;
        }
    }
    Ok(num_safe)
}

fn part2(filepath: &str) -> eyre::Result<isize> {
    let mut num_safe = 0;
    for l in fs::read_to_string(filepath)?.lines() {
        if Report::from_str(l)?.is_dampsafe() {
            num_safe += 1;
        }
    }
    Ok(num_safe)
}

  • Nim

     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).

    • Cool to see another solution in Nim here =)

      (0..<record.len).anyIt(record.dup(delete(it)).validate)

      That's smart. I haven't thought of using iterators to loop over indexes (except in a for loop).

      I got stuck on part 2 trying to check everything inside a single loop, which kept getting more ugly.

      Yeah I've thought of simple ways to do this and found none. And looking at the input - it's too easy to bruteforce, especially in compiled lang like Nim.

  • Smalltalk

    Discovered a number of frustrations with this supposedly small and elegant language

    1. Smalltalk's block based iteration has NO control flow
    2. blocks are very dissimilar to functions
    3. You cannot early return from blocks (leading to a lot of horrible nested ifs or boolean operations)
    4. Smalltalk's messages (~functions) cannot take multiple arguments, instead it has these sort of combined messages, so instead of a function with three arguments, you would send 3 combined messages with one argument. This is fine until you try to chain messages with arguments, as smalltalk will interpret them as a combined message and fail, forcing you to either break into lots of temp variables, or do lisp-like parenthesis nesting, both of which I hate
    5. Smalltalk's order of operations, while nice and simple, is also quite frustrating at times, similar to #4, forcing you to break into lots of temp variables, or do lisp-like parenthesis nesting. For instance (nums at: i) - (nums at: i+1) which would be nums[i] - nums[i+1] in most languages

    Part 1 

     smalltalk
        
day2p1: input
    ^ (input lines 
        collect: [ :l | l substrings collect: [ :s | s asInteger ]])
        
        count: [ :nums |
            (nums = nums sorted or: nums = nums sorted reverse) 
                and: [
                    (1 to: nums size-1) allSatisfy: [ :i | 
                        ((nums at: i) - (nums at: i+1)) abs between: 1 and: 3
        ]    ]    ]

    Part 2 

     smalltalk
        
day2p2: input    
    | temp |
    
    ^ (input lines 
        collect: [ :l | (l substrings collect: [ :s | s asInteger ]) asOrderedCollection ])
         
        count: [ :nums |
            
            (self day2p2helper: nums)
            or: [ 
                ((1 to: nums size) anySatisfy: [ :i |
                    temp := nums copy.
                    temp removeAt: i.
                    self day2p2helper: temp
                ])
            
            
            or: [(self day2p2helper: nums reversed)
            or: [
                (1 to: nums size) anySatisfy: [ :i |
                    temp := nums reversed.
                    temp removeAt: i.
                    self day2p2helper: temp
                ]
            ]]] .
        ]


  
     smalltalk
        
day2p2helper: nums
    ^ (1 to: nums size - 1) allSatisfy: [ :i | 
        ((nums at: i+1) - (nums at: i)) between: 1 and: 3    
    ].

  • #Zig 

     zig
        
const std = @import("std");
const List = std.ArrayList;

const splitScalar = std.mem.splitScalar;
const parseInt = std.fmt.parseInt;
const print = std.debug.print;
const concat = std.mem.concat;

var gpa = std.heap.GeneralPurposeAllocator(.{}){};
const alloc = gpa.allocator();

const Answer = struct {
    safe: u32,
    tolerated: u32,
};

pub fn isSafe(levels: []i32) bool {
    if (levels.len == 0) {
        return false;
    }
    // slide window in pairs, advancing by one
    var it = std.mem.window(i32, levels, 2, 1);
    const first = it.first();
    const decreasing = first[0] - first[1] > 0;
    it.reset(); // rewind the iterator

    while (it.next()) |slice| {
        const lhs: i32 = slice[0];
        const rhs: i32 = slice[1];
        if (decreasing) {
            if (lhs <= rhs) return false;
            if (lhs - rhs < 1 or lhs - rhs > 3) return false;
        } else {
            if (rhs <= lhs) return false;
            if (rhs - lhs < 1 or rhs - lhs > 3) return false;
        }
    }
    return true;
}

pub fn solve(input: []const u8) !Answer {
    var rows = splitScalar(u8, input, '\n');

    // PART 1

    // determine how many reports are safe
    var safe_reports: u32 = 0;
    var tolerated_reports: u32 = 0;
    var unsafe_reports = List([]i32).init(alloc);
    defer unsafe_reports.deinit();

    while (rows.next()) |row| {
        var levels = splitScalar(u8, row, ' ');

        var report = List(i32).init(alloc);
        defer report.deinit();

        while (levels.next()) |level| {
            const value = parseInt(i32, level, 10) catch continue;
            report.append(value) catch continue;
        }

        if (isSafe(report.items)) {
            safe_reports += 1;
        } else {
            try unsafe_reports.append(try alloc.dupe(i32, report.items));
        }
    }

    // PART 2

    // determine how many unsafe reports can be tolerated
    for (unsafe_reports.items) |report| {
        var index: usize = 0;
        while (index < report.len) : (index += 1) {
            // mutate report by removing one level
            const mutated_report = concat(
                alloc,
                i32,
                &[_][]const i32{ report[0..index], report[index + 1 ..] },
            ) catch report;
            defer alloc.free(mutated_report);

            if (isSafe(mutated_report)) {
                tolerated_reports += 1;
                break;
            }
        }
    }

    return Answer{ .safe = safe_reports, .tolerated = safe_reports + tolerated_reports };
}

pub fn main() !void {
    const answer = try solve(@embedFile("input.txt"));
    print("Part 1: {d}\n", .{answer.safe});
    print("Part 2: {d}\n", .{answer.tolerated});
}

test "test input" {
    const answer = try solve(@embedFile("test.txt"));
    try std.testing.expectEqual(2, answer.safe);
    try std.testing.expectEqual(4, answer.tolerated);

  • Kotlin:

     undefined
        
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.

  • R (R-Wasm) 

     undefined
        
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

  • Elixir

     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. 

     undefined
        
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
}
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