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

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

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
                ]
            ]]] .
        ]

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

#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:

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)

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 ;

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))))

#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

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();
}

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

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!();

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

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).