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()

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
}

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    
    ].

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)

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}

#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);