Day2.lua

local function ProcessInput(input)
	local ProcessedInputs = {}
	for match, _ in input:gmatch("(%a+)\n") do
		table.insert(ProcessedInputs,match)
	end
	return ProcessedInputs
end

local function GetLetters(id)
	local Letters = {}
	for letter in id:gmatch("%a") do
		table.insert(Letters,letter)
	end
	return Letters
end

local function CountLetters(id)
	local CountMap = {}
	for idx,letter in ipairs(GetLetters(id)) do
		CountMap[letter] = (CountMap[letter] or 0) + 1
	end
	return CountMap
end

local function HasLetterCount(id,countToCheck)
	for letter,count in pairs(CountLetters(id)) do
		if count == countToCheck then
			return true
		end
	end
	return false
end

local function HasOneDifferentInSameSpot(id1,id2)
	local Id1Letters = GetLetters(id1)
	local Id2Letters = GetLetters(id2)
	
	local Diffs = 0
	local IdxOfDifference = nil
	
	for idx,letter in ipairs(Id1Letters) do
		if Id2Letters[idx] ~= letter then
			Diffs = Diffs + 1
			IdxOfDifference = idx
		end
		if Diffs > 1 then
			return false
		end
	end
	
	return (Diffs == 1), IdxOfDifference
end

local function CheckSum(input,countsToCheck)
	local ProcessedInput = ProcessInput(input)
	local CountsIndex = {}
	local CheckSum = 1
	
	for _,id in pairs(ProcessedInput) do
		for _,countToCheck in pairs(countsToCheck) do
			if HasLetterCount(id,countToCheck) then
				--print("id:",id,"has letters that appear at least",countToCheck,"times.")
				CountsIndex[countToCheck] = (CountsIndex[countToCheck] or 0) + 1
			end
		end
	end
	
	for count,howManyContained in pairs(CountsIndex) do
		CheckSum = CheckSum * howManyContained
	end
	
	return CheckSum
end

local function FindCorrectBox(input)
	local ProcessedInput = ProcessInput(input)
	
	for idx1,id1 in ipairs(ProcessedInput) do
		for idx2,id2 in ipairs(ProcessedInput) do
			if idx1 ~= idx2 then
				local IsDiff, DiffIdx = HasOneDifferentInSameSpot(id1,id2)
				
				if IsDiff then
					local TrimmedString = id1:sub(1,DiffIdx-1)..id1:sub(DiffIdx+1,-1)
					return TrimmedString
				end
			end
		end
	end
end

local Input = require(script.Input)

print("Day 2 | Part 1")
print("Checksum:",CheckSum(Input, {2,3}))
print("Day 2 | Part 2")
print("Correct box:",FindCorrectBox(Input))

Input.lua

local Input = [[
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]]

return Input