_output.txt

Number of valid passwords:
	814

Day4.lua

local lowest = 234208
local highest = 765869

local function numDigits(number)
	return math.floor( math.log(number, 10) + 1 )
end

-- gross
local function digitAt(number, x)
	local stringified = tostring(number)
	return tonumber(stringified:sub(x,x))
end

local function toDigitTable(number)
	local digitTable = {}
	for i = 1, numDigits(number) do
		table.insert(digitTable, digitAt(number, i))
	end

	return digitTable
end

local function isValid(number)
	local digits = toDigitTable(number)

	local isSixDigit = #digits == 6
	local digitsDecrease = false
	local containsPair = false
	local validPairs = {}

	local lastDigit = 0

	for _, digit in ipairs(digits) do
		-- check if the digits decreased
		if not (digit >= lastDigit) then
			digitsDecrease = true
			break -- could just return false here
		end
		
		-- mark pairs, if the pair already exists current digit is guarenteed to be part of a larger group
		if digit == lastDigit then
			-- hoo boy
			-- if the validPairs[digit] is false we have already determined that this is a group larger than 2
			-- if the validPairs[digit] is true this is a group of digits larger than 2, set to false
			-- if the validPairs[digit] is nil this pair has not been seen, set to true.
			if validPairs[digit] ~= false then
				if validPairs[digit] == true then
					validPairs[digit] = false
				elseif validPairs[digit] == nil then
					validPairs[digit] = true
				end
			end
		end
		lastDigit = digit
	end
	
	-- if we have a valid pair set the flag
	for _, isValid in pairs(validPairs) do
		if isValid then
			containsPair = true
			break
		end
	end

	return (
		isSixDigit and
		containsPair and
		not digitsDecrease
	)
end

-- count valid answers
local validAnswers = 0
for i = lowest, highest do
	if isValid(i) then
		validAnswers = validAnswers + 1
	end
end

print("Number of valid passwords:")
print("\t"..validAnswers)