aoc-2024/day2/part2.lua

-- see if the file exists
local function file_exists(file)
	local f = io.open(file, "rb")
	if f then
		f:close()
	end
	return f ~= nil
end

-- get all lines from a file, returns an empty
-- list/table if the file does not exist
local function lines_from(file)
	if not file_exists(file) then
		return {}
	end
	local lines = {}
	for line in io.lines(file) do
		lines[#lines + 1] = line
	end
	return lines
end

local function splitstr(inputstr, sep)
	if sep == nil then
		sep = "%s"
	end
	local t = {}
	for str in string.gmatch(inputstr, "([^" .. sep .. "]+)") do
		t[#t + 1] = str
	end
	return t
end

-- tests the functions above
local file = "input"
local lines = lines_from(file)

local reports = {}
for _, line in ipairs(lines) do
	local raw_report = splitstr(line)
	for i, v in ipairs(raw_report) do
		raw_report[i] = tonumber(v)
	end

	reports[#reports + 1] = raw_report
end

-- return the sign of a number
local function sign(number)
	if number > 0 then
		return 1
	elseif number == 0 then
		return 0
	else
		return -1
	end
end

function table.shallow_copy(t)
	local t2 = {}
	for k, v in pairs(t) do
		t2[k] = v
	end
	return t2
end

local function is_safe(report)
	local s = nil -- keeps track of the sign over the course of a report
	local unsafe = false
	local unsafe_idx_1, unsafe_idx_2
	for i, v1 in ipairs(report) do
		local v2 = report[i + 1]
		-- if v2 doesn't exist, we are at the end of the report
		if v2 ~= nil then
			if s == nil then
				-- if we are on the first iteration, we need to set the direction for
				-- the rest of the report
				s = sign(v1 - v2)
			else
				-- we check that the sign matches the rest of the report, breaking early if not
				if sign(v1 - v2) ~= s then
					unsafe = true
					unsafe_idx_1, unsafe_idx_2 = i, i + 1
					break
				end
			end

			-- Checking if the rate of change in the report is within acceptable bounds
			local distance = math.abs(v1 - v2)
			if distance < 1 or distance > 3 then
				unsafe = true
				unsafe_idx_1, unsafe_idx_2 = i, i + 1
				break
			end
		end
	end

	return unsafe, unsafe_idx_1, unsafe_idx_2
end

local function test_altered_report(report, idx)
	local test_report = table.shallow_copy(report)
	table.remove(test_report, idx)

	return is_safe(test_report)
end

local safe_count = 0
for _, report in ipairs(reports) do
	local unsafe, idx1, idx2 = is_safe(report)

	if unsafe then
		if not test_altered_report(report, idx1) or not test_altered_report(report, idx2) then
			unsafe = false
		end
	end

	if not unsafe then
		safe_count = safe_count + 1
	end
end

print(safe_count)

local function test()
	return true, nil, nil
end

if test() then
	print("uwu")
end