From e9bb9c84680af652791c4135e9c2b1a3f5681af9 Mon Sep 17 00:00:00 2001
From: Zynh Ludwig <zynh0722@gmail.com>
Date: Mon, 2 Dec 2024 11:47:13 -0800
Subject: [PATCH] day2 part 2

---
 day2/part2.lua | 131 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 131 insertions(+)
 create mode 100644 day2/part2.lua

diff --git a/day2/part2.lua b/day2/part2.lua
new file mode 100644
index 0000000..60b7706
--- /dev/null
+++ b/day2/part2.lua
@@ -0,0 +1,131 @@
+-- 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