day 4 part 2: doesn't work

day4/part2.lua

@@ -0,0 +1,91 @@
+-- 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
+
+-- tests the functions above
+local file = "input"
+local lines = lines_from(file)
+
+-- Solution Starts here
+
+---@diagnostic disable-next-line: redefined-local
+local function count_xmas_at_index(lines, line_idx, char_idx)
+	local line = lines[line_idx]
+
+	local up_idx = line_idx - 1
+	local down_idx = line_idx + 1
+	local right_idx = char_idx + 1
+	local left_idx = char_idx - 1
+
+	local char_count = {}
+	char_count["M"] = 0
+	char_count["S"] = 0
+	if up_idx >= 1 then
+		if right_idx <= #line then
+			local key = lines[up_idx]:sub(right_idx, right_idx)
+			if char_count[key] ~= nil then
+				char_count[key] = char_count[key] + 1
+			end
+		end
+
+		if left_idx >= 1 then
+			local key = lines[up_idx]:sub(left_idx, left_idx)
+			if char_count[key] ~= nil then
+				char_count[key] = char_count[key] + 1
+			end
+		end
+	end
+
+	if down_idx <= #lines then
+		if right_idx <= #line then
+			local key = lines[down_idx]:sub(right_idx, right_idx)
+			if char_count[key] ~= nil then
+				char_count[key] = char_count[key] + 1
+			end
+		end
+
+		if left_idx >= 1 then
+			local key = lines[down_idx]:sub(left_idx, left_idx)
+			if char_count[key] ~= nil then
+				char_count[key] = char_count[key] + 1
+			end
+		end
+	end
+
+	if char_count["M"] == 2 and char_count["S"] == 2 then
+		return true
+	end
+
+	return false
+end
+
+local xmas_count = 0
+-- print all line numbers and their contents
+---@diagnostic disable-next-line: unused-local
+for line_idx, line in ipairs(lines) do
+	for char_idx in line:gmatch("()A") do
+		if count_xmas_at_index(lines, line_idx, char_idx) then
+			xmas_count = xmas_count + 1
+		end
+	end
+end
+
+io.output():write(xmas_count)