day1

day1/part1.lua

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+-- 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)
+
+local total = 0
+
+local left_list = {}
+local right_list = {}
+
+-- https://www.lua.org/manual/5.4/manual.html#pdf-ipairs
+-- print all line numbers and their contents
+for _, v in ipairs(lines) do
+	for n1, n2 in string.gmatch(v, "(%d+)%s+(%d+)") do
+		-- [](https://stackoverflow.com/a/27434198)
+		-- [Lua Length Operator](https://www.lua.org/manual/5.2/manual.html#3.4.6)
+		left_list[#left_list + 1] = n1
+		right_list[#right_list + 1] = n2
+	end
+end
+
+-- https://www.lua.org/manual/5.2/manual.html#pdf-table.sort
+table.sort(left_list)
+table.sort(right_list)
+
+for i, v in ipairs(left_list) do
+	total = total + math.abs(v - right_list[i])
+end
+
+io.output():write(total)

day1/part2.lua

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+-- 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)
+
+local left_list = {}
+local right_list = {}
+
+-- https://www.lua.org/manual/5.4/manual.html#pdf-ipairs
+-- print all line numbers and their contents
+for _, v in ipairs(lines) do
+	for n1, n2 in string.gmatch(v, "(%d+)%s+(%d+)") do
+		-- [](https://stackoverflow.com/a/27434198)
+		-- [Lua Length Operator](https://www.lua.org/manual/5.2/manual.html#3.4.6)
+		left_list[#left_list + 1] = n1
+		right_list[#right_list + 1] = n2
+	end
+end
+
+local right_list_count_map = {}
+
+for _, v in ipairs(right_list) do
+	if right_list_count_map[v] ~= nil then
+		right_list_count_map[v] = right_list_count_map[v] + 1
+	else
+		right_list_count_map[v] = 1
+	end
+end
+
+local total = 0
+
+for _, v in ipairs(left_list) do
+	local count
+	if right_list_count_map[v] ~= nil then
+		count = right_list_count_map[v]
+	else
+		count = 0
+	end
+	total = total + (v * count)
+end
+
+io.output():write(total)

template.lua

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+-- 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 = "test.lua"
+local lines = lines_from(file)
+
+-- print all line numbers and their contents
+for k, v in pairs(lines) do
+	print("line[" .. k .. "]", v)
+end