adventofcode-2021/day05/part1.lua

#!/usr/bin/env luajit

function readline()
	local x1 = io.read("*n")
	io.read(1) -- ","
	local y1 = io.read("*n")
	io.read(4) -- " -> "
	local x2 = io.read("*n")
	io.read(1) -- ","
	local y2 = io.read("*n")
	io.read("*l") -- "...\n"
	return x1, y1, x2, y2
end

io.input(arg[3])
local x1, y1, x2, y2 = readline()
local field = {}
while x1 ~= nil do
	if x1 == x2 then
		for y = math.min(y1, y2), math.max(y1, y2) do
			local row = field[x1] or {}
			row[y] = (row[y] or 0) + 1
			field[x1] = row
		end
	elseif y1 == y2 then
		for x = math.min(x1, x2), math.max(x1, x2) do
			local row = field[x] or {}
			row[y1] = (row[y1] or 0) + 1
			field[x] = row
		end
	end
	x1, y1, x2, y2 = readline()
end

local count = 0
for r, row in pairs(field) do
	for c, col in pairs(row) do
		if col >= 2 then
			count = count + 1
		end
	end
end
print(count)