adventofcode-2021/day05/part2.lua

#!/usr/bin/env luajit
require("utils")

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

function mark(field, x, y)
	local row = field[x] or {}
	row[y] = (row[y] or 0) + 1
	field[x] = row
end

local input = io.open(arg[3]):read("*a")
local lines = split(input, "\n")

local field = {}
for _, line in pairs(lines) do
	local x1, y1, x2, y2 = unpack(split(line, "[^0-9]+"))
	x1, y1, x2, y2 = tonumber(x1), tonumber(y1), tonumber(x2), tonumber(y2)
	local x, y, dx, dy = x1, y1, sign(x2 - x1), sign(y2 - y1)
	mark(field, x, y)
	while x ~= x2 or y ~= y2 do
		x, y = x + dx, y + dy
		mark(field, x, y)
	end
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)