adventofcode-2021/day15/part2.lua

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

local grid = {}
for line in io.lines(arg[3]) do
	local row = {}
	for char in chars(line) do
		table.insert(row, tonumber(char))
	end
	table.insert(grid, row)
end
local h, w = #grid, #grid[1]

local function cost(r, c)
	if r <= 0 or c <= 0 or r > 5 * h or c > 5 * w then
		return math.huge
	else
		return (grid[(r-1)%h+1][(c-1)%w+1]
		        + math.floor((r-1)/h)
		        + math.floor((c-1)/w)
		        - 1
		       ) % 9 + 1
	end
end

local pq = pqueue(function(a, b)
	return a[3] < b[3]
end)

local distances = {}

local n = { 1, 1, 0 }
while n ~= nil and (distances[5*h] == nil or distances[5*h][5*w] == nil) do
	local r, c, d = unpack(n)
	local row = distances[r] or {}
	if d < (row[c] or math.huge) then
		row[c] = d
		distances[r] = row
		if cost(r-1, c) < math.huge then pq:add({ r-1, c, d + cost(r-1, c) }) end
		if cost(r, c-1) < math.huge then pq:add({ r, c-1, d + cost(r, c-1) }) end
		if cost(r+1, c) < math.huge then pq:add({ r+1, c, d + cost(r+1, c) }) end
		if cost(r, c+1) < math.huge then pq:add({ r, c+1, d + cost(r, c+1) }) end
	end
	n = pq:pop()
end

print(distances[5*h] and distances[5*h][5*w])