aoc20_10.awk

{
	adapters[$0]
}

$0 > max {
	max = $0
}

END {
	adapters[0]
	count[max] = 1

	for (adapter = max; adapter > 0; adapter--) {
		if (!(adapter in adapters) || adapter in seen)
			continue

		for (i = 1; i <= 3; i++)
			if (adapter-i in adapters)
				count[adapter-i] += count[adapter]

		seen[adapter]
	}

	print count[0]
}