# Simple way to store an `HashMap[K1, HashMap[K2, HashMap[K3, HashMap[K4, V]]]]`
#
# ~~~~
# var hm4 = new HashMap4[Int, String, Int, String, Float]
# hm4[1, "one", 11, "un"] = 1.0
# hm4[2, "two", 22, "deux"] = 2.0
# assert hm4[1, "one", 11, "un"] == 1.0
# assert hm4[2, "not-two", 22, "deux"] == null
# ~~~~
class HashMap4[K1, K2, K3, K4, V]

	private var level1 = new HashMap[K1, HashMap3[K2, K3, K4, V]]

	# Return the value associated to the keys `k1`, `k2`, `k3` and `k4`.
	# Return `null` if no such a value.
	fun [](k1: K1, k2: K2, k3: K3, k4: K4): nullable V
	do
		var level1 = self.level1
		var level2 = level1.get_or_null(k1)
		if level2 == null then return null
		return level2[k2, k3, k4]
	end

	# Set `v` the value associated to the keys `k1`, `k2`, `k3` and `k4`.
	fun []=(k1: K1, k2: K2, k3: K3, k4: K4, v: V)
	do
		var level1 = self.level1
		var level2 = level1.get_or_null(k1)
		if level2 == null then
			level2 = new HashMap3[K2, K3, K4, V]
			level1[k1] = level2
		end
		level2[k2, k3, k4] = v
	end

	# Remove the item at `k1`, `k2`, `k3` and `k4`
	fun remove_at(k1: K1, k2: K2, k3: K3, k4: K4)
	do
		var level1 = self.level1
		var level2 = level1.get_or_null(k1)
		if level2 == null then return
		level2.remove_at(k2, k3, k4)
	end

	# Is there a value at `k1, k2, k3, k4`?
	fun has(k1: K1, k2: K2, k3: K3, k4: K4): Bool
	do
		if not level1.keys.has(k1) then return false
		return level1[k1].has(k2, k3, k4)
	end

	# Remove all items
	fun clear do level1.clear
end