-- SPDX-FileCopyrightText: © 2022—2026 Vladimir Zorin <vladimir@deviant.guru>
-- SPDX-License-Identifier: LicenseRef-OWL-1.0-or-later
-- Licensed under OWL v1.0+. See LICENSE.

--[[
Bounded two-generation cache: `get` promotes hits from the stale
generation into the fresh one; once the fresh generation reaches `cap`
entries it becomes the stale one and a new table starts. At most 2*cap
entries are live, and cold entries age out within two rotations without
any per-entry bookkeeping.
]]

local put = function(state, key, value)
	if state.fresh[key] == nil then
		state.count = state.count + 1
	end
	state.fresh[key] = value
	if state.count >= state.cap then
		state.stale = state.fresh
		state.fresh = {}
		state.count = 0
	end
end

local get = function(self, key)
	local state = self.__state
	local value = state.fresh[key]
	if value == nil then
		value = state.stale[key]
		if value ~= nil then
			put(state, key, value)
		end
	end
	return value
end

local set = function(self, key, value)
	put(self.__state, key, value)
end

local new = function(cap)
	return {
		cfg = { cap = cap },
		__state = { fresh = {}, stale = {}, count = 0, cap = cap },
		get = get,
		set = set,
	}
end

return {
	new = new,
}