util.cache: Ordered key->value data structure, with size limit (same as pubsub)

1 files changed, 91 insertions, 0 deletions

diff --git a/util/cache.lua b/util/cache.lua
new file mode 100644
index 00000000..9453697c
--- /dev/null
+++ b/util/cache.lua
@@ -0,0 +1,91 @@
+local cache_methods = {};
+local cache_mt = { __index = cache_methods };
+
+local function new(size)
+	assert(size > 0, "cache size must be greater than zero");
+	local data = {};
+	return setmetatable({ data = data, count = 0, size = size, head = nil, tail = nil }, cache_mt);
+end
+
+local function _remove(list, m)
+	if m.prev then
+		m.prev.next = m.next;
+	end
+	if m.next then
+		m.next.prev = m.prev;
+	end
+	if list.tail == m then
+		list.tail = m.prev;
+	end
+	if list.head == m then
+		list.head = m.next;
+	end
+	list.count = list.count - 1;
+end
+
+local function _insert(list, m)
+	if list.head then
+		list.head.prev = m;
+	end
+	m.prev, m.next = nil, list.head;
+	list.head = m;
+	if not list.tail then
+		list.tail = m;
+	end
+	list.count = list.count + 1;
+end
+
+function cache_methods:set(k, v)
+	local m = self.data[k];
+	if m then
+		-- Key already exists
+		if v ~= nil then
+			-- Bump to head of list
+			_remove(self, m);
+			_insert(self, m);
+			m.value = v;
+		else
+			-- Remove from list
+			_remove(self, m);
+			self.data[k] = nil;
+		end
+		return;
+	end
+	-- New key
+	if v == nil then
+		return;
+	end
+	-- Check whether we need to remove oldest k/v
+	if self.count == self.size then
+		self.data[self.tail.key] = nil;
+		_remove(self, self.tail);
+	end
+
+	m = { key = k, value = v, prev = nil, next = nil };
+	self.data[k] = m;
+	_insert(self, m);
+end
+
+function cache_methods:get(k)
+	local m = self.data[k];
+	if m then
+		return m.value;
+	end
+	return nil;
+end
+
+function cache_methods:items()
+	local m = self.head;
+	return function ()
+		if not m then
+			return;
+		end
+		local k, v = m.key, m.value;
+		m = m.next;
+		return k, v;
+	end
+end
+
+return {
+	new = new;
+}