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

diff --git a/util/cache.lua b/util/cache.lua
new file mode 100644 (file)
index 0000000..9453697
--- /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;
+}