From 81957070f9549446f13da2b31a4b138df7efe5ae Mon Sep 17 00:00:00 2001
From: Starbeamrainbowlabs <sbrl@starbeamrainbowlabs.com>
Date: Tue, 13 Jul 2021 23:42:08 +0100
Subject: [PATCH] Implement an LRU cache

---
 worldeditadditions/init.lua        |  1 +
 worldeditadditions/utils/lru.lua   | 80 ++++++++++++++++++++++++++++++
 worldeditadditions/utils/queue.lua | 19 +++++--
 3 files changed, 96 insertions(+), 4 deletions(-)
 create mode 100644 worldeditadditions/utils/lru.lua

diff --git a/worldeditadditions/init.lua b/worldeditadditions/init.lua
index 182bdaa..6342b07 100644
--- a/worldeditadditions/init.lua
+++ b/worldeditadditions/init.lua
@@ -13,6 +13,7 @@ worldeditadditions.Mesh,
 worldeditadditions.Face = dofile(worldeditadditions.modpath.."/utils/mesh.lua")
 
 worldeditadditions.Queue = dofile(worldeditadditions.modpath.."/utils/queue.lua")
+worldeditadditions.LRU = dofile(worldeditadditions.modpath.."/utils/lru.lua")
 
 
 dofile(worldeditadditions.modpath.."/utils/strings/init.lua")
diff --git a/worldeditadditions/utils/lru.lua b/worldeditadditions/utils/lru.lua
new file mode 100644
index 0000000..0ec0111
--- /dev/null
+++ b/worldeditadditions/utils/lru.lua
@@ -0,0 +1,80 @@
+local Queue
+if worldeditadditions then
+	Queue = dofile(worldeditadditions.modpath.."/utils/queue.lua")
+else
+	Queue = require("queue")
+end
+
+--- A least-recently-used cache implementation.
+-- @class
+local LRU = {}
+LRU.__index = LRU
+
+--- Creates a new LRU cache.
+-- Optimal sizes: 8, 16, 32, 64 or any value above
+-- @param	max_size=32		number	The maximum number of items to store in the cache.
+-- @returns	LRU				A new LRU cache.
+function LRU.new(max_size)
+	if not max_size then max_size = 32 end
+	local result = {
+		max_size = max_size,
+		cache = {  },
+		size = 0,
+		queue = Queue.new()
+	}
+	setmetatable(result, LRU)
+	return result
+end
+
+--- Determines whether the given key is present in this cache object.
+-- Does NOT update the most recently used status of said key.
+-- @param	key		string	The key to check.
+-- @returns	bool	Whether the given key exists in the cache or not.
+function LRU:has(key)
+	return self.cache[key] ~= nil
+end
+
+--- Gets the value associated with the given key.
+-- @param	key		string	The key to retrieve the value for.
+-- @returns	any|nil	The value associated with the given key, or nil if it doesn't exist in this cache.
+function LRU:get(key)
+	if not self.cache[key] then return nil end
+	
+	-- Put it to the end of the queue
+	self.queue:remove_index(self.cache[key].index)
+	self.cache[key].index = self.queue:enqueue(key)
+	
+	return self.cache[key].value
+end
+
+--- Adds a given key-value pair to this cache.
+-- Note that this might (or might not) result in the eviction of another item.
+-- @param	key		string	The key of the item to add.
+-- @param	value	any		The value to associate with the given key.
+-- @returns nil
+function LRU:set(key, value)
+	if self.cache[key] ~= nil then
+		-- It's already present in the cache - update it
+		-- Put it to the end of the queue
+		self.queue:remove_index(self.cache[key].index)
+		local new_index = self.queue:enqueue(key)
+		-- Update the cache entry
+		self.cache[key] = {
+			value = value,
+			index = new_index
+		}
+	else
+		-- It's not in the cache -- add it
+		self.cache[key] = { value = value, index = self.queue:enqueue(key) }
+		
+		self.size = self.size + 1
+		if self.size > self.max_size then
+			-- The cache is full, delete the oldest item
+			local oldest_key = self.queue:dequeue()
+			self.cache[oldest_key] = nil
+			self.size = self.size - 1
+		end
+	end
+end
+
+return LRU
diff --git a/worldeditadditions/utils/queue.lua b/worldeditadditions/utils/queue.lua
index 126b7ae..0bbfafc 100644
--- a/worldeditadditions/utils/queue.lua
+++ b/worldeditadditions/utils/queue.lua
@@ -15,6 +15,7 @@ function Queue:enqueue(value)
 	local new_last = self.last + 1
 	self.last = new_last
 	self.items[new_last] = value
+	return new_last
 end
 
 function Queue:contains(value)
@@ -30,15 +31,25 @@ function Queue:is_empty()
 	return self.first > self.last
 end
 
+function Queue:remove_index(index)
+	self.items[index] = nil
+end
+
 function Queue:dequeue()
-	local first = self.first
 	if Queue.is_empty(self) then
 		error("Error: The self is empty!")
 	end
 	
-	local value = self.items[first]
-	self.items[first] = nil -- Help the garbage collector out
-	self.first = first + 1
+	local first = self.first
+	-- Find the next non-nil item
+	local value
+	while value == nil do
+		if first >= self.last then return nil end
+		value = self.items[first]
+		self.items[first] = nil -- Help the garbage collector out
+		first = first + 1
+	end
+	self.first = first
 	return value
 end