Mostly done, I think

init.lua

@@ -1 +1,83 @@
--- Nothing yet
+-- Copyright (c) raymoo 2016
+-- Licensed under Apache 2.0 license. See COPYING for details.
+
+-- Any documentation here are internal details, please avoid using them in your
+-- mod.
+
+player_monoids = {}
+
+-- Needed for removing players from monoid lists when they leave.
+local player_maps = {}
+
+local mon_meta = {}
+
+mon_meta.__index = mon_meta
+
+-- A monoid object is a table with the following fields:
+--   player_map: A map from player names to their effect tables. Effect tables
+--     are maps from effect IDs to values.
+--   value_cache: A map from player names to the cached value for the monoid.
+--   next_id: The next unique ID to assign an effect.
+
+local function monoid(def)
+        local mon = {}
+
+        local player_map = {}
+        mon.players = player_map
+        table.insert(player_maps, player_map)
+
+        mon.next_id = 1
+
+        setmetatable(mon, mon_methods)
+
+        return mon
+end
+
+player_monoids.make_monoid = monoid
+
+function mon_meta:add_change(player, value)
+        local p_name = player:get_player_name()
+        
+        local p_effects = self.player_map[p_name]
+        if p_effects == nil then
+                p_effects = {}
+                self.player_map[p_name] = p_effects
+        end
+
+        local actual_id
+
+        if id then
+                actual_id = id
+        else
+                actual_id = self.next_id
+                self.next_id = actual_id + 1
+        end
+
+        local old_total = self.value_cache[p_name]
+        p_effects[actual_id] = value
+        local new_total = self.fold(p_effects)
+        self.value_cache[p_name] = new_total
+        
+        self.apply(new_total, player)
+        self.on_change(old_total, new_total, player)
+end
+
+function mon_meta:del_change(player, id)
+        local p_name = player:get_player_name()
+
+        local p_effects = self.player_map[p_name]
+        if p_effects == nil then return end
+
+        local old_total = self.value_cache[p_name]
+        p_effects[id] = nil
+        local new_total = self.fold(p_effects)
+        self.value_cache[p_name] = new_total
+
+        self.apply(new_total, player)
+        self.on_change(old_total, new_total, player)
+end
+
+function mon_meta:value(player)
+        local p_name = player:get_player_name()
+        return self.value_cache[p_name] or self.identity
+end