Lua: Persistent variables

Add a way for the lua scripts to set and retrieve variables
that are persistent - saved and restored with the game.

Invalidates saves.

dat/nhcore.lua

@@ -4,6 +4,15 @@
 -- This file contains lua code used by NetHack core.
 -- Is it loaded once, at game start, and kept in memory until game exit.
 
+-- Data in nh_lua_variables table can be set and queried with nh.variable()
+-- This table is saved and restored along with the game.
+nh_lua_variables = {
+};
+
+-- wrapper to simplify calling from nethack core
+function get_variables_string()
+   return "nh_lua_variables=" .. table_stringify(nh_lua_variables) .. ";";
+end
 
 -- This is an example of generating an external file during gameplay,
 -- which is updated periodically.

dat/nhlib.lua

@@ -107,3 +107,35 @@ end
 function pline(fmt, ...)
    nh.pline(string.format(fmt, table.unpack({...})));
 end
+
+-- wrapper to make calling from nethack core easier
+function nh_set_variables_string(key, tbl)
+   return "nh_lua_variables[\"" .. key .. "\"]=" .. table_stringify(tbl) .. ";";
+end
+
+-- wrapper to make calling from nethack core easier
+function nh_get_variables_string(tbl)
+   return "return " .. table_stringify(tbl) .. ";";
+end
+
+-- return the (simple) table tbl converted into a string
+function table_stringify(tbl)
+   local str = "";
+   for key, value in pairs(tbl) do
+      local typ = type(value);
+      if (typ == "table") then
+         str = str .. "[\"" .. key .. "\"]=" .. table_stringify(value);
+      elseif (typ == "string") then
+         str = str .. "[\"" .. key .. "\"]=[[" .. value .. "]]";
+      elseif (typ == "boolean") then
+         str = str .. "[\"" .. key .. "\"]=" .. tostring(value);
+      elseif (typ == "number") then
+         str = str .. "[\"" .. key .. "\"]=" .. value;
+      elseif (typ == "nil") then
+         str = str .. "[\"" .. key .. "\"]=nil";
+      end
+      str = str .. ",";
+   end
+   -- pline("table_stringify:(%s)", str);
+   return "{" .. str .. "}";
+end