Add a hashtable file save method

src/Breve/UrlTable.hs

@@ -1,24 +1,41 @@
 module Breve.UrlTable
 ( UrlTable
-, records
+, load
 , insert
 , extract
 ) where
 
 import Breve.Generator
+
+import Control.Monad       (forever)
+import Control.Concurrent  (forkIO, threadDelay)
+import Text.Read           (readMaybe)
 import qualified Data.HashTable.IO as H
 
 type UrlTable = H.CuckooHashTable Word Url
 
---Empty url hash table
-records :: IO UrlTable
-records = H.new
+-- Periodically write a url table to a file
+sync :: UrlTable -> FilePath -> IO ()
+sync table file = forever $ do
+  threadDelay (round 3.0e8)
+  content <- fmap show (H.toList table)
+  writeFile file content
 
--- Insert the url in the table and return the word
+-- Load a url table from a file
+load :: FilePath -> IO UrlTable
+load file = do
+  content <- readFile file
+  table <- case readMaybe content of
+    Just list -> H.fromList list
+    Nothing   -> H.new
+  forkIO (sync table file)
+  return table
+
+-- Insert the url in a table and return the word
 insert :: UrlTable -> Url -> IO Word
 insert table url = H.insert table new url >> return new
   where new = wordID url
 
--- Lookup the table for the associated url
+-- Lookup a table for the associated url
 extract :: UrlTable -> Word -> IO (Maybe Url)
 extract = H.lookup