fix(holidays): add proper memoization to yet unused function

src/Utils/Holidays.hs

@@ -105,24 +105,35 @@ isBankHolidayArea land dd = dd `Set.member` holidays
 -}
 
 -- | Returns whether a day is a bank holiday for years >= 1995
--- | Repeated calls are handled efficiently using a lazy cache for 2020--2075
+-- | Repeated calls are handled efficiently using lazy memoization 
 isBankHolidayArea :: Feiertagsgebiet -> Day -> Bool
 isBankHolidayArea land dd = dd `Set.member` holidays
   where 
     (year, _, _) = toGregorian dd
     holidays 
-      | year >= cacheMinYear
-      , year <= cacheMaxYear
-      , (Just hys) <- Map.lookup land cacheHolidays 
-      , (Just hds) <- Map.lookup year hys = hds
+      | (Just hys) <- Map.lookup land memoHolidays
+      , (Just hds) <- index hys $ fromInteger $ year2index year = hds
       | otherwise = bankHolidaysAreaSet land year 
 
-cacheMinYear, cacheMaxYear :: Integer
-cacheMinYear = 2020
-cacheMaxYear = 2075
+-- memoize holidays 
+memoHolidays :: Map.Map Feiertagsgebiet [Set.Set Day]
+memoHolidays = Map.fromList  [(land,
+                                  [bankHolidaysAreaSet land year
+                                  | i <- [0..], let year = index2year i]) 
+                              | land <- universeF]    
 
-cacheHolidays :: Map.Map Feiertagsgebiet (Map.Map Integer (Set.Set Day))
-cacheHolidays = Map.fromList  [(land, Map.fromList 
-                                  [(year, bankHolidaysAreaSet land year) 
-                                  | year <- [cacheMinYear..cacheMaxYear]]) 
-                              | land <- universeF]
+-- year with fastest access
+memoTip :: Integer
+memoTip = 2030
+
+year2index :: Integer -> Integer
+year2index y 
+  | y < memoTip = 2 * (memoTip - y) - 1  
+  | otherwise   = 2 * (y - memoTip)
+
+index2year :: Integer -> Integer
+index2year y = result
+  where 
+    (x,r) = y `divMod` 2 
+    result | r == 0    = memoTip + x 
+           | otherwise = memoTip - x - 1