I'm trying the add-a-gram challenge from here: http://www.itasoftware.com/careers/programmers-archive.php ... and I'm also experiencing runaway memory consumption. If I load the supplied list of words (a 1.6M file) and search for shorter strings, things are OK. Memory consumption increases dramatically as I search for longer and longer trails. The function searchWord starts with a particular map and searches the maps below it (i.e. those containing shorter words), so when I search for longer trails, a larger number of maps must be examined. When I search for short trails, only a few (2-3 say) of the maps are examined. I think this implies some kind of lazy evaluation problem, but I'm not sure where to go from here. Was it a bad idea to use a Map of Maps as a data structure? Initially I had a list of Maps, but this wouldn't even load the full file (so I suppose lazy evaluation has allowed me to get a bit further...). Maps seemed like the right data structures to use for this problem. Is there some way I could force evaluation of the maps as the file is loaded? (Or is blaming the maps a red herring?) Program below:

module Main where

import Data.FiniteMap import Data.List import System.IO

Data structure is a Map of Maps. The top-level Map holds Maps indexed by Int (for words of size n). Each of these Maps holds words of that size, where a key is the sorted list of chars for a word, and the value is the list of words whose sorted characters are that key i.e. all words constructed from the same set of characters.

type WordMaps = FiniteMap Int WordMap type WordMap = FiniteMap String [String] type MyWord = String

ail + s = sail + n = nails + e = aliens + t = salient + r = entrails + c = clarinets + e = interlaces + d = CREDENTIALS (length 11) mar + c = cram + h = march + s = charms + o = chromas + n = monarchs + i = harmonics + a = maraschino + n = ANACHRONISM (length 11) -----------------------------------------------------------

addWordToMap :: String -> WordMap -> WordMap addWordToMap word m = case lookupFM m key of Just wlist -> addToFM m key (word:wlist) Nothing -> addToFM m key [word] where key = sort word

addWord :: String -> WordMaps -> WordMaps addWord word maps = addToFM maps key newmap where key = length word newmap = addWordToMap word oldmap oldmap = case (lookupFM maps key) of Just m -> m Nothing -> emptyFM

populateMaps :: WordMaps -> IO WordMaps populateMaps maps = do eof <- isEOF if eof then return maps else do word <- getLine if (length word) > 0 -- test for blank lines then populateMaps (addWord word maps) else populateMaps maps

-----------------------------------------------------------

getMap :: WordMaps -> Int -> WordMap getMap maps key = case (lookupFM maps key) of Nothing -> emptyFM Just m -> m

showWordList :: [String] -> String showWordList = concat . intersperse ", "

printTrail :: WordMaps -> [String] -> String printTrail _ [] = "" printTrail maps (w:ws) = (showWordList elts) ++ "\n" ++ (printTrail maps ws) where m = getMap maps (length w) elts = lookupWithDefaultFM m [""] w

removeEachChar :: String -> [String] removeEachChar word = removeEachChar' word (length word) []

removeEachChar' :: String -> Int -> [String] -> [String] removeEachChar' [] _ list = list removeEachChar' _ 0 list = list removeEachChar' word n list = removeEachChar' word (n-1) (newword:list) where newword = (take (n-1) word) ++ (drop n word)

unfussyHead :: [[String]] -> [String] unfussyHead [] = [] unfussyHead l = head l

Given a word (and a trail so far) this will return either an empty trail, or a trail to the first 3-char word that completes the search. A trail is just the list of keys, shortest first.

searchWord :: WordMaps -> [String] -> String -> [String] searchWord maps trail word | length word <= 3 = word:trail | otherwise = case lookupFM m word of Nothing -> [] Just _ -> unfussyHead $ filter ([] /=) $ map cont (removeEachChar word) where cont = searchWord maps (word:trail) m = getMap maps (length word)

For each word (key) in a level, call searchWord. The first to return a non-empty list is the winner. If the list for this level is empty, then recurse with the next level down.

searchLevel :: WordMaps -> Int -> [String] searchLevel _ 3 = [] searchLevel maps level = case (lookupFM maps level) of Nothing -> [] Just m -> let result = filter ([] /= ) $ map (searchWord maps []) (keysFM m) in if result == [] then searchLevel maps (level-1) else head result

Kick off search at top-level.

start :: WordMaps -> [String] start maps = searchLevel maps (last (keysFM maps))

Look for a specific trail. For testing/debugging.

findw :: WordMaps -> [String] findw maps = searchWord maps [] (sort "entrails")

findw maps = searchWord maps [] (sort "nails") findw maps = searchWord maps [] (sort "salient") findw maps = searchWord maps [] (sort "entrails") findw maps = searchWord maps [] (sort "clarinets")

readWords :: IO String readWords = do wordMaps <- populateMaps emptyFM return (printTrail wordMaps (findw wordMaps)) -- This is how we'd normally start: --return (printTrail wordMaps (start wordMaps))

main :: IO () main = do s <- readWords putStrLn s return ()

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