[Haskell-cafe] Trying to reduce memory costs of String duplicates
wren ng thornton
wren at freegeek.org
Sat Sep 5 20:31:28 EDT 2009
Günther Schmidt wrote:
> Hi all,
> I'm reading in a data of 216k records into a map of Key, Values Pairs,
> the values being strings.
> As it happens out of 216k String values there really are only about 6.6k
> distinct string values, so I could save a lot of RAM if I was able to
> "insert" only actually *new* string values into the map and use
> references to (string) values that already are in memory instead.
> Is there a container that would, if I wanted to insert an element,
> return a pair of either the previously inserted, equal value and the
> container unchanged, or the new, previously unknown value and the new
> container amended by that element?
If by "strings" you allow ByteStrings, then you could use the
bytestring-trie package. This will be more worthwhile than other
approaches if your 6.6k strings have a lot of repeated prefixes, since
repeated prefixes will be shared among the unique strings.
Something like the following should work:
intern :: ByteString -> Trie ByteString
-> (ByteString, Trie ByteString)
intern s t = (fromJust (lookup s t'), t')
t' = alterBy (\_ _ -> maybe (Just s) Just) s s t
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