Hello.
I am new to clojure, but try it.
code:
(defn get-size [filename]
(println 'size filename)
(count filename))
(defn get-quickhash [filename]
(println 'quickhash filename)
(hash (take 3 filename)))
(defn get-hash [filename]
(println 'hash filename)
(hash filename))
(def get-info (memoize
(fn [filename]
(map #(% filename) [get-size get-quickhash get-hash]))))
(println "A")
(nth (get-info "abc") 0)
(nth (get-info "abc") 0)
(println "B")
(nth (get-info "abc") 0)
(nth (get-info "abc") 1)
(println "C")
(nth (get-info "abc") 0)
(nth (get-info "abc") 1)
(nth (get-info "abc") 2)
(println "D")
(nth (get-info "abc") 0)
(nth (get-info "abc") 1)
(nth (get-info "abc") 2)
result:
A
size abc
B
quickhash abc
C
hash abc
D
For memory efficiency, I suppose you may use state-monads and trie.
But it will need a lot of lines of code, and too hard for me.
Regards,
---------
Mikio Hokari
2009/5/28 Korny Sietsma <[email protected]>:
>
> Hi all,
>
> I have some ruby code that I'm thinking of porting to clojure, but I'm
> not sure how to translate this idiom to a functional world:
> I have objects that are externally immutable, but have internal
> mutable state they use for optimisation, specifically in this case to
> defer un-needed calculations.
>
> Basically, I have a FileInfo class that wraps a data file, used to
> compare lots of files on my system.
> It has an "exact_match" method similar to:
> def exact_match(other)
> return false if size != other.size
> return false if quickhash() != other.quickhash()
> return hash() != other.hash()
> end
>
> quickhash and hash store their results in instance variables so they
> only need to do the expensive calculations once - and quite often they
> never need to get calculated at all; I'm looking for duplicate files,
> but many files have no duplicate, so probably never need to have their
> contents hashed.
>
> How would I do this in a functional way? My first effort would be
> something like
> (defn hash [filename] (memoize (... hash function ...)))
> but I have a couple of problems with this:
> - it doesn't seem to store the hash value with the rest of the file
> information, which feels a bit ugly
> - I assume it means storing the full filename three times, once in
> the original file info structure, once in the memoized hash function,
> and once in the memoized quickhash function. My program struggles to
> get enough RAM to track as many files as I'd like already - storing
> the filename multiple times would blow out memory quite badly.
>
> I guess I could define a unique key for each filename, and define hash
> as a function on that key, but then hash would need to be able to
> access the list of filenames somehow. It's starting to get beyond me
> - I'm hoping there's a simpler option!
>
> Any suggestions? I'd hope this is not an uncommon idiom.
>
> - Korny
>
> --
> Kornelis Sietsma korny at my surname dot com
> "Every jumbled pile of person has a thinking part
> that wonders what the part that isn't thinking
> isn't thinking of"
>
> >
>
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