Great catch, Gijs!
On Nov 7, 9:32 am, "Gijs S." <gijsstuur...@gmail.com> wrote:
> The problem is not the freqs function. The implementation in Python
> and in Clojure are different algorithms.
>
> Both produce a graph with the number of blocks per layer per type, for
> 6 particular types. These six types are the Clay, Coal ore, Diamond
> ore, Gold ore, Iron ore, Obsidian, and Redstone ore that are plotted
> in the graphs in Pepijn's blogpost.
>
> The Python approach only calculates the frequencies for these 6 types.
> The Clojure approach calculates the frequency for ALL the types and
> then only displays the plot of the 6 types. This is why the freqs
> function take such a long time as it does too much.
>
> The implementation in Clojure below only counts the frequencies for
> the 6 types. It also only traverses the blocks array only once,
> similarly to the suggestion from Benny Tsai.
>
> Counting the frequencies takes 3 seconds on my machine, using Pepijn's
> example level as input. This shows that there is no need to apologize
> for using Clojure :)
>
> Counting the frequencies of all 92 types takes 40 seconds on my
> machine on the example level. I bet this is faster than the Python
> algorithm for all types, because the Clojure approach traverses the
> blocks array only once. The Python algorithm will traverse each of the
> 128 layers 92 times.
>
> ;; blocks is one big byte array of all the blocks, created by
> concatenating all the files
> (defn freqs [^bytes blocks]
>
> (let [freq-layer (vec (repeatedly 128 #(transient {})))
>
> types #{(byte 56) (byte 49) (byte 16) (byte 15) (byte 14)
> (byte 73)}
>
> size (alength blocks)]
>
> (loop [idx (int 0)]
>
> (when (< idx size)
>
> (let [block (byte (aget blocks idx))]
>
> (when (types block)
>
> ;;the work below is only done when block is one of the six
> types
> (let [layer (unchecked-remainder idx 128)
>
> fl (nth freq-layer layer)]
>
> (assoc! fl block (inc (get fl block (int 0))))))
>
> (recur (unchecked-inc idx)))))
>
> (doall (map persistent! freq-layer))))
>
> The whole file:https://gist.github.com/666228
>
> -Gijs
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