Stream-Based Processing of Range Chunks in D

qznc qznc at web.de
Tue Dec 10 02:28:39 PST 2013


On Tuesday, 10 December 2013 at 09:57:44 UTC, Nordlöw wrote:
> I'm looking for an elegant way to perform chunk-stream-based 
> processing of arrays/ranges. I'm building a file 
> indexing/search engine in D that calculates various kinds of 
> statistics on files such as histograms and SHA1-digests. I want 
> these calculations to be performed in a single pass with 
> regards to data-access locality.
>
> Here is an excerpt from the engine
>
>     /** Process File in Cache Friendly Chunks. */
>     void calculateCStatInChunks(immutable (ubyte[]) src,
>                                 size_t chunkSize, bool doSHA1, 
> bool doBHist8) {
>         if (!_cstat.contentsDigest[].allZeros) { doSHA1 = 
> false; }
>         if (!_cstat.bhist8.allZeros) { doBHist8 = false; }
>
>         import std.digest.sha;
>         SHA1 sha1;
>         if (doSHA1) { sha1.start(); }
>
>         import std.range: chunks;
>         foreach (chunk; src.chunks(chunkSize)) {
>             if (doSHA1) { sha1.put(chunk); }
>             if (doBHist8) { /*...*/ }
>         }
>
>         if (doSHA1) {
>             _cstat.contentsDigest = sha1.finish();
>         }
>     }
>
> Seemingly this is not a very elegant (functional) approach as I 
> have to spread logic for each statistics (reducer) across three 
> different places in the code, namely `start`, `put` and 
> `finish`.
>
> Does anybody have suggestions/references on Haskell-monad-like 
> stream based APIs that can make this code more D-style 
> component-based?

You could make a range step for each kind of statistic, which 
outputs the input range unchanged and does its job as a side 
effect.

   SHA1 sha1;
   src.chunks(chunkSize)
      .add_sha1(doSHA1, &sha1)
      .add_bhist(doBHist8)
      .strict_consuming();

You could try to use constructor/destructor mechanisms for 
sha1.start and sha1.finish. Or at least scope guards:

SHA1 sha1;
if (doSHA1) { sha1.start(); }
scope(exit) if (doSHA1) { _cstat.contentsDigest = sha1.finish(); }


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