Re: C++ programming challenge

James Kanze <>
Mon, 15 Jun 2009 02:38:53 -0700 (PDT)
On Jun 12, 10:22 pm, Ioannis Vranos <> wrote:

Peter Jansson wrote:

On Fri, 12 Jun 2009, fifoforlifo wrote:

I thought this competition sounded fun, so I gave it a
shot. The following is a 2-threaded program that overlaps
reads and computation. I had to use boost::thread for
this, but this should all be doable in C++0x -- but I'll
let you be the final judge of whether it's an acceptable
entry. As many pointed out, the problem is I/O bound, so
that's what I tackled first. The computation part is
nearly identical to Chris' code, I could not beat the
simple accumulate-into-256-wide-table. So cheers for
finding the best solution there :-)


I have included your code in the list of results at

My Ubuntu machine had Boost 1.37 so that is what was used
during compilation.

I think you are relaxing the rules. I know Qt programming, and
most (if not all) Linux distributions provide the Qt

May I use Qt?

If so... I have a class which manages memory mapping, portably
to Windows and Unix/Linux. Can I use that? (I rather suspect
that a mmap'ed solution would be faster than any of the
alternatives, at least on Linux.)

More generally, what are the rules? In particular:

 -- What assumptions can I make concerning the input encoding?
    Single byte? Derived from ASCII? None of the proposed
    solutions I've seen handle case folding correctly for UTF-8.

 -- What assumptions can I make with regards to portability?
    (What about a system which doesn't support files, for

 -- What's the largest file size the program must handle? (The
    test data is only 1.1GB, which will fit into system cache on
    some systems.) If the entire file fits into memory, things
    can be done a lot faster.

 -- What's the real criteria for "fastest"? If it's only to
    read one input file (the one given), then obviously,
    pre-calculating the results is the fastest solution;
    presumably, that would be considered cheating. But things
    like buffer size and the effects of parallelization will
    depend on the system; for large enough files, the optimal
    solution obviously involves forking/spawning, with separate
    processes handling each block, but where the cutoffs will
    occur will depend largely on the configuration of the
    system: how much real memory is available, etc. (One of the
    systems I have available has 32 cores and 48GB main memory.
    The trade offs for a 500 GB file on that system are
    obviously different than those for a 1 GB file on a twin
    core with 2GB main memory.)

James Kanze (GABI Software)
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