Scanning byte array as uint array

Question

I have a large byte array with mostly 0's but some values that I need to process. If this was C++ or unsafe C# I would use a 32bit pointer and only if the current 32bit were not 0, I would look at the individual bytes. This enables much faster scanning through the all 0 blocks. Unfortunately this must be safe C# :-)

I could use an uint array instead of a byte array and then manipulate the individual bytes but it makes what I'm doing much more messy than I like. I'm looking for something simpler, like the pointer example (I miss pointers sigh)

Thanks!

score 2 · Answer 1 · edited May 23 '17 at 12:29

2

I'd follow what this guy said: Using SSE in c# is it possible?

Basically, write a little bit of C/C++, possibly using SSE, to implement the scanning part efficiently, and call it from C#.

edited May 23 '17 at 12:29

Community

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answered Apr 24 '11 at 18:03

John Zwinck

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In some ways that is "unsafe via stealth: put the unsafe code where we don't see it"... Up to the OP whether that is sensible, of course. – Marc Gravell Apr 24 '11 at 18:23
I'd like to but it has to run on Windows Phone 7 so it's not an option :( – Rabbit Apr 24 '11 at 19:44
@Marc: it's not just stealth, it allows the application of SSE instructions where none would otherwise be used (since they cannot be explicitly coded in .NET itself, to my knowledge). – John Zwinck Apr 24 '11 at 21:09

Marc Gravell · Accepted Answer · 2011-04-24T18:16:21.547

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If the code must be safe, and you don't want to use a larger type and "shift", them you'll have to iterate each byte.

(edit) If the data is sufficiently sparse, you could use a dictionary to store the non-zero values; then finding the non-zeros is trivial (and enormous by sparse arrays become cheap).

edited Apr 24 '11 at 18:16

answered Apr 24 '11 at 18:05

Marc Gravell

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You are probably right, I don't know of any way to do it either. A shame, it's a very easy perf gain. Shifting etc. would actually make it slower in the case where most bytes were non-zero (but usually they are mostly zero). – Rabbit Apr 24 '11 at 19:49
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@user72185 - one thing to check; does WP7 support explicit-layout on structs? I don't *think* it does, but worth a try? – Marc Gravell Apr 24 '11 at 20:22

score 0 · Answer 3 · answered Apr 24 '11 at 18:02

0

You can access the characters

string.ToCharArray()

Or you can access the raw byte[]

Text.Encoding.UTF8Encoding.GetBytes(stringvalue)

Ultimately, what I think you'd need here is

MemoryStream stream;
stream.Write(...)

then you will be able to directly hadnle the memory's buffer

There is also UnmanagedMemoryStream but I'm not sure whether it'd use unsafe calls inside

answered Apr 24 '11 at 18:02

sehe

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Mmmm I suppose I can say: "I did" LOL – sehe Apr 24 '11 at 18:09

score 0 · Answer 4 · answered Apr 24 '11 at 20:28

You can use the BitConverter class:

byte[] byteArray = GetByteArray(); // or whatever
for (int i = 0; i < b.Length; I += 2)
{
    uint x = BitConverter.ToUInt32(byteArray, i);
    // do what you want with x
}

Another option is to create a MemoryStream from the byte array, and then use a BinaryReader to read 32-bit values from it.

Scanning byte array as uint array

4 Answers4