BIT banging help

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DotNetNewbie - 17 Jan 2008 19:59 GMT

I want to store the maximum amount of permissions in a bit mask.

From what I know, I should use the C# long data type and use the
bigint in sqlserver.

How would I initialize a variable of type long to represent a 64bit
mask?

I think it is something like:

0x0..............0

But I am unsure as to the exact number of 0's, can someone clear this
up for me (with as much detail as possible).

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Peter Duniho - 17 Jan 2008 20:04 GMT

> [...]
> How would I initialize a variable of type long to represent a 64bit
[quoted text clipped - 6 lines]
> But I am unsure as to the exact number of 0's, can someone clear this
> up for me (with as much detail as possible).

long grbitMask = 0;

You can write as many zeros in your literal as you like. The number is
still zero.

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Alberto Poblacion - 17 Jan 2008 20:17 GMT

>I want to store the maximum amount of permissions in a bit mask.
>
[quoted text clipped - 7 lines]
>
> 0x0..............0

When entering the digits in hexadecimal (prefix 0x), every digit
represents four bits. Therefore, you would need 16 hex digits to represent a
64-bit value. However, if it is all zeroes that you want, you don't have to
write all of them. Just writing value=0 will set to zero all the bits in
value.

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DotNetNewbie - 17 Jan 2008 21:23 GMT

On Jan 17, 3:17 pm, "Alberto Poblacion" <earthling-
quitaestoparacontes...@poblacion.org> wrote:

> >I want to store the maximum amount of permissions in a bit mask.
>
[quoted text clipped - 13 lines]
> write all of them. Just writing value=0 will set to zero all the bits in
> value.

I want to make an enumeration that will represent all my permission
values for each role, then I will get all the users roles and then OR
them together to get the users final permission value.

So something like:

enum Permisions : long
{
CanLogIn = 0,
CanXXXX = 0x0000000000000001,
CanAAA = 0x0000000000000002,
CanBBB = 0x0000000000000004,
etc.

}

1. how many enum values can I store with a long? (which I will then
store as a bigint in sqlserver).

Reply to this Message

Peter Duniho - 17 Jan 2008 21:44 GMT

> [...]
> 1. how many enum values can I store with a long? (which I will then
> store as a bigint in sqlserver).

Do you know how many bits are in a long?

Do you know how many bits it takes to store one permission flag?

You _should_ already have the answers to both of those questions. And
from those answers, the answer to "how many enum values can I store with a
long" is trivial to answer.

Pete

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DotNetNewbie - 17 Jan 2008 21:53 GMT

On Jan 17, 4:44 pm, "Peter Duniho" <NpOeStPe...@nnowslpianmk.com>
wrote:

> > [...]
> > 1. how many enum values can I store with a long? (which I will then
[quoted text clipped - 9 lines]
>
> Pete

long is 64 bits, or 8 bytes.

I want to be able to AND and OR each permission enum value, so there
can't be any overlap.
From what I understand that I will use 1 bit per permission right?

Reply to this Message

Peter Duniho - 17 Jan 2008 22:57 GMT

> long is 64 bits, or 8 bytes.

Yes.

> I want to be able to AND and OR each permission enum value, so there
> can't be any overlap.

Yes. Obviously you cannot use the same bit for two different values.

> From what I understand that I will use 1 bit per permission right?

Assuming each permission is "on" or "off", yes.

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Alun Harford - 17 Jan 2008 22:21 GMT

> On Jan 17, 3:17 pm, "Alberto Poblacion" <earthling-
> quitaestoparacontes...@poblacion.org> wrote:
[quoted text clipped - 30 lines]
> 1. how many enum values can I store with a long? (which I will then
> store as a bigint in sqlserver).

You can store 64 bits if you use a ulong and bigint unsigned.
You can store 63 bits if you use a long and bigint.

You don't need to worry about setting the values - just use:

[Flags]
enum Permissions : ulong
{
    CanLogIn,
    CanXXXX,
    CanAAA,
    CanBBB
}

Alun Harford

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Hans Kesting - 18 Jan 2008 08:48 GMT

Alun Harford formulated the question :

> You don't need to worry about setting the values - just use:
>
[quoted text clipped - 8 lines]
>
> Alun Harford

No, that would still use underlying values of 0, 1, 2, 3 instead of 1,
2, 4, 8

Hans Kesting

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Ian Semmel - 18 Jan 2008 19:00 GMT

You can do something like this

    public class Permissions
    {
        ulong val = 0;

        public enum Attr
        {
            CanLogIn,
            CanXXXX,
            CanAAA,
            CanBBB
        }

        public Permissions (params Attr[] attr)
        {
            val = 0;
            foreach (Attr a in attr)
                val |= ( 1U << (int) a );
        }

        public bool IsSet(Attr a)
        {
            return ((val & ( 1U << (int) a )) != 0);
        }

        // etc
    }
and then

Permissions p = new Permissions(Permissions.Attr.CanAAA,
Permissions.Attr.CanBBB);

bool b = p.IsSet(Permissions.Attr.CanBBB);
b = p.IsSet(Permissions.Attr.CanLogIn);

Could be done better but I know C++ better than C#. Checkout [Flags] for
Attr.

> Alun Harford formulated the question :
>
[quoted text clipped - 15 lines]
>
> Hans Kesting

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