#bit-manipulation

How to: Should I use #define, enum or const?

Should I use #define, enum or const?

In a C++ project I’m working on, I have a flag kind of value which can have four values. Those four flags can be combined. Flags describe the records in database and can be:

new record

deleted record

modified record

existing record

Now, for each record I wish to keep this attribute, so I could use an enum:

enum { xNew, xDeleted, xModified, xExisting }

Howeve…

Fixed What does (x ^ 0x1) != 0 mean? #dev #it #asnwer

What does (x ^ 0x1) != 0 mean?

I came across the following code snippet

if( 0 != ( x ^ 0x1 ) ) encode( x, m );

What does x ^ 0x1 mean ? Is this some standard technique ?

Answer: What does (x ^ 0x1) != 0 mean?

There are a lot of good answers but I like to think of it in a simpler way.

if ( 0 != ( x ^ 0x1 ) );

First of all. An if statement is only false if the argument is zero. This means that…

How to: Precedence and bitmask operations

Precedence and bitmask operations

I’ve come across a (seemingly) very strange case.

Take the number 2 (0b10) and bitmask it with 1 (0b01)

This should produce 0b00 which is equivalent to 0.

However, here’s where Mr Schrödinger comes in:

var_dump(0b10 & 0b01); // int(0) var_dump(0b10 & 0b01 == 0); // int(0) var_dump(0b10 & 0b01 != 0); // int(0)

Whiskey. Tango. Foxtrot.

I am, admittedly, not the…

Fixed Extracting bits with a single multiplication #dev #it #asnwer

Extracting bits with a single multiplication

I saw an interesting technique used in an answer to another question, and would like to understand it a little better.

We’re given an unsigned 64-bit integer, and we are interested in the following bits:

1.......2.......3.......4.......5.......6.......7.......8.......

Specifically, we’d like to move them to the top eight positions, like so:

12345678.…

How to: What is “2's Complement”?

What is “2’s Complement”?

I’m in a computer systems course and have been struggling, in part, with Two’s Complement. I want to understand it but everything I’ve read hasn’t brought the picture together for me. I’ve read the wikipedia article and various other articles, including my text book.

Hence, I wanted to start this community wikipost to define what Two’s Complement is, how to use it and…

Fixed How to count the number of set bits in a 32-bit integer? #dev #it #asnwer

How to count the number of set bits in a 32-bit integer?

8 bits representing the number 7 look like this:

00000111

Three bits are set.

What are algorithms to determine the number of set bits in a 32-bit integer?

Answer: How to count the number of set bits in a 32-bit integer?

This is known as the ‘Hamming Weight‘, ‘popcount’ or ‘sideways addition’.

The ‘best’ algorithm really depends on which…

How to: Best Algorithm for Bit Reversal ( from MSB->LSB to LSB->MSB) in C

Best Algorithm for Bit Reversal ( from MSB->LSB to LSB->MSB) in C

What is the best algorithm to achieve the following:

0010 0000 => 0000 0100

The conversion is from MSB->LSB to LSB->MSB. All bits must be reversed; that is, this is not endianness-swapping.

Answer: Best Algorithm for Bit Reversal ( from MSB->LSB to LSB->MSB) in C

I’m not sure if it the most efficient, but this should work:

//retu…

How to: Most common C# bitwise operations on enums

Most common C# bitwise operations on enums

For the life of me, I can’t remember how to set, delete, toggle or test a bit in a bitfield. Either I’m unsure or I mix them up because I rarely need these. So a “bit-cheat-sheet” would be nice to have.

For example:

flags = flags | FlagsEnum.Bit4; // Set bit 4.

or

if ((flags & FlagsEnum.Bit4)) == FlagsEnum.Bit4) // Is there a less verbose way?

Can you…