Bitová manipulácia
An excellent example of D's ability to generate code on compile-time with mixins,
is bit manipulation.
Simple bit manipulation
D offers the following operators for bit manipulation:
& bitwise and
| bitwise or
~ bitwise negative
<< bitwise signed left-shift
>> bitwise signed right-shift (preserves the sign of the high-order bit)
>>> bitwise unsigned right-shift
A practical example
A common example for bit manipulation is to read the value of a bit.
D provides core.bitop.bt for most common tasks, however to get used to bit
manipulation, let's start with a verbose implementation of testing a bit:
enum posA = 1;
enum maskA = (1 << posA);
bool getFieldA()
{
return _data & maskA;
}
A generalization is to test for blocks that are longer than 1. Hence
a special read mask with the length of the block is needed
and the data block is shifted accordingly before applying the mask:
enum posA = 1;
enum lenA = 3;
enum maskA = (1 << lenA) - 1; // ...0111
uint getFieldA()
{
return (_data >> posA) & maskA;
}
Setting such a block can equivalently be defined by negating the mask and thus
only allowing writes within the specified block:
void setFieldA(bool b);
{
return (_data & ~maskAWrite) | ((b << aPos) & maskAWrite);
}
std.bitmanip to the rescue
It's a lot of fun to write ones' custom bit manipulation code and
D provides the full toolbox to do so. However in most cases one doesn't want to
copy&paste such bit manipulation code as this is very error-prone and hard to maintain.
Hence in D std.bitmanip helps you to write maintainable, easy-to-read bit manipulations
with std.bitmanip and the power of mixins - without sacrificing performance.
Have a look at the exercise section. A BitVector is defined, but it still uses
just X bits and is nearly indistinguishable from a regular struct.
std.bitmanip and core.bitop contain more helpers that are greatly helpful
for applications that require low-memory consumption.
Padding and alignment
As the compiler will add padding for variables with a size lower than the current
OS memory layout (size_t.sizeof) e.g. bool, byte, char, it is recommended
to start with fields of high alignments.
In-depth
