On Wednesday, 27 December 2017 at 18:08:19 UTC, Bastiaan Veelo
wrote:
I suppose the following is not a bug, but confusing it is:
```
void main()
{
import std.stdio;
import std.bitmanip;
BitArray ba = [1, 1, 1, 1, 1, 1, 1, 1];
writeln(ba); // [1, 1, 1, 1, 1, 1, 1, 1]
ba >>= 4; // right shift
writeln(ba); // [1, 1, 1, 1, 0, 0, 0, 0] bits shifted left
}```
I suppose this is because the array is printed left-to-right,
whereas the bits in a byte are typically ordered right-to-left.
I suppose I should interpret the bits in the array to increase
in significance with increasing index (little endian) and that
right-shift means a shift towards less significance (which is
to the right in big endian).
The documentation of <<= and >>= [1] however just talks about
left and right, without defining left and right or clarifying
that the directions are reversed from how the array is printed.
Is there something I have missed?
[1]
https://dlang.org/phobos/std_bitmanip.html#.BitArray.opOpAssign.2
BitArray is apparently a mess. As you've pointed out it prints
the bits in the wrong order. I won't mince words here, since D
has binary literals on the form 0b10011110. Put that in a
BitArray and print it with the format string "0b%b", and you'll
get 0b01111001. While it may have been intentional, it's bug
prone and confusing, and so definitely a bug.
It also fucks up royally when it has an exact multiple of 32 bits
in its buffer, overwriting the last word with 0s when you try and
shift it in any way.
It also doesn't remove set bits outside of its covered area when
cast to size_t[]. That is, if I do
cast(size_t[])(BitArray([1,1,1,1,1,1,1,1]) << 4), the result will
be something like [4080], which corresponds to
[0b0000_1111_1111_0000].
Lastly (and this is pointed out explicitly in the documentation,
but still smells if you ask me), it will overwrite bits in the
words it covers, even if it does not cover those exact bits.
The first two are definitely bugs. The last two are results of
the intended use case for BitArray, I believe. The documentation
doesn't explicitly point this out, but it seems BitArray is
intended to give a bit-by-bit view of an area of memory that is
actually some other type. Something like this:
struct S {
int n;
float f;
}
void foo(ref S s) {
import std.bitmanip;
auto a = BitArray((&s)[0..1], S.sizeof);
a[7] = true; // Actually sets the corresponding bit in s.
}
--
Biotronic
