repack ubyte[] to use only 7 bits

[Charles Hixson via Digitalmars-d-learn]

Your comments would be reasonable if this were destined for a library, 
but I haven't even finished checking it (and probably won't since I've 
switched to a simple zero elimination scheme).  But this is a bit 
specialized for a library...a library should probably deal with 
arbitrary ints from 8 to 64 or 128 [but I don't think that the 128 bit 
type is yet standard, only reserved].  I just thought that something 
like that should be available, possibly along the lines of Python's pack 
and unpack, and wondered where it was and what it was called.)

Additionally, I'm clearly not the best person to write the library 
version, as I still have LOTS of trouble with D templates.  And I have 
not successfully wrapped my mind around D ranges...which is odd, because 
neither Ruby nor Python ranges give me much trouble. Perhaps its the syntax.

As for " pure", "@safe", and "nothrow" ... I'd like to understand that I 
COULD use those annotations.  (The "in" I agree should be applied.  I 
understand that one.)

As for size_t for indexes...here I think we disagree.  It would be a bad 
mistake to use an index that size.  I even considered using short or 
ushort, but I ran into a comment awhile back saying that one should 
never use those for local variables.  This *can't* be an efficient 
enough way that it would be appropriate to use it for a huge array...but 
that should probably be documented if it were for a library.  (If I were 
to use size_t indexing, I'd want to modify things so that I could feed 
it a file as input, and that's taking it well away from what I was 
building it for:  converting input to a redis database so that I could 
feed it raw serial data streams without first converting it into human 
readable formats.  I wanted to make everything ASCII-7 binary data, 
which, when I thought about it more, was overkill.  All I need to do is 
eliminate internal zeros, since C handles various extended character 
formats by ignoring them.

I'm not clear what you mean by a "final switch".  fBit must adopt 
various different values during execution.  If you mean it's the same as 
a nest of if...else if ... statements, that's all I was really 
expecting, but I thought switch was a bit more readable.

Binary literals would be more self-documenting, but would make the code 
harder to read.  If I'd though of them I might have used them...but 
maybe not.

Output range?  Here I'm not sure what you're suggesting, probably 
because I don't understand D ranges.

The formatting got a bit messed up during pasting from the editor to the 
mail message.  I should have looked at it more carefully.  My standard 
says that unless the entire block is on a single line, the closing brace 
should align with the opening brace.  And I use tabs for spacing which 
works quite well in the editor, but I *do* need to remember to convert 
it to spaces before doing a cut and paste.

Thanks for your comments.  I guess that means that there *isn't* a 
standard function that does this.
Charles

On 12/06/2014 03:01 PM, bearophile via Digitalmars-d-learn wrote:
> Charles Hixson:
>
>> byte[]    x8to7 (ubyte[] bin)
>
> Better to add some annotations, like pure, @safe, nothrow, if you can, 
> and to annotate the bin with an "in".
>
>
>>     int    fByte, fBit;
>
> It's probably better to define them as size_t.
>
>
>
>>         switch (fBit)
>
> I think D doesn't yet allow this switch to be _meaningfully_ a final 
> switch.
>
>
>>                 b    =    bin[fByte] & 0x7f;
>
> D allows binary number literals as 0b100110010101.
>
>
>>                 b    ~=    (b1 | b2);
>
> Perhaps an output range is better?
>
>
>>         if    (b == 0)    bout    ~= 0x80;
>>         else            bout    ~=    b;
>>         fBit    =    fBit + 7;
>>         if    (fBit > 7)
>>         {    fByte++;
>>             fBit -=    7;
>
> The formatting seems a bit messy.
>
> Bye,
> bearophile
>