Re: Reading and converting binary file 2 bits at a time

[Gary Willoughby via Digitalmars-d-learn]

On Thursday, 27 August 2015 at 09:38:52 UTC, Andrew Brown wrote:
On Thursday, 27 August 2015 at 09:26:55 UTC, rumbu wrote:
On Thursday, 27 August 2015 at 09:00:02 UTC, Andrew Brown 
wrote:
Hi,
I need to read a binary file, and then process it two bits at 
a time. But I'm a little stuck on the first step. So far I 
have:

import std.file;
import std.stdio;

void main(){
  auto f = std.file.read("binaryfile");
  auto g = cast(bool[]) f;
  writeln(g);
}

but all the values of g then are just true, could you tell me 
what I'm doing wrong? I've also looked at the bitmanip 
module, I couldn't get it to help, but is that the direction 
I should be looking?

Thanks very much

Andrew

auto bytes = cast(ubyte[])read("binaryfile");
foreach(b; bytes)
{
    writeln((b & 0xC0) >> 6);  //bits 7, 6
    writeln((b & 0x30) >> 4);  //bits 5, 4
    writeln((b & 0x0C) >> 2);  //bits 3, 2
    writeln((b & 0x03));       //bits 1, 0
}

That's lovely, thank you. One quick question, the length of the 
file is not a multiple of the length of ubyte,  but the cast 
still seems to work. Do you know how it converts a truncated 
final section?

Thanks again

Andrew

You can also avoid the bitshifts to make the code a little more 
readable like this:

import std.stdio;
import std.bitmanip;

struct Crumbs
{
        mixin(bitfields!(
                uint, "one", 2,
                uint, "two", 2,
                uint, "three", 2,
                uint, "four", 2
        ));
}

void main(string[] args)
{
        ubyte[] buffer = [123, 12, 126, 244, 35];

        foreach (octet; buffer)
        {
                auto crumbs = Crumbs(octet);
                ubyte* representation = cast(ubyte*)&crumbs;

                writefln("Crumb:       %08b", *representation);
                writefln("Crumb one:   %s", crumbs.one);
                writefln("Crumb two:   %s", crumbs.two);
                writefln("Crumb three: %s", crumbs.three);
                writefln("Crumb four:  %s", crumbs.four);
                writeln("---------------------");
        }
}

Now you can read a crumb at a time.