Nick Nick - 3 months ago 12
C Question

How do I read this software phase lock loop code?

In my search for an example of a software phase lock loop I came across the following question

Software Phase Locked Loop example code needed

In the answer by Adam Davis a site is given that is broken and I have tried the new link that is given in a comment but I cant get that to work either.

The answer from Kragen Javier Sitaker gave the following code as a simple example of a software phase locked loop.

main(a,b){for(;;)a+=((b+=16+a/1024)&256?1:-1)*getchar()-a/512,putchar(b);}


Also included in his answer was a link to what should be a much more readable example but this link is also broken. Thus I have been trying to translate the above code into simpler and more readable code.

I have come this far:

main(a,b){
for(;;){
// here I need to break up the code somehow into a if() statement('s).
if(here I get lost){
a = a+1;
if(here i get lost some more){
b = b+1;
}
}
}


Thanks to the SO question What does y -= m < 3 mean?

I know it is possible to break up the
a+=
and
b+=
into if statements.
But the
(&256? 1 : -1)*getchar()-a/512,putchar(b);
part in the code is killing me. I have been looking on Google and on SO to the meaning of the symbols and functions that are used.

I know the & sign indicates an address in memory.

I know that the : sign declares a bit field OR can be used in combination with the ? sign which is a conditional operator. The combination of the two I can use like sirgeorge answer in what does the colon do in c?

Theory Behind getchar() and putchar() Functions

I know that getchar() reads one character

I know that putchar() displays the character

But the combination of all these in the example code is not readable for me and . I can not make it readable for my self even do I know what they all separately do.

So my question: How do I read this software phase lock loop code?

main(a,b){for(;;)a+=((b+=16+a/1024)&256?1:-1)*getchar()-a/512,putchar(b);}

Answer

What I get is:

main (a, b)
{
    char c;
    for (;;)
    {
        c = getchar();
        b = (b + 16 + (a / 1024));
        if(!(b & 256)) 
        {
            c = c * -1;
        }
        a = a + c - (a/512);
        putchar(b);
    }
}

I had to add a c variable to not get lost.

What the program does:

Take a and b.
Infinite loop:
    get a char input in c
    calculate b
    If (b bitwise AND 256)
        c = -c
    Calculate a
    Print b

It seems it translate input into something else, I have to see the code in action to understand better myself.

Hope it helped!

Hint:

https://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B

a+= => a = a +

a?b:c => if(a){return b;} else {return c;} (As a function itself, it don t truly return)

Add parentheses, it help.

a & b is bitwise AND:

a/b |0|1|
   0|0|0|
   1|0|1|
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