Re: lun - Lucky Number

Richard Heathfield in sci.crypt:

On 15/03/2025 19:44, Marcel Logen wrote:

[...]

Okay. In C, ! is the `not' operator.

I was aware of that. So I thought that the double exclamation
mark was superfluous.

0 is false, 1 is true, non-zero is true, but not true is false. So:

                        ^^^^^^^^^^^^^^^^
This is probably the crucial point.

x = 6;
printf("%d", x); prints 6.
printf("%d", !x); prints 0 because not non-zero is false.
printf("%d", !!x); prints 1 because not not non-zero is true.
>
So !!x maps 0 to 0 and everything else to 1. If you're printing
bits, it's just the job.

A good trick! So a kind of normalization, AFAICS.

[...]

#define BIT_QRY(x,i) ((x[(i)>>3] & (1<<((i)&7)))!=0)
#define BIT_SET(x,i) (x)[(i)>>3]|=(1<<((i)&7))
#define BIT_CLR(x,i) (x)[(i)>>3]&=(1<<((i)&7))^0xFF

[...]

Say i is 77, which is
>
01001101
>
i >> 3 gives us 00001001, or 9.
i & 7 gives us 00000101, or 5.
>
So these macros respectively query, set, or clear bit 5 of byte 9
(counting from x).
>
A somewhat hurried explanation, so feel free to ask further.

I'll take a closer look at this. For now, your explanation is
good enough for me. Thanks.

Marcel
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