Messages in this thread | | | Subject | Re: [PATCH] strings: helper for maximum decimal encoding of an unsigned integer | From | Bernd Petrovitsch <> | Date | Fri, 14 Sep 2012 15:18:12 +0200 |
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On Fre, 2012-09-14 at 08:30 -0400, Jim Rees wrote: > Bernd Petrovitsch wrote: [...] > A pure K&R-C version would use a string: > ---- snip ---- > #define base10len(i) "\0x1\0x3\0x5\0x8\0x0A\0x0D\0x0F\0x11\0x14"[sizeof(i)] > ---- snip ---- > (if I converted them properly into hexadecimal) and that gives a "char" > which is happily promoted to whatever one needs in that place. > > 1. That may give you a signed char on some architectures, which is not what > you want (although it doesn't matter since the values are all < 128)
And it depends on compiler options BTW.
But we can easily cast it: #define base10len(i) ((unsigned char)"\x1\x3\x5\x8\xA\xD\xF\x11\x14"[sizeof(i)])
> 2. If you put this in a .h, you'll get multiple copies of the array
That depends on the compiler.
> 3. No bounds checking (but in ninja K&R style you never check bounds)
Well, I assumed that we don't use VLAs as parameter for the sizeof() so the value is compile time known and the better C compilers can check it. And then, there is no reason to store the string as such too.
[....] > Pure K&R:
We can (and should) make it "const" too.
> base10.h: > extern unsigned char base10len_vals[]; extern const unsigned char base10len_vals[]; > #define base10len(i) (base10len_vals[sizeof(i)]) > > base10.c: > unsigned char base10len_vals[] = {1,3,5,8,10,13,15,17,20}; const unsigned char base10len_vals[] = {1,3,5,8,10,13,15,17,20}; > But I still like my way better.
The 8 wasted bytes probably do not matter ....
Bernd -- Bernd Petrovitsch Email : bernd@petrovitsch.priv.at LUGA : http://www.luga.at
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