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Re: Hex to NSString or NSData
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Re: Hex to NSString or NSData

  • Subject: Re: Hex to NSString or NSData
  • From: Clark Cox <email@hidden>
  • Date: Sat, 9 May 2009 18:05:47 -0700
On Sat, May 9, 2009 at 4:05 PM, Gwynne Raskind <email@hidden> wrote:
> On May 9, 2009, at 5:20 PM, Marcel Weiher wrote:
>>>
>>> int hexDigitToInt(char d)
>>> {
>>>  int result;
>>>  switch (d) {
>>>      case '0': result = 0; break;
>>>      case '1': result = 1; break;
>>> [snip]
>>>      case 'E': result = 14; break;
>>>      case 'F': result = 15; break;
>>>
>>>      default:
>>>          result = 0xFF;
>>>  }
>>>
>>>  return result;
>>> }
>>> Although it might look ugly, this is one of the fasted methods using
>>> standard C that converts a hex digit (nibble) to an int (2 machine
>>> instructions).
>>
>> OK, how do you get it to compile to just two instructions?  I've been
>> looking at various optimization options, and as far as I can tell it is at
>> least doing the following (non-contiguous):
>>
>> 00001e67        subl    $0x30,êx
>> 00001e6d        cmpl    $0x36,êx
>> 00001e70        jal     0x00001f67
>> 00001e76        movl    0x00000038(ëx,êx,4),êx
>> 00001e7d        addl    ëx,êx
>> 00001e7f        jmp     *êx
>>
>> So that's 6 instructions to set up and perform the indexed jump to which
>> the switch/case is being compiled.
>>
>> At the jump sites, typically another 2 instructions:
>>
>> 00001f60        movl    $0x0000000f,êx
>> 00001f65        jmp     0x00001fd2
>>
>> So that's a total of 8 instructions, with 2 taken and one typically
>> non-taken jump and one of the taken jumps computed, so probably not
>> predictable.  I must be missing something obvious.
>
> The smallest (and likely fastest) would probably be the lookup table
> version:
>
> char hexCharToNibbleL(char nibble)
> {
>        const char lt[255] = { 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06,
> 0x07, 0x08, 0x09, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x0A, 0x0B,
> 0x0C, 0x0D, 0x0E, 0x0F, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
> 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF };
>        return lt[nibble];
> }

That last line needs to be:

    return lt[(unsigned char)nibble];

otherwise, you're (at best) returning garbage for any non-ASCII characters.

--
Clark S. Cox III
email@hidden
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References: 
 >Hex to NSString or NSData (From: "Mr. Gecko" <email@hidden>)
 >Re: Hex to NSString or NSData (From: Jerry Krinock <email@hidden>)
 >Re: Hex to NSString or NSData (From: "Mr. Gecko" <email@hidden>)
 >Re: Hex to NSString or NSData (From: Gwynne Raskind <email@hidden>)
 >Re: Hex to NSString or NSData (From: Andreas Grosam <email@hidden>)
 >Re: Hex to NSString or NSData (From: Marcel Weiher <email@hidden>)
 >Re: Hex to NSString or NSData (From: Gwynne Raskind <email@hidden>)

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