Re: [iPhone] How to create a unique string
Re: [iPhone] How to create a unique string
- Subject: Re: [iPhone] How to create a unique string
- From: Thomas Wetmore <email@hidden>
- Date: Thu, 13 May 2010 11:13:12 -0400
UUID is the way to go. Here is a category I use to generate unique ids. It uses core foundation to generate a 128-bit UUID and converts that to a 22-character string. The conventional string form of a UUID is 36 characters long. A goal of this code was to minimize the size of the id since my application generates billions of them.
------------------------
static unichar x (unsigned int);
// This category uses Apple's core foundation to generate a 128-bit universal id and then
// encodes the bits into a 22 character string. The conventional string form of a universal
// id is 36 characters long, so this saves 14 characters.
//----------------------------------------------------------------------------------------
@implementation NSString (TWUUID)
+ (NSString*) stringWithUniqueId
{
CFUUIDRef uuid = CFUUIDCreate(NULL);
CFUUIDBytes b = CFUUIDGetUUIDBytes(uuid);
unichar unichars[22];
unichar* c = unichars;
*c++ = x(b.byte0 >> 2);
*c++ = x((b.byte0 & 3 << 4) + (b.byte1 >> 4));
*c++ = x((b.byte1 & 15 << 2) + (b.byte2 >> 6));
*c++ = x(b.byte2 & 63);
*c++ = x(b.byte3 >> 2);
*c++ = x((b.byte3 & 3 << 4) + (b.byte4 >> 4));
*c++ = x((b.byte4 & 15 << 2) + (b.byte5 >> 6));
*c++ = x(b.byte5 & 63);
*c++ = x(b.byte6 >> 2);
*c++ = x((b.byte6 & 3 << 4) + (b.byte7 >> 4));
*c++ = x((b.byte7 & 15 << 2) + (b.byte8 >> 6));
*c++ = x(b.byte8 & 63);
*c++ = x(b.byte9 >> 2);
*c++ = x((b.byte9 & 3 << 4) + (b.byte10 >> 4));
*c++ = x((b.byte10 & 15 << 2) + (b.byte11 >> 6));
*c++ = x(b.byte11 & 63);
*c++ = x(b.byte12 >> 2);
*c++ = x((b.byte12 & 3 << 4) + (b.byte13 >> 4));
*c++ = x((b.byte13 & 15 << 2) + (b.byte14 >> 6));
*c++ = x(b.byte14 & 63);
*c++ = x(b.byte15 >> 2);
*c = x(b.byte15 & 3);
CFRelease(uuid);
return [NSString stringWithCharacters: unichars length: 22];
}
@end
// Convert six-bit values into letters, numbers or _ or $ (64 characters in that set).
//------------------------------------------------------------------------------------
unichar x (unsigned int c)
{
if (c < 26) return 'a' + c;
if (c < 52) return 'A' + c - 26;
if (c < 62) return '0' + c - 52;
if (c == 62) return '$';
return '_';
}
On May 13, 2010, at 11:05 AM, Michael Ash wrote:
>
> CFUUID includes the MAC address, so unless your MAC address is cloned
> or you manage to generate two UUIDs on the same device in the same
> 100ns time interval or the calendar rolls over (which will take about
> 3700 years), they are entirely unique within the universe of CFUUID
> strings.
>
> Mike
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Tom Wetmore, Chief Bottle Washer, DeadEnds Software
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