Create Random Django Compatible Usernames in Objective-C

June 28, 2012

I’m working on an app right now that lets anonymous users update data. Only I don’t quite want them to be anonymous. I want to be able to keep track of “who” data belongs too in order to provide individual service. So I needed to create unique usernames to register with the server. I extended the NSString class and add a UUID method. Problem with that is the resulting string was too long for Django. So I added another method to convert it to a base 64 ASCII string of alphanumeric characters (and two special characters). Here’s my implementation file.

#import "NSString+UUID.h"

@implementation NSString (UUID)

+ (NSString *)Base64Encode:(NSData *)data{
    //Point to start of the data and set buffer sizes
    int inLength = [data length];
    int outLength = ((((inLength * 4)/3)/4)*4) + (((inLength * 4)/3)%4 ? 4 : 0);
    const char *inputBuffer = [data bytes];
    char *outputBuffer = malloc(outLength);
    outputBuffer[outLength] = 0;

    //64 digit code
    static char Encode[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_-";

    //start the count
    int cycle = 0;
    int inpos = 0;
    int outpos = 0;
    char temp;

    //Pad the last to bytes, the outbuffer must always be a multiple of 4
    outputBuffer[outLength-1] = '=';
    outputBuffer[outLength-2] = '=';

    while (inpos < inLength){
        switch (cycle) {
            case 0:
                outputBuffer[outpos++] = Encode[(inputBuffer[inpos]&0xFC)>>2];
                cycle = 1;
                break;
            case 1:
                temp = (inputBuffer[inpos++]&0x03)<<4;
                outputBuffer[outpos] = Encode[temp];
                cycle = 2;
                break;
            case 2:
                outputBuffer[outpos++] = Encode[temp|(inputBuffer[inpos]&0xF0)>> 4];
                temp = (inputBuffer[inpos++]&0x0F)<<2;
                outputBuffer[outpos] = Encode[temp];
                cycle = 3;
                break;
            case 3:
                outputBuffer[outpos++] = Encode[temp|(inputBuffer[inpos]&0xC0)>>6];
                cycle = 4;
                break;
            case 4:
                outputBuffer[outpos++] = Encode[inputBuffer[inpos++]&0x3f];
                cycle = 0;
                break;
            default:
                cycle = 0;
                break;
        }
    }

    NSString *datastring = [NSString stringWithUTF8String:outputBuffer];
    free(outputBuffer);

    return datastring;
}

+ (NSString *)uuid
{
    NSString *uuidString = nil;
    CFUUIDRef uuid = CFUUIDCreate(NULL);
    if (uuid) {
        CFUUIDBytes bytes = CFUUIDGetUUIDBytes(uuid);
        NSData *uidBytes = [NSData dataWithBytes:&bytes length:sizeof(bytes)];

        uuidString = [self Base64Encode:uidBytes];
        CFRelease(uuid);
    }
    return [uuidString substringToIndex:[uuidString length] - 2];
}

@end

Working well for me. Now I need to look into getting Django TastyPie APIs setup to use this.

Base 64 implementation credit goes to StackOverflow. I’m afraid that I don’t remember where I pulled the UUID method from, though I have tweaked it a fair amount since.