Two functions to convert a base 10 NSNumber to a base 36 NSString and the other way around

base36.m

/* Arno Richter, May 2014 */

- (NSNumber *)getIndexOf:(NSString *)c alphabet:(NSString*)string {
    
	for(int i=0; i<[string length]; i++) {
        
		if([c characterAtIndex:0] == [string characterAtIndex:i]) {
			return [NSNumber numberWithInteger:i];
        }
    }
	
	return [NSNumber numberWithInt:-1];
}

- (NSNumber *)base36To10:(NSString *)fromValue {
    
    NSString *alphabet = @"0123456789abcdefghijklmnopqrstuvwxyz";
    NSInteger total = 0;
    NSInteger power = 1;
    NSInteger sum = 0;
    
    for(int i=0; i<[fromValue length]; i++) {
        NSString *character = [fromValue substringWithRange:NSMakeRange(i, 1)];
        NSNumber *decimal = [self getIndexOf:character alphabet:alphabet];
        
        if(i == ([fromValue length]-1)) {
            power = 1;
        } else {
            power = pow(36, [fromValue length]-i-1);
        }
        
        sum = [decimal intValue] * power;
        total = total + sum;
    }
    
    return [NSNumber numberWithInteger:total];
}

- (NSString *)base10To36:(NSNumber *)fromValue {
    
    NSString *alphabet = @"0123456789abcdefghijklmnopqrstuvwxyz";
    NSMutableArray *returnArray = [[NSMutableArray alloc] init];
    NSMutableString *returnString = [[NSMutableString alloc] init];
    NSInteger manual = [fromValue intValue];
    
    while(manual > 0) {
        NSInteger rest = manual % 36;
        [returnArray addObject:[NSNumber numberWithInteger:rest]];
        
        manual = manual / 36;
    }
    
    NSArray *reversed = [[returnArray reverseObjectEnumerator] allObjects];
    
    for(int i=0; i<[reversed count]; i++) {
        NSString *character = [alphabet substringWithRange:NSMakeRange([[reversed objectAtIndex:i] intValue], 1)];
        [returnString appendString:character];
    }
    
    return returnString;
}