Firebase push algorithm in Objective-C (ref: https://www.firebase.com/blog/2015-02-11-firebase-unique-identifiers.html)

generatePushId.m

/**
 * Based on https://gist.github.com/mikelehen/3596a30bd69384624c11#file-generate-pushid-js
 *
 * Fancy ID generator that creates 20-character string identifiers with the following properties:
 *
 * 1. They're based on timestamp so that they sort *after* any existing ids.
 * 2. They contain 72-bits of random data after the timestamp so that IDs won't collide with other clients' IDs.
 * 3. They sort *lexicographically* (so the timestamp is converted to characters that will sort properly).
 * 4. They're monotonically increasing.  Even if you generate more than one in the same timestamp, the
 *    latter ones will sort after the former ones.  We do this by using the previous random bits
 *    but "incrementing" them by 1 (only in the case of a timestamp collision).
 */
+(NSString *)generatePushID
{
    static uint64_t lastPushTime = 0; // uint64_t instead of NSUInteger to work correctly on 32-bit systems like Apple Watch.
    
    static NSString *PUSH_CHARS = @"-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";
    
    static NSMutableArray *lastRandChars = nil;
    if (!lastRandChars) {
        lastRandChars = [NSMutableArray array];
        for (int i = 0; i < 12; i++) [lastRandChars addObject:@0];
    }
    
    uint64_t now = round([[NSDate date] timeIntervalSince1970] * 1000);
    BOOL duplicateTime = (now == lastPushTime);
    lastPushTime = now;
    
    NSMutableString *timeStampChars = [[NSMutableString alloc] initWithCapacity:8];
    
    int i;
    for (i = 7; i >= 0; i--) {
        [timeStampChars insertString:[PUSH_CHARS substringWithRange:NSMakeRange(now % 64, 1)] atIndex:0];
        now >>=6;
    }
    
    NSAssert(now == 0, @"We should have converted the entire timestamp");
    
    NSMutableString *pushId = [NSMutableString stringWithString:timeStampChars];
    
    if (!duplicateTime) {
        for (i = 0; i < 12; i++) {
            lastRandChars[i] = [NSNumber numberWithUnsignedInteger:arc4random_uniform(64)];
        }
    } else {
        // If the timestamp hasn't changed since last push, use the same random number, except incremented by 1.
        
        for (i = 11; i >= 0 && [lastRandChars[i] isEqual: @63]; i--) {
            lastRandChars[i] = @0;
        }
        
        i = i == -1 ? 11 : i;
        lastRandChars[i] = @([(NSNumber *)lastRandChars[i] unsignedIntegerValue]+1);
    }
    
    for (i = 0; i < 12; i++) {
        [pushId appendString:[PUSH_CHARS substringWithRange:NSMakeRange([lastRandChars[i] unsignedIntegerValue], 1)]];
    }
    
    NSAssert([pushId length] == 20, @"Length should be 20.");
    
    return pushId;
}