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package com.firebase.utils;
import java.util.Date;
/**
* 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).
*
* @author jfbyers@about.me
* @see <a href="https://gist.github.com/mikelehen/3596a30bd69384624c11#file-generate-pushid-js">
Original port by mikelehen </a>
*/
public class FireBasePushIdGenerator {
// Modeled after base64 web-safe chars, but ordered by ASCII.
private final static String PUSH_CHARS = "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";
public static String generatePushId() {
// Timestamp of last push, used to prevent local collisions if you push twice in one ms.
long lastPushTime = 0L;
// We generate 72-bits of randomness which get turned into 12 characters and
// appended to the timestamp to prevent collisions with other clients. We store the last
// characters we generated because in the event of a collision, we'll use those same
// characters except "incremented" by one.
char[] lastRandChars = new char[72];
long now = new Date().getTime();
boolean duplicateTime = (now == lastPushTime);
char[] timeStampChars = new char[8];
for (int i = 7; i >= 0; i--) {
final long module = now % 64;
timeStampChars[i] = PUSH_CHARS.charAt(Long.valueOf(module).intValue());
now = (long) Math.floor(now / 64);
}
if (now != 0)
throw new AssertionError("We should have converted the entire timestamp.");
String id = new String(timeStampChars);
if (!duplicateTime) {
for (int i = 0; i < 12; i++) {
final double times = Math.random() * 64;
lastRandChars[i] = (char) Math.floor(Double.valueOf(times).intValue());
}
} else {
// If the timestamp hasn't changed since last push, use the same random number,
//except incremented by 1.
int lastValueOfInt=0;
for (int i = 11; i >= 0 && lastRandChars[i] == 63; i--) {
lastValueOfInt = i;
lastRandChars[i] = 0;
}
lastRandChars[lastValueOfInt]++;
}
for (int i = 0; i < 12; i++) {
id += PUSH_CHARS.charAt(lastRandChars[i]);
}
if (id.length() != 20)
throw new AssertionError("Length should be 20.");
return id;
};
}
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