@@ -0,0 +1,57 @@
/**
* 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).
*/
generatePushID = ( function ( ) {
// Modeled after base64 web-safe chars, but ordered by ASCII.
var PUSH_CHARS = '-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz' ;
// Timestamp of last push, used to prevent local collisions if you push twice in one ms.
var lastPushTime = 0 ;
// 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.
var lastRandChars = [ ] ;
return function ( ) {
var now = new Date ( ) . getTime ( ) ;
var duplicateTime = ( now === lastPushTime ) ;
lastPushTime = now ;
var timeStampChars = new Array ( 8 ) ;
for ( var i = 7 ; i >= 0 ; i -- ) {
timeStampChars [ i ] = PUSH_CHARS . charAt ( now % 64 ) ;
// NOTE: Can't use << here because javascript will convert to int and lose the upper bits.
now = Math . floor ( now / 64 ) ;
}
if ( now !== 0 ) throw new Error ( 'We should have converted the entire timestamp.' ) ;
var id = timeStampChars . join ( '' ) ;
if ( ! duplicateTime ) {
for ( i = 0 ; i < 12 ; i ++ ) {
lastRandChars [ i ] = Math . floor ( Math . random ( ) * 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 ] === 63 ; i -- ) {
lastRandChars [ i ] = 0 ;
}
lastRandChars [ i ] ++ ;
}
for ( i = 0 ; i < 12 ; i ++ ) {
id += PUSH_CHARS . charAt ( lastRandChars [ i ] ) ;
}
if ( id . length != 20 ) throw new Error ( 'Length should be 20.' ) ;
return id ;
} ;
} ) ( ) ;
mikelehen created this gist