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###*
* 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 = (->
# Modeled after base64 web-safe chars, but ordered by ASCII.
pushChars = "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz"
# Timestamp of last push, used to prevent local collisions if you push twice in one ms.
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.
lastRandChars = []
return ->
now = new Date().getTime()
duplicateTime = now is lastPushTime
lastPushTime = now
timeStampChars = [0..7].map((e)->
r = pushChars.charAt(now % 64)
now =Math.floor(now / 64)
return r
).reverse().join ''
throw new Error('We should have converted the entire timestamp.') if now isnt 0
lastRandChars = [0...12].map (e)-> ~~(Math.random()*64)
if duplicateTime
# If the timestamp hasn't changed since last push, use the same random number, incremented by 1
lastRandChars[lastRandChars.length-1]++
lastRandChars[lastRandChars.length-1] = 0 if lastRandChars[lastRandChars.length-1] > 64
throw new Error('Length should be 20.') if lastRandChars.length+timeStampChars.length isnt 20
return timeStampChars+lastRandChars.map((e)->pushChars.charAt(e)).join('')
)()
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