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adamrothman/firebase_id.py

Created June 3, 2016 06:27
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  1. adamrothman created this gist Jun 3, 2016.
    64 changes: 64 additions & 0 deletions firebase_id.py
    View file
    Original file line number Diff line number Diff line change
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    """Firebase-style 120-bit unique identifiers.
    https://www.firebase.com/blog/2015-02-11-firebase-unique-identifiers.html
    """
    from math import floor
    from random import random
    from string import ascii_lowercase
    from string import ascii_uppercase
    from string import digits
    from time import time


    # Modeled after web-safe base64 alphabet, but in ASCII ordered.
    ALPHABET = '-' + digits + ascii_uppercase + '_' + ascii_lowercase
    ALPHABET_SIZE = len(ALPHABET)

    # Timestamp of last ID generated. Used to prevent local collisions more than
    # one ID is generated in the same ms.
    _LAST_TS = 0

    # We generate 72 random bits which are encoded into 12 characters and
    # appended to the timestamp. In the event of a time collision, we use the last
    # set of random characters "incremented" by one.
    _LAST_RANDOM_CHARS = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]


    def firebase_id():
    now = floor(time() * 1000) # whole ms
    time_collision = (now == _LAST_TS)

    global _LAST_TS
    _LAST_TS = now

    # 48 time bits become 8 characters
    timestamp_chars = [0, 0, 0, 0, 0, 0, 0, 0]
    for i in range(len(timestamp_chars), 0, -1):
    timestamp_chars[i - 1] = ALPHABET[now % ALPHABET_SIZE]
    now = floor(now / 64)

    if now != 0:
    # Interestingly enough, this only works until some time in 2109...
    raise Exception('Failed to convert entire timestamp')

    uid = ''.join(timestamp_chars)

    if time_collision is False:
    for i in range(12):
    _LAST_RANDOM_CHARS[i] = floor(random() * ALPHABET_SIZE)
    else:
    # If the timestamp hasn't changed since the last ID we generated, use
    # the same random number, incremented by 1.
    for i in range(11, -1, -1):
    if _LAST_RANDOM_CHARS[i] != ALPHABET_SIZE - 1:
    break
    else:
    _LAST_RANDOM_CHARS[i] = 0
    _LAST_RANDOM_CHARS[i] += 1

    for i in range(12):
    uid += ALPHABET[_LAST_RANDOM_CHARS[i]]

    if len(uid) != 20:
    raise Exception('Generated ID should have length 20')

    return uid