# vim: tabstop=4 shiftwidth=4 softtabstop=4
#
#    Licensed under the Apache License, Version 2.0 (the "License"); you may
#    not use this file except in compliance with the License. You may obtain
#    a copy of the License at
#
#         http://www.apache.org/licenses/LICENSE-2.0
#
#    Unless required by applicable law or agreed to in writing, software
#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
#    License for the specific language governing permissions and limitations
#    under the License.

'''
Utilities for creating short ID strings based on a random UUID. The IDs
each comprise 12 (lower-case) alphanumeric characters.
'''

import base64
import uuid


def _to_byte_string(value, num_bits):
    '''
    Convert an integer to a big-endian string of bytes, with any padding
    required added at the end (i.e. after the least-significant bit).
    '''
    shifts = xrange(num_bits - 8, -8, -8)
    byte_at = lambda off: (value >> off if off >= 0 else value << -off) & 0xff
    return ''.join(chr(byte_at(offset)) for offset in shifts)


def get_id(source_uuid):
    '''
    Derive a short (12 character) id from a random UUID.

    The supplied UUID must be a version 4 UUID object.
    '''
    if isinstance(source_uuid, basestring):
        source_uuid = uuid.UUID(source_uuid)
    if source_uuid.version != 4:
        raise ValueError('Invalid UUID version (%d)' % source_uuid.version)

    # The "time" field of a v4 UUID contains 60 random bits
    # (see RFC4122, Section 4.4)
    random_bytes = _to_byte_string(source_uuid.time, 60)
    # The first 12 bytes (= 60 bits) of base32-encoded output is our data
    encoded = base64.b32encode(random_bytes)[:12]

    return encoded.lower()


def generate_id():
    '''
    Generate a short (12 character), random id.
    '''
    return get_id(uuid.uuid4())