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# Copyright (C) 2009 by Michael Fogleman

# 

# Permission is hereby granted, free of charge, to any person obtaining a copy

# of this software and associated documentation files (the "Software"), to deal

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# copies of the Software, and to permit persons to whom the Software is

# furnished to do so, subject to the following conditions:

# 

# The above copyright notice and this permission notice shall be included in

# all copies or substantial portions of the Software.

# 

# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR

# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,

# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE

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# THE SOFTWARE.

'''

Short URL Generator

===================

Python implementation for generating Tiny URL- and bit.ly-like URLs.

A bit-shuffling approach is used to avoid generating consecutive, predictable 

URLs.  However, the algorithm is deterministic and will guarantee that no 

collisions will occur.

The URL alphabet is fully customizable and may contain any number of 

characters.  By default, digits and lower-case letters are used, with 

some removed to avoid confusion between characters like o, O and 0.  The 

default alphabet is shuffled and has a prime number of characters to further 

improve the results of the algorithm.

The block size specifies how many bits will be shuffled.  The lower BLOCK_SIZE 

bits are reversed.  Any bits higher than BLOCK_SIZE will remain as is.

BLOCK_SIZE of 0 will leave all bits unaffected and the algorithm will simply 

be converting your integer to a different base.

The intended use is that incrementing, consecutive integers will be used as 

keys to generate the short URLs.  For example, when creating a new URL, the 

unique integer ID assigned by a database could be used to generate the URL 

by using this module.  Or a simple counter may be used.  As long as the same 

integer is not used twice, the same short URL will not be generated twice.

The module supports both encoding and decoding of URLs. The min_length 

parameter allows you to pad the URL if you want it to be a specific length.

Sample Usage:

>>> import short_url

>>> url = short_url.encode_url(12)

>>> print url

LhKA

>>> key = short_url.decode_url(url)

>>> print key

12

Use the functions in the top-level of the module to use the default encoder. 

Otherwise, you may create your own UrlEncoder object and use its encode_url 

and decode_url methods.

Author: Michael Fogleman

License: MIT

Link: http://code.activestate.com/recipes/576918/

'''

DEFAULT_ALPHABET = 'mn6j2c4rv8bpygw95z7hsdaetxuk3fq'

DEFAULT_BLOCK_SIZE = 24

MIN_LENGTH = 5

class UrlEncoder(object):

    def __init__(self, alphabet=DEFAULT_ALPHABET, block_size=DEFAULT_BLOCK_SIZE):

        self.alphabet = alphabet

        self.block_size = block_size

        self.mask = (1 << block_size) - 1

        self.mapping = range(block_size)

        self.mapping.reverse()

    def encode_url(self, n, min_length=MIN_LENGTH):

        return self.enbase(self.encode(n), min_length)

    def decode_url(self, n):

        return self.decode(self.debase(n))

    def encode(self, n):

        return (n & ~self.mask) | self._encode(n & self.mask)

    def _encode(self, n):

        result = 0

        for i, b in enumerate(self.mapping):

            if n & (1 << i):

                result |= (1 << b)

        return result

    def decode(self, n):

        return (n & ~self.mask) | self._decode(n & self.mask)

    def _decode(self, n):

        result = 0

        for i, b in enumerate(self.mapping):

            if n & (1 << b):

                result |= (1 << i)

        return result

    def enbase(self, x, min_length=MIN_LENGTH):

        result = self._enbase(x)

        padding = self.alphabet[0] * (min_length - len(result))

        return '%s%s' % (padding, result)

    def _enbase(self, x):

        n = len(self.alphabet)

        if x < n:

            return self.alphabet[x]

        return self._enbase(x / n) + self.alphabet[x % n]

    def debase(self, x):

        n = len(self.alphabet)

        result = 0

        for i, c in enumerate(reversed(x)):

            result += self.alphabet.index(c) * (n ** i)

        return result

DEFAULT_ENCODER = UrlEncoder()

def encode(n):

    return DEFAULT_ENCODER.encode(n)

def decode(n):

    return DEFAULT_ENCODER.decode(n)

def enbase(n, min_length=MIN_LENGTH):

    return DEFAULT_ENCODER.enbase(n, min_length)

def debase(n):

    return DEFAULT_ENCODER.debase(n)

def encode_url(n, min_length=MIN_LENGTH):

    return DEFAULT_ENCODER.encode_url(n, min_length)

def decode_url(n):

    return DEFAULT_ENCODER.decode_url(n)

if __name__ == '__main__':

    for a in range(0, 200000, 37):

        b = encode(a)

        c = enbase(b)

        d = debase(c)

        e = decode(d)

        assert a == e

        assert b == d

        c = (' ' * (7 - len(c))) + c

        print '%6d %12d %s %12d %6d' % (a, b, c, d, e)