Counting but with characters

Question

Sites like youtube have their ID's using the full character range to make an ID eg dQw4w9WgXcQ. I am wondering how this would be done by selecting a number and turning it into a compressed number using the full range of basic characters like this.

a>b>c>d>e> ect >azy>azz>aAa>aAb

I have written this in javascript and it was hideous for how I did it was embedded for loops going through an array of possible characters. I really hope I don't need to do that again but the best I can find in an efficient manner is int to hex but hex does not cover the full range of characters I need.

http://stackoverflow.com/questions/2257441/random-string-generation-with-upper-case-letters-and-digits-in-python — Bryan Newman, May 06 '17 at 19:06
That gives a random sequence - the OP wants an integer->string mapping. — PaulMcG, May 06 '17 at 19:07
If you want a securely random integer → string mapping, that’s not really easy: https://crypto.stackexchange.com/questions/712/converting-a-stream-cipher-into-a-block-cipher If you’re okay with 1 → a, though, just encode the number in base 62 by dividing it by 62 repeatedly and taking the remainder. — Ry-, May 06 '17 at 19:09

PaulMcG · Accepted Answer · 2017-05-07T18:22:30.507

This might be close to what you have in mind:

def int_to_string_fns(chars):
    num_chars = len(chars)
    char_to_i = dict((c,i) for i, c in enumerate(chars))

    # create 2 functions for the given characters, 
    # one to convert an int to a string, and one to 
    # convert a string back to the corresponding int
    def _to_string(i, minsize=0):

        pad = []
        ret = []
        if minsize:
            maxval = num_chars ** (minsize-1)
            while maxval > max(i, 1):
                pad.append(chars[0])
                maxval /= num_chars

        if i > 0:
            while i:
                i, c = divmod(i, num_chars)
                ret.append(chars[c])
        else:
            ret.append(chars[0])

        return ''.join(pad + ret[::-1])

    def _from_string(s):
        mult = num_chars
        ret = 0
        for c in s:
            ret *= mult
            ret += char_to_i[c]
        return ret

    return _to_string, _from_string

enc,dec = int_to_string_fns("ABCDEFGHIJKLMNOP")
print(enc(100))
print(dec("GE"))

for i in range(100,110):
    print(i, enc(i), dec(enc(i)))

prints

Thanks! Works perfectly. – Hello World May 06 '17 at 19:54 — Hello World, May 06 '17 at 19:54

Counting but with characters

1 Answers1