challenge.py

#!/usr/bin/env python
'''Traverses JSON objects to retrieve a secret string.

The JSON objects form a tree structure. The hashes identifying an object's
children are stored in the object's `next` field. Some objects have a
`secret` value, which holds one character. In-order traversal of the object
tree gives the secret string, which when reversed yields the challenge's
solution.

Objects are retrieved from a web server by their hash (see `get_page()`).

Examples of objects:
>>> get_page('34ffe00db65f4576b5add43dda39ff99', session_id)
{"depth": 1,
 "id": "34ffe00db65f4576b5add43dda39ff99",
 "next": ["81dd32039f2f4bfea3ab35f40c994a0e",
          "adad9a3a96464d2ba2977d9863e33794",
          "33aaccd7d4fe40019160c49b689c7358"]}
>>> get_page('d9f9a7c4ac0a48959f0de596c91ee90b', session_id)
{"depth": 4,
 "id": "d9f9a7c4ac0a48959f0de596c91ee90b",
 "secret": "A"}

'''

import requests
import sys


def get_session_id():
    req = requests.get('http://challenge.shopcurbside.com/get-session')
    return req.text


def get_page(page_hash, session_id):
    req = requests.get('http://challenge.shopcurbside.com/{}'.format(page_hash),
                       headers={'Session': session_id})
    return req.json()


def main():
    nrequests = 0
    stack = list()
    stack.append('start')
    secrets = list()
    status_fmt = '\r#processed: {}, stack size: {}, #secrets: {}, last hash: {}'

    while len(stack) > 0:
        if nrequests % 10 == 0:
            session_id = get_session_id()

        page_hash = stack.pop()
        response = get_page(page_hash, session_id)
        nrequests += 1
        # Convert all keys to lower case, since the responses are inconsistent
        # in their use of upper- and lower-case.
        response = {key.lower(): value for (key, value) in response.iteritems()}
        if 'next' in response:
            if isinstance(response['next'], basestring):
                stack.append(response['next'])
            else:
                for next_page_hash in response['next']:
                    stack.append(next_page_hash)
        if 'secret' in response:
            secrets.append(response['secret'])

        print status_fmt.format(nrequests, len(stack), len(secrets), page_hash),
        sys.stdout.flush()

    secrets = ''.join(secrets)
    print '\nSecrets: {}'.format(secrets)
    print 'Reversed: {}'.format(secrets[::-1])


if __name__ == '__main__':
    main()