import hashlib
import random
import string

class RandomContentFile(object):
    def __init__(self, size, seed):
        self.seed = seed
        self.random = random.Random(self.seed)
        self.offset = 0
        self.size = size
        self.hash = hashlib.md5()
        self.digest_size = self.hash.digest_size
        self.digest = None

    def seek(self, offset):
        assert offset == 0
        self.random.seed(self.seed)
        self.offset = offset

    def tell(self):
        return self.offset

    def read(self, size=-1):
        if size < 0:
            size = self.size - self.offset

        r = []

        random_count = min(size, self.size - self.offset - self.digest_size)
        if random_count > 0:
            self.offset += random_count
            size -= random_count
            data = ''.join(chr(self.random.getrandbits(8)) for _ in xrange(random_count))
            if self.hash is not None:
                self.hash.update(data)
            r.append(data)

        digest_count = min(size, self.size - self.offset)
        if digest_count > 0:
            if self.digest is None:
                self.digest = self.hash.digest()
                self.hash = None
            self.offset += digest_count
            size -= digest_count
            data = self.digest[:digest_count]
            r.append(data)

        return ''.join(r)

class FileVerifier(object):
    def __init__(self):
        self.size = 0
        self.hash = hashlib.md5()
        self.buf = ''

    def write(self, data):
        self.size += len(data)
        self.buf += data
        digsz = -1*self.hash.digest_size
        new_data, self.buf = self.buf[0:digsz], self.buf[digsz:]
        self.hash.update(new_data)

    def valid(self):
        """
        Returns True if this file looks valid. The file is valid if the end
        of the file has the md5 digest for the first part of the file.
        """
        return self.buf == self.hash.digest()

def files(mean, stddev, seed=None):
    """
    Yields file-like objects with effectively random contents, where
    the size of each file follows the normal distribution with `mean`
    and `stddev`.

    Beware, the file-likeness is very shallow. You can use boto's
    `key.set_contents_from_file` to send these to S3, but they are not
    full file objects.

    The last 128 bits are the MD5 digest of the previous bytes, for
    verifying round-trip data integrity. For example, if you
    re-download the object and place the contents into a file called
    ``foo``, the following should print two identical lines:

	python -c 'import sys, hashlib; data=sys.stdin.read(); print hashlib.md5(data[:-16]).hexdigest(); print "".join("%02x" % ord(c) for c in data[-16:])' <foo

    Except for objects shorter than 16 bytes, where the second line
    will be proportionally shorter.
    """
    rand = random.Random(seed)
    while True:
        while True:
            size = int(rand.normalvariate(mean, stddev))
            if size >= 0:
                break
        yield RandomContentFile(size=size, seed=rand.getrandbits(32))

def names(mean, stddev, charset=None, seed=None):
    """
    Yields strings that are somewhat plausible as file names, where
    the lenght of each filename follows the normal distribution with
    `mean` and `stddev`.
    """
    if charset is None:
        charset = string.ascii_lowercase
    rand = random.Random(seed)
    while True:
        while True:
            length = int(rand.normalvariate(mean, stddev))
            if length >= 0:
                break
        name = ''.join(rand.choice(charset) for _ in xrange(length))
        yield name