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)

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