# Copyright 2013-2019 Lawrence Livermore National Security, LLC and other
# Spack Project Developers. See the top-level COPYRIGHT file for details.
#
# SPDX-License-Identifier: (Apache-2.0 OR MIT)

import sys
import hashlib

import llnl.util.tty as tty


#: Set of hash algorithms that Spack can use, mapped to digest size in bytes
hashes = {
    'md5': 16,
    'sha1': 20,
    'sha224': 28,
    'sha256': 32,
    'sha384': 48,
    'sha512': 64
}


#: size of hash digests in bytes, mapped to algoritm names
_size_to_hash = dict((v, k) for k, v in hashes.items())


#: List of deprecated hash functions. On some systems, these cannot be
#: used without special options to hashlib.
_deprecated_hash_algorithms = ['md5']


#: cache of hash functions generated
_hash_functions = {}


class DeprecatedHash(object):
    def __init__(self, hash_alg, alert_fn, disable_security_check):
        self.hash_alg = hash_alg
        self.alert_fn = alert_fn
        self.disable_security_check = disable_security_check

    def __call__(self, disable_alert=False):
        if not disable_alert:
            self.alert_fn("Deprecation warning: {0} checksums will not be"
                          " supported in future Spack releases."
                          .format(self.hash_alg))
        if self.disable_security_check:
            return hashlib.new(self.hash_alg, usedforsecurity=False)
        else:
            return hashlib.new(self.hash_alg)


def hash_fun_for_algo(algo):
    """Get a function that can perform the specified hash algorithm."""
    hash_gen = _hash_functions.get(algo)
    if hash_gen is None:
        if algo in _deprecated_hash_algorithms:
            try:
                hash_gen = DeprecatedHash(
                    algo, tty.debug, disable_security_check=False)

                # call once to get a ValueError if usedforsecurity is needed
                hash_gen(disable_alert=True)
            except ValueError:
                # Some systems may support the 'usedforsecurity' option
                # so try with that (but display a warning when it is used)
                hash_gen = DeprecatedHash(
                    algo, tty.warn, disable_security_check=True)
        else:
            hash_gen = getattr(hashlib, algo)
        _hash_functions[algo] = hash_gen

    return hash_gen


def hash_algo_for_digest(hexdigest):
    """Gets name of the hash algorithm for a hex digest."""
    bytes = len(hexdigest) / 2
    if bytes not in _size_to_hash:
        raise ValueError(
            'Spack knows no hash algorithm for this digest: %s' % hexdigest)
    return _size_to_hash[bytes]


def hash_fun_for_digest(hexdigest):
    """Gets a hash function corresponding to a hex digest."""
    return hash_fun_for_algo(hash_algo_for_digest(hexdigest))


def checksum(hashlib_algo, filename, **kwargs):
    """Returns a hex digest of the filename generated using an
       algorithm from hashlib.
    """
    block_size = kwargs.get('block_size', 2**20)
    hasher = hashlib_algo()
    with open(filename, 'rb') as file:
        while True:
            data = file.read(block_size)
            if not data:
                break
            hasher.update(data)
    return hasher.hexdigest()


class Checker(object):
    """A checker checks files against one particular hex digest.
       It will automatically determine what hashing algorithm
       to used based on the length of the digest it's initialized
       with.  e.g., if the digest is 32 hex characters long this will
       use md5.

       Example: know your tarball should hash to 'abc123'.  You want
       to check files against this.  You would use this class like so::

          hexdigest = 'abc123'
          checker = Checker(hexdigest)
          success = checker.check('downloaded.tar.gz')

       After the call to check, the actual checksum is available in
       checker.sum, in case it's needed for error output.

       You can trade read performance and memory usage by
       adjusting the block_size optional arg.  By default it's
       a 1MB (2**20 bytes) buffer.
    """

    def __init__(self, hexdigest, **kwargs):
        self.block_size = kwargs.get('block_size', 2**20)
        self.hexdigest = hexdigest
        self.sum = None
        self.hash_fun = hash_fun_for_digest(hexdigest)

    @property
    def hash_name(self):
        """Get the name of the hash function this Checker is using."""
        return self.hash_fun().name

    def check(self, filename):
        """Read the file with the specified name and check its checksum
           against self.hexdigest.  Return True if they match, False
           otherwise.  Actual checksum is stored in self.sum.
        """
        self.sum = checksum(
            self.hash_fun, filename, block_size=self.block_size)
        return self.sum == self.hexdigest


def prefix_bits(byte_array, bits):
    """Return the first <bits> bits of a byte array as an integer."""
    if sys.version_info < (3,):
        b2i = ord          # In Python 2, indexing byte_array gives str
    else:
        b2i = lambda b: b  # In Python 3, indexing byte_array gives int

    result = 0
    n = 0
    for i, b in enumerate(byte_array):
        n += 8
        result = (result << 8) | b2i(b)
        if n >= bits:
            break

    result >>= (n - bits)
    return result


def bit_length(num):
    """Number of bits required to represent an integer in binary."""
    s = bin(num)
    s = s.lstrip('-0b')
    return len(s)
