import struct
from hashlib import md5
from typing import Union

from pyhanko.pdf_utils import generic
from pyhanko.pdf_utils.crypt._util import rc4_encrypt

_encryption_padding = (
    b'\x28\xbf\x4e\x5e\x4e\x75\x8a\x41\x64\x00\x4e\x56'
    b'\xff\xfa\x01\x08\x2e\x2e\x00\xb6\xd0\x68\x3e\x80\x2f\x0c'
    b'\xa9\xfe\x64\x53\x69\x7a'
)


def derive_legacy_file_key(
    password, rev, keylen, owner_entry, p_entry, id1_entry, encrypt_metadata
):
    """
    Implementation of algorithm 3.2 of the PDF standard security handler,
    section 3.5.2 of the PDF 1.6 reference.
    """

    # 1. Pad or truncate the password string to exactly 32 bytes.  If the
    # password string is more than 32 bytes long, use only its first 32 bytes;
    # if it is less than 32 bytes long, pad it by appending the required number
    # of additional bytes from the beginning of the padding string
    # (_encryption_padding).
    password = (password + _encryption_padding)[:32]
    # 2. Initialize the MD5 hash function and pass the result of step 1 as
    # input to this function.
    # NOTE: Suppress LGTM warning here, we have to do what the spec says
    m = md5(password)  # lgtm
    # 3. Pass the value of the encryption dictionary's /O entry to the MD5 hash
    # function.
    m.update(owner_entry)
    # 4. Treat the value of the /P entry as an unsigned 4-byte integer and pass
    # these bytes to the MD5 hash function, low-order byte first.
    p_entry = struct.pack('<i', p_entry)
    m.update(p_entry)
    # 5. Pass the first element of the file's file identifier array to the MD5
    # hash function.
    m.update(id1_entry)
    # 6. (Revision 4 or greater) If document metadata is not being encrypted,
    # pass 4 bytes with the value 0xFFFFFFFF to the MD5 hash function.
    if rev >= 4 and not encrypt_metadata:
        m.update(b"\xff\xff\xff\xff")
    # 7. Finish the hash.
    md5_hash = m.digest()
    # 8. (Revision 3 or greater) Do the following 50 times: Take the output
    # from the previous MD5 hash and pass the first n bytes of the output as
    # input into a new MD5 hash, where n is the number of bytes of the
    # encryption key as defined by the value of the encryption dictionary's
    # /Length entry.
    if rev >= 3:
        for i in range(50):
            md5_hash = md5(md5_hash[:keylen]).digest()
    # 9. Set the encryption key to the first n bytes of the output from the
    # final MD5 hash, where n is always 5 for revision 2 but, for revision 3 or
    # greater, depends on the value of the encryption dictionary's /Length
    # entry.
    return md5_hash[:keylen]


def legacy_normalise_pw(password: Union[str, bytes]) -> bytes:
    if isinstance(password, str):
        return generic.encode_pdfdocencoding(password[:32])
    else:
        return password[:32]


def compute_o_value_legacy(owner_pwd, user_pwd, rev, keylen):
    """
    Implementation of algorithm 3.3 of the PDF standard security handler,
    section 3.5.2 of the PDF 1.6 reference.
    """

    # steps 1 - 4
    key = compute_o_value_legacy_prep(owner_pwd, rev, keylen)
    # 5. Pad or truncate the user password string as described in step 1 of
    # algorithm 3.2.
    user_pwd = (user_pwd + _encryption_padding)[:32]
    # 6. Encrypt the result of step 5, using an RC4 encryption function with
    # the encryption key obtained in step 4.
    val = rc4_encrypt(key, user_pwd)
    # 7. (Revision 3 or greater) Do the following 19 times: Take the output
    # from the previous invocation of the RC4 function and pass it as input to
    # a new invocation of the function; use an encryption key generated by
    # taking each byte of the encryption key obtained in step 4 and performing
    # an XOR operation between that byte and the single-byte value of the
    # iteration counter (from 1 to 19).
    if rev >= 3:
        for i in range(1, 20):
            new_key = bytes(b ^ i for b in key)
            val = rc4_encrypt(new_key, val)
    # 8. Store the output from the final invocation of the RC4 as the value of
    # the /O entry in the encryption dictionary.
    return val


def compute_o_value_legacy_prep(password, rev, keylen):
    """
    Steps 1-4 of algorithm 3.3
    """
    # 1. Pad or truncate the owner password string as described in step 1 of
    # algorithm 3.2.  If there is no owner password, use the user password
    # instead.
    password = (password + _encryption_padding)[:32]
    # 2. Initialize the MD5 hash function and pass the result of step 1 as
    # input to this function.
    # NOTE: Suppress LGTM warning here, we have to do what the spec says
    m = md5(password)  # lgtm
    # 3. (Revision 3 or greater) Do the following 50 times: Take the output
    # from the previous MD5 hash and pass it as input into a new MD5 hash.
    md5_hash = m.digest()
    if rev >= 3:
        for i in range(50):
            md5_hash = md5(md5_hash).digest()
    # 4. Create an RC4 encryption key using the first n bytes of the output
    # from the final MD5 hash, where n is always 5 for revision 2 but, for
    # revision 3 or greater, depends on the value of the encryption
    # dictionary's /Length entry.
    key = md5_hash[:keylen]
    return key


def compute_u_value_r2(password, owner_entry, p_entry, id1_entry):
    """
    Implementation of algorithm 3.4 of the PDF standard security handler,
    section 3.5.2 of the PDF 1.6 reference.
    """
    # 1. Create an encryption key based on the user password string, as
    # described in algorithm 3.2.
    key = derive_legacy_file_key(
        password, 2, 5, owner_entry, p_entry, id1_entry, encrypt_metadata=True
    )
    # 2. Encrypt the 32-byte padding string shown in step 1 of algorithm 3.2,
    # using an RC4 encryption function with the encryption key from the
    # preceding step.
    u = rc4_encrypt(key, _encryption_padding)
    # 3. Store the result of step 2 as the value of the /U entry in the
    # encryption dictionary.
    return u, key


def compute_u_value_r34(
    password,
    rev,
    keylen,
    owner_entry,
    p_entry,
    id1_entry,
    encrypt_metadata: bool,
):
    """
    Implementation of algorithm 3.4 of the PDF standard security handler,
    section 3.5.2 of the PDF 1.6 reference.
    """

    # 1. Create an encryption key based on the user password string, as
    # described in Algorithm 3.2.
    key = derive_legacy_file_key(
        password,
        rev,
        keylen,
        owner_entry,
        p_entry,
        id1_entry,
        encrypt_metadata=encrypt_metadata,
    )
    # 2. Initialize the MD5 hash function and pass the 32-byte padding string
    # shown in step 1 of Algorithm 3.2 as input to this function.
    m = md5()
    m.update(_encryption_padding)
    # 3. Pass the first element of the file's file identifier array (the value
    # of the ID entry in the document's trailer dictionary; see Table 3.13 on
    # page 73) to the hash function and finish the hash.  (See implementation
    # note 25 in Appendix H.)
    m.update(id1_entry)
    md5_hash = m.digest()
    # 4. Encrypt the 16-byte result of the hash, using an RC4 encryption
    # function with the encryption key from step 1.
    val = rc4_encrypt(key, md5_hash)
    # 5. Do the following 19 times: Take the output from the previous
    # invocation of the RC4 function and pass it as input to a new invocation
    # of the function; use an encryption key generated by taking each byte of
    # the original encryption key (obtained in step 2) and performing an XOR
    # operation between that byte and the single-byte value of the iteration
    # counter (from 1 to 19).
    for i in range(1, 20):
        new_key = bytes(b ^ i for b in key)
        val = rc4_encrypt(new_key, val)
    # 6. Append 16 bytes of arbitrary padding to the output from the final
    # invocation of the RC4 function and store the 32-byte result as the value
    # of the U entry in the encryption dictionary.
    # (implementer note: I don't know what "arbitrary padding" is supposed to
    # mean, so I have used null bytes.  This seems to match a few other
    # people's implementations)
    return val + (b'\x00' * 16), key


def legacy_derive_object_key(
    shared_key: bytes, idnum: int, generation: int, use_aes=False
) -> bytes:
    """
    Function that does the key derivation for PDF's legacy security handlers.

    :param shared_key:
        Global file encryption key.
    :param idnum:
        ID of the object being written.
    :param generation:
        Generation number of the object being written.
    :param use_aes:
        Boolean indicating whether the security handler uses RC4 or AES(-128).
    :return:
    """
    pack1 = struct.pack("<i", idnum)[:3]
    pack2 = struct.pack("<i", generation)[:2]
    key = shared_key + pack1 + pack2
    assert len(key) == (len(shared_key) + 5)
    if use_aes:
        key += b'sAlT'
    md5_hash = md5(key).digest()
    return md5_hash[: min(16, len(shared_key) + 5)]