"""
This module contains a function to send a newsletter from HTML content.  

???+ warning "Pre-requisites" 
    To use this module, the following variables must be defined in your .env file:  
    `smtpHost`, `smtpPort`, `smtpKey`, `smtpSecret`  
    See the [.env file configuration][configure-your-env-file] section if needed

To use this module in your script, import the module like so.

```
from djagitit.mailing import smtp
```
"""

import smtplib
from email.message import EmailMessage
from email.headerregistry import Address
import os
from itertools import zip_longest
from dotenv import load_dotenv
load_dotenv()


def _safely_getenv(var):
    """Checks if environment variable is defined before returning it"""
    if not os.getenv(var):
        raise EnvironmentError(f'Environment variable {var} not defined!')

    return os.getenv(var)


def _attach_file(email, file_path):

    with open(file_path, 'rb') as file:
        email.add_attachment(
            file.read(), 
            maintype='application', 
            subtype='octet-stream', 
            filename=os.path.basename(file_path)
        )
    
    return email


def send_newsletter(subject, html, sender, to=None, cc=None, bcc=None, sender_alias=None, reply_to=None, attachments=None):
    """
    Sends a newsletter email from HTML content to a list of recipients.
    
    Args:
        subject (str): The subject of the newsletter email.
        html (str): The HTML content of the newsletter email.
        sender (str): The email address of the sender.
        to (list or str, optional): A list of email addresses or a single email address of the recipients. If omitted, defaults to sender email.
        cc (list or str, optional): A list of email addresses or a single email address of the cc recipients.
        bcc (list or str, optional): A list of email addresses or a single email address of the bcc recipients.
        sender_alias (str, optional): An optional alias for the sender's name.
        reply_to (str, optional): An optional email address to set as the reply-to address.
        attachments (list or str, optional): Optional paths of files to add as attachments.
    
    Returns:
        r (dict): The response from the smtp server. Empty if no issue occured. Contains the issue message otherwise.

    Example:
        ```
        subject = 'The Infamous Newsletter'
        html = '<h1>Check it out now!</h1>'
        sender = 'julien.comte@sonymusic.com'
        to = ['julien.comte@sonymusic.com','matti.karjalainen@sonymusic.com']
        cc = 'alikutay.bengi@sonymusic.com'
        sender_alias = 'CEA Data Team'
        reply_to = 'julien.comte@sonymusic.com'
        attachments = ['path/to/a/cool/spreadsheet.xlsx', 'path/to/another/nice/file.txt']

        smtp.send_newsletter(subject, html, sender, diffusion_list, sender_alias, reply_to, attachments)
        ```
    """
    # load smtp credentials
    smtp_server = _safely_getenv('smtpHost')
    smtp_port = _safely_getenv('smtpPort')
    username = _safely_getenv('smtpKey')
    password = _safely_getenv('smtpSecret')

    email = EmailMessage()
    email['Subject'] = subject
    if sender_alias:
        email['From'] = Address(display_name=sender_alias, addr_spec=sender)
    else:
        email['From'] = sender
    if reply_to:
        email['Reply-To'] = reply_to

    # If recipients not defined, send to yourself
    if not to:
         to = [sender]
    # if recipients is a single email address, convert it to a list
    if not isinstance(to, list):
            to = [to]
    if cc and not isinstance(cc, list):
            cc = [cc]
    if bcc and not isinstance(bcc, list):
            bcc = [bcc]


    # if attachment path is a string, convert it to a list
    if attachments and not isinstance(attachments, list):
            attachments = [attachments]

    email.set_content('This is a plain text body.')
    email.add_alternative(html, subtype = 'html')

    if attachments:
         for a in attachments:
            email = _attach_file(email, a)

    max_recipients = 49
    to_lists = [to[i:i+max_recipients] for i in range(0, len(to), max_recipients)]
    cc_lists = [cc[i:i+max_recipients] for i in range(0, len(cc), max_recipients)] if cc else [None]
    bcc_lists = [bcc[i:i+max_recipients] for i in range(0, len(bcc), max_recipients)] if bcc else [None]

    for t, c, b in zip_longest(to_lists, cc_lists, bcc_lists):
        email['To'] = to
        if c:
            email['Cc'] = c
        if b:
            email['Bcc'] = b
        try: 
            with smtplib.SMTP(host=smtp_server, port=smtp_port,) as server:
                server.starttls()
                server.login(username, password)
                r = server.send_message(email)
                server.quit()
        except smtplib.SMTPResponseException as e:
            error_code = e.smtp_code
            error_message = e.smtp_error
            r = {error_code: error_message}
            print(f'{error_code}: {error_message}')
        
    return r