import re

from nntp.tiny.message import Message

class MBoxReaderError(Exception):
    pass

class MBoxReaderBuffer():
    def __init__(self):
        self.lines = [None, None, None, None]
        self.index = None

    def add(self, line: str):
        if self.lines[0] is None:
            self.lines[0] = line
            self.index = 0
        elif self.lines[1] is None:
            self.lines[1] = line
            self.index = 1
        elif self.lines[2] is None:
            self.lines[2] = line
            self.index = 2
        elif self.lines[3] is None:
            self.lines[3] = line
            self.index = 3
        else:
            self.lines[0] = self.lines[1]
            self.lines[1] = self.lines[2]
            self.lines[2] = self.lines[3]
            self.lines[3] = line
            self.index = 3

        if self.index is None:
            self.index = 0
        elif self.index < 3:
            self.index += 1

    def is_empty_line(self, line):
        return self.lines[line] == '\n'

    def is_from_line(self, line):
        return self.lines[line][0:5] == 'From '

    def is_header_line(self, line):
        return re.match(Message.HEADER_REGEX, self.lines[line]) is not None

    def is_start(self):
        if self.lines[0] is None or self.lines[1] is None:
            return
            
        if self.is_from_line(0) and self.is_header_line(1):
            pass
        elif self.is_empty_line(0) \
         and self.is_empty_line(1) \
         and self.is_from_line(2) \
         and self.is_header_line(3):
            return 3

class MBoxReader():
    __slots__ = 'path', 'fh', 'line', 'buf', 'message',

    def __init__(self, path: str):
        self.path    = path
        self.fh      = open(path, 'r', newline='')
        self.line    = 0
        self.buf     = MBoxReaderBuffer()
        self.message = None

    def get_message(self):
        while True:
            line = self.fh.readline()

            if line is None or line == '':
                ret = self.message

                self.message = None

                return ret

            self.line += 1

            self.buf.add(line)

            if self.buf.is_start():
                if self.message is None:
                    self.message = Message()
                else:
                    ret = self.message

                    self.message = Message()
                    self.message.add(line)

                    return ret

            if self.message:
                self.message.add(line)

    def messages(self):
        while True:
            message = self.get_message()

            if message is None:
                break

            yield message