import re
import enum
import sqlite3

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('^([^:]+): (.*)$', 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 MBoxMessageState(enum.Enum):
    EMPTY  = 0
    HEADER = 1
    BODY   = 2

class MBoxMessage():
    __slots__ = 'state', 'headers', 'line', 'body', 'key',

    def __init__(self):
        self.state   = MBoxMessageState.EMPTY
        self.headers = dict()
        self.line    = None
        self.body    = ''
        self.key     = None

    def add(self, line: str):
        if self.state is MBoxMessageState.EMPTY:
            self.state = MBoxMessageState.HEADER

        if self.state is MBoxMessageState.HEADER:
            if line == '\n' or line == '\r\n':
                self.state = MBoxMessageState.BODY
                self.body  = ''
            elif line[0] == ' ' or line[0] == '\t':
                self.headers[self.key] += ' ' + line.strip()
            else:
                match = re.match('^([^:]+): (.*)$', line)

                if match:
                    self.key = match[1]

                    self.headers[self.key] = match[2].rstrip()
        elif self.state is MBoxMessageState.BODY:
            if self.line is None:
                self.body = line
            else:
                self.body += self.line

            self.line = line

    def is_first_line(self):
        return len(self.headers) == 1 and self.body == ''

    @staticmethod
    def each_line(text: str):
        start = 0
        end   = len(text)

        while True:
            try:
                index = text.index('\n', start, end)

                yield text[start:index+1]

                start = index + 1

                if start == end:
                    break
            except ValueError:
                yield text[start:end]
                break

    @staticmethod
    def parse(text: str):
        message = MBoxMessage()

        for line in MBoxMessage.each_line(text):
            message.add(line)

        return message

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 = MBoxMessage()
                else:
                    ret = self.message

                    self.message = MBoxMessage()
                    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