"""A readline()-style interface to the parts of a multipart message.

The MultiFile class makes each part of a multipart message "feel" like
an ordinary file, as long as you use fp.readline().  Allows recursive
use, for nested multipart messages.  Probably best used together
with module mimetools.

Suggested use:

real_fp = open(...)
fp = MultiFile(real_fp)

"read some lines from fp"
fp.push(separator)
while 1:
        "read lines from fp until it returns an empty string" (A)
        if not fp.next(): break
fp.pop()
"read remaining lines from fp until it returns an empty string"

The latter sequence may be used recursively at (A).
It is also allowed to use multiple push()...pop() sequences.

If seekable is given as 0, the class code will not do the bookkeeping
it normally attempts in order to make seeks relative to the beginning of the
current file part.  This may be useful when using MultiFile with a non-
seekable stream object.
"""

__all__ = ["MultiFile","Error"]

class Error(Exception):
    pass

class MultiFile:

    seekable = 0

    def __init__(self, fp, seekable=1):
        self.fp = fp
        self.stack = []
        self.level = 0
        self.last = 0
        if seekable:
            self.seekable = 1
            self.start = self.fp.tell()
            self.posstack = []

    def tell(self):
        if self.level > 0:
            return self.lastpos
        return self.fp.tell() - self.start

    def seek(self, pos, whence=0):
        here = self.tell()
        if whence:
            if whence == 1:
                pos = pos + here
            elif whence == 2:
                if self.level > 0:
                    pos = pos + self.lastpos
                else:
                    raise Error("can't use whence=2 yet")
        if not 0 <= pos <= here or \
                        self.level > 0 and pos > self.lastpos:
            raise Error('bad MultiFile.seek() call')
        self.fp.seek(pos + self.start)
        self.level = 0
        self.last = 0

    def readline(self):
        if self.level > 0:
            return ''
        line = self.fp.readline()
        # Real EOF?
        if not line:
            self.level = len(self.stack)
            self.last = (self.level > 0)
            if self.last:
                raise Error('sudden EOF in MultiFile.readline()')
            return ''
        assert self.level == 0
        # Fast check to see if this is just data
        if self.is_data(line):
            return line
        else:
            # Ignore trailing whitespace on marker lines
            marker = line.rstrip()
        # No?  OK, try to match a boundary.
        # Return the line (unstripped) if we don't.
        for i, sep in enumerate(reversed(self.stack)):
            if marker == self.section_divider(sep):
                self.last = 0
                break
            elif marker == self.end_marker(sep):
                self.last = 1
                break
        else:
            return line
        # We only get here if we see a section divider or EOM line
        if self.seekable:
            self.lastpos = self.tell() - len(line)
        self.level = i+1
        if self.level > 1:
            raise Error('Missing endmarker in MultiFile.readline()')
        return ''

    def readlines(self):
        list = []
        while 1:
            line = self.readline()
            if not line: break
            list.append(line)
        return list

    def read(self): # Note: no size argument -- read until EOF only!
        return ''.join(self.readlines())

    def next(self):
        while self.readline(): pass
        if self.level > 1 or self.last:
            return 0
        self.level = 0
        self.last = 0
        if self.seekable:
            self.start = self.fp.tell()
        return 1

    def push(self, sep):
        if self.level > 0:
            raise Error('bad MultiFile.push() call')
        self.stack.append(sep)
        if self.seekable:
            self.posstack.append(self.start)
            self.start = self.fp.tell()

    def pop(self):
        if self.stack == []:
            raise Error('bad MultiFile.pop() call')
        if self.level <= 1:
            self.last = 0
        else:
            abslastpos = self.lastpos + self.start
        self.level = max(0, self.level - 1)
        self.stack.pop()
        if self.seekable:
            self.start = self.posstack.pop()
            if self.level > 0:
                self.lastpos = abslastpos - self.start

    def is_data(self, line):
        return line[:2] != '--'

    def section_divider(self, str):
        return "--" + str

    def end_marker(self, str):
        return "--" + str + "--"