##############################################################################
#
# Copyright (c) 2002 Zope Foundation and Contributors.
#
# This software is subject to the provisions of the Zope Public License,
# Version 2.1 (ZPL).  A copy of the ZPL should accompany this distribution.
# THIS SOFTWARE IS PROVIDED "AS IS" AND ANY AND ALL EXPRESS OR IMPLIED
# WARRANTIES ARE DISCLAIMED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
# WARRANTIES OF TITLE, MERCHANTABILITY, AGAINST INFRINGEMENT, AND FITNESS
# FOR A PARTICULAR PURPOSE
#
##############################################################################
"""HTTP Range support utilities.

The RFC 2616 specification defines the 'Range' and 'If-Range' headers for
enabeling partial download of published resources. This module provides a
flag-interface and some support functions for implementing this functionality.

For an implementation example, see the File class in OFS/Image.py.
"""

import re
import sys

from zope.interface import Interface


WHITESPACE = re.compile(r'\s*', re.MULTILINE)


def parseRange(header):
    """RFC 2616 (HTTP 1.1) Range header parsing.

    Convert a range header to a list of slice indexes, returned as (start, end)
    tuples. If no end was given, end is None. Note that the RFC specifies the
    end offset to be inclusive, we return python convention indexes, where the
    end is exclusive. Syntactically incorrect headers are to be ignored, so if
    we encounter one we return None.
    """

    ranges = []
    add = ranges.append

    # First, clean out *all* whitespace. This is slightly more tolerant
    # than the spec asks for, but hey, it makes this function much easier.
    header = WHITESPACE.sub('', header)

    # A range header only can specify a byte range
    try:
        spec, sets = header.split('=')
    except ValueError:
        return None

    if spec != 'bytes':
        return None

    # The sets are delimited by commas.
    sets = sets.split(',')
    # Filter out empty values, things like ',,' are allowed in the spec
    sets = [_set for _set in sets if _set]
    # We need at least one set
    if not sets:
        return None

    for set in sets:
        try:
            start, end = set.split('-')
        except ValueError:
            return None

        # Catch empty sets
        if not start and not end:
            return None

        # Convert to integers or None (which will raise errors if
        # non-integers were used (which is what we want)).
        try:
            if start == '':
                start = None
            else:
                start = int(start)
            if end == '':
                end = None
            else:
                end = int(end)
        except ValueError:
            return None

        # Special case: No start means the suffix format was used, which
        # means the end value is actually a negative start value.
        # Convert this by making it absolute.
        # A -0 range is converted to sys.maxsize, which will result in a
        # Unsatisfiable response if no other ranges can by satisfied either.
        if start is None:
            start, end = -end, None
            if not start:
                start = sys.maxsize
        elif end is not None:
            end = end + 1  # Make the end of the range exclusive

        if end is not None and end <= start:
            return None

        # And store
        add((start, end))

    return ranges


def expandRanges(ranges, size):
    """Expand Range sets, given those sets and the length of the resource.

    Expansion means relative start values and open ends
    """

    expanded = []
    add = expanded.append
    for start, end in ranges:
        if start < 0:
            start = size + start
        end = end or size
        if end > size:
            end = size
        # Only use satisfiable ranges
        if start < size:
            add((start, end))

    return expanded


class HTTPRangeInterface(Interface):
    """Objects implementing this Interface support the HTTP Range header.

    Objects implementing support for the HTTP Range header will return partial
    content as specified in RFC 2616. Note that the'If-Range' header must
    either be implemented correctly or result in a normal '200 OK' response at
    all times.

    This interface specifies no methods, as this functionality can either be
    implemented in the index_html or __call__ methods of a published object.
    """
