# -*- coding: utf-8 -*-
#
# Copyright (C) 2007-2009 Edgewall Software
# All rights reserved.
#
# This software is licensed as described in the file COPYING, which
# you should have received as part of this distribution. The terms
# are also available at http://genshi.edgewall.org/wiki/License.
#
# This software consists of voluntary contributions made by many
# individuals. For the exact contribution history, see the revision
# history and logs, available at http://genshi.edgewall.org/log/.

"""A filter for functional-style transformations of markup streams.

The `Transformer` filter provides a variety of transformations that can be
applied to parts of streams that match given XPath expressions. These
transformations can be chained to achieve results that would be comparitively
tedious to achieve by writing stream filters by hand. The approach of chaining
node selection and transformation has been inspired by the `jQuery`_ Javascript
library.

 .. _`jQuery`: http://jquery.com/

For example, the following transformation removes the ``<title>`` element from
the ``<head>`` of the input document:

>>> from genshi.builder import tag
>>> html = HTML('''<html>
...  <head><title>Some Title</title></head>
...  <body>
...    Some <em>body</em> text.
...  </body>
... </html>''',
... encoding='utf-8')
>>> print(html | Transformer('body/em').map(unicode.upper, TEXT)
...                                    .unwrap().wrap(tag.u))
<html>
  <head><title>Some Title</title></head>
  <body>
    Some <u>BODY</u> text.
  </body>
</html>

The ``Transformer`` support a large number of useful transformations out of the
box, but custom transformations can be added easily.

:since: version 0.5
"""

import re
import sys

from genshi.builder import Element
from genshi.core import Stream, Attrs, QName, TEXT, START, END, _ensure, Markup
from genshi.path import Path

__all__ = ['Transformer', 'StreamBuffer', 'InjectorTransformation', 'ENTER',
           'EXIT', 'INSIDE', 'OUTSIDE', 'BREAK']


class TransformMark(str):
    """A mark on a transformation stream."""
    __slots__ = []
    _instances = {}

    def __new__(cls, val):
        return cls._instances.setdefault(val, str.__new__(cls, val))


ENTER = TransformMark('ENTER')
"""Stream augmentation mark indicating that a selected element is being
entered."""

INSIDE = TransformMark('INSIDE')
"""Stream augmentation mark indicating that processing is currently inside a
selected element."""

OUTSIDE = TransformMark('OUTSIDE')
"""Stream augmentation mark indicating that a match occurred outside a selected
element."""

ATTR = TransformMark('ATTR')
"""Stream augmentation mark indicating a selected element attribute."""

EXIT = TransformMark('EXIT')
"""Stream augmentation mark indicating that a selected element is being
exited."""

BREAK = TransformMark('BREAK')
"""Stream augmentation mark indicating a break between two otherwise contiguous
blocks of marked events.

This is used primarily by the cut() transform to provide later transforms with
an opportunity to operate on the cut buffer.
"""


class PushBackStream(object):
    """Allows a single event to be pushed back onto the stream and re-consumed.
    """
    def __init__(self, stream):
        self.stream = iter(stream)
        self.peek = None

    def push(self, event):
        assert self.peek is None
        self.peek = event

    def __iter__(self):
        while True:
            if self.peek is not None:
                peek = self.peek
                self.peek = None
                yield peek
            else:
                try:
                    event = self.stream.next()
                    yield event
                except StopIteration:
                    if self.peek is None:
                        raise


class Transformer(object):
    """Stream filter that can apply a variety of different transformations to
    a stream.

    This is achieved by selecting the events to be transformed using XPath,
    then applying the transformations to the events matched by the path
    expression. Each marked event is in the form (mark, (kind, data, pos)),
    where mark can be any of `ENTER`, `INSIDE`, `EXIT`, `OUTSIDE`, or `None`.

    The first three marks match `START` and `END` events, and any events
    contained `INSIDE` any selected XML/HTML element. A non-element match
    outside a `START`/`END` container (e.g. ``text()``) will yield an `OUTSIDE`
    mark.

    >>> html = HTML('<html><head><title>Some Title</title></head>'
    ...             '<body>Some <em>body</em> text.</body></html>',
    ...             encoding='utf-8')

    Transformations act on selected stream events matching an XPath expression.
    Here's an example of removing some markup (the title, in this case)
    selected by an expression:

    >>> print(html | Transformer('head/title').remove())
    <html><head/><body>Some <em>body</em> text.</body></html>

    Inserted content can be passed in the form of a string, or a markup event
    stream, which includes streams generated programmatically via the
    `builder` module:

    >>> from genshi.builder import tag
    >>> print(html | Transformer('body').prepend(tag.h1('Document Title')))
    <html><head><title>Some Title</title></head><body><h1>Document
    Title</h1>Some <em>body</em> text.</body></html>

    Each XPath expression determines the set of tags that will be acted upon by
    subsequent transformations. In this example we select the ``<title>`` text,
    copy it into a buffer, then select the ``<body>`` element and paste the
    copied text into the body as ``<h1>`` enclosed text:

    >>> buffer = StreamBuffer()
    >>> print(html | Transformer('head/title/text()').copy(buffer)
    ...     .end().select('body').prepend(tag.h1(buffer)))
    <html><head><title>Some Title</title></head><body><h1>Some Title</h1>Some
    <em>body</em> text.</body></html>

    Transformations can also be assigned and reused, although care must be
    taken when using buffers, to ensure that buffers are cleared between
    transforms:

    >>> emphasis = Transformer('body//em').attr('class', 'emphasis')
    >>> print(html | emphasis)
    <html><head><title>Some Title</title></head><body>Some <em
    class="emphasis">body</em> text.</body></html>
    """

    __slots__ = ['transforms']

    def __init__(self, path='.'):
        """Construct a new transformation filter.

        :param path: an XPath expression (as string) or a `Path` instance
        """
        self.transforms = [SelectTransformation(path)]

    def __call__(self, stream, keep_marks=False):
        """Apply the transform filter to the marked stream.

        :param stream: the marked event stream to filter
        :param keep_marks: Do not strip transformer selection marks from the
                           stream. Useful for testing.
        :return: the transformed stream
        :rtype: `Stream`
        """
        transforms = self._mark(stream)
        for link in self.transforms:
            transforms = link(transforms)
        if not keep_marks:
            transforms = self._unmark(transforms)
        return Stream(transforms,
                      serializer=getattr(stream, 'serializer', None))

    def apply(self, function):
        """Apply a transformation to the stream.

        Transformations can be chained, similar to stream filters. Any callable
        accepting a marked stream can be used as a transform.

        As an example, here is a simple `TEXT` event upper-casing transform:

        >>> def upper(stream):
        ...     for mark, (kind, data, pos) in stream:
        ...         if mark and kind is TEXT:
        ...             yield mark, (kind, data.upper(), pos)
        ...         else:
        ...             yield mark, (kind, data, pos)
        >>> short_stream = HTML('<body>Some <em>test</em> text</body>',
        ...                      encoding='utf-8')
        >>> print(short_stream | Transformer('.//em/text()').apply(upper))
        <body>Some <em>TEST</em> text</body>
        """
        transformer = Transformer()
        transformer.transforms = self.transforms[:]
        if isinstance(function, Transformer):
            transformer.transforms.extend(function.transforms)
        else:
            transformer.transforms.append(function)
        return transformer

    #{ Selection operations

    def select(self, path):
        """Mark events matching the given XPath expression, within the current
        selection.

        >>> html = HTML('<body>Some <em>test</em> text</body>', encoding='utf-8')
        >>> print(html | Transformer().select('.//em').trace())
        (None, ('START', (QName('body'), Attrs()), (None, 1, 0)))
        (None, ('TEXT', u'Some ', (None, 1, 6)))
        ('ENTER', ('START', (QName('em'), Attrs()), (None, 1, 11)))
        ('INSIDE', ('TEXT', u'test', (None, 1, 15)))
        ('EXIT', ('END', QName('em'), (None, 1, 19)))
        (None, ('TEXT', u' text', (None, 1, 24)))
        (None, ('END', QName('body'), (None, 1, 29)))
        <body>Some <em>test</em> text</body>

        :param path: an XPath expression (as string) or a `Path` instance
        :return: the stream augmented by transformation marks
        :rtype: `Transformer`
        """
        return self.apply(SelectTransformation(path))

    def invert(self):
        """Invert selection so that marked events become unmarked, and vice
        versa.

        Specificaly, all marks are converted to null marks, and all null marks
        are converted to OUTSIDE marks.

        >>> html = HTML('<body>Some <em>test</em> text</body>', encoding='utf-8')
        >>> print(html | Transformer('//em').invert().trace())
        ('OUTSIDE', ('START', (QName('body'), Attrs()), (None, 1, 0)))
        ('OUTSIDE', ('TEXT', u'Some ', (None, 1, 6)))
        (None, ('START', (QName('em'), Attrs()), (None, 1, 11)))
        (None, ('TEXT', u'test', (None, 1, 15)))
        (None, ('END', QName('em'), (None, 1, 19)))
        ('OUTSIDE', ('TEXT', u' text', (None, 1, 24)))
        ('OUTSIDE', ('END', QName('body'), (None, 1, 29)))
        <body>Some <em>test</em> text</body>

        :rtype: `Transformer`
        """
        return self.apply(InvertTransformation())

    def end(self):
        """End current selection, allowing all events to be selected.

        Example:

        >>> html = HTML('<body>Some <em>test</em> text</body>', encoding='utf-8')
        >>> print(html | Transformer('//em').end().trace())
        ('OUTSIDE', ('START', (QName('body'), Attrs()), (None, 1, 0)))
        ('OUTSIDE', ('TEXT', u'Some ', (None, 1, 6)))
        ('OUTSIDE', ('START', (QName('em'), Attrs()), (None, 1, 11)))
        ('OUTSIDE', ('TEXT', u'test', (None, 1, 15)))
        ('OUTSIDE', ('END', QName('em'), (None, 1, 19)))
        ('OUTSIDE', ('TEXT', u' text', (None, 1, 24)))
        ('OUTSIDE', ('END', QName('body'), (None, 1, 29)))
        <body>Some <em>test</em> text</body>

        :return: the stream augmented by transformation marks
        :rtype: `Transformer`
        """
        return self.apply(EndTransformation())

    #{ Deletion operations

    def empty(self):
        """Empty selected elements of all content.

        Example:

        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body>Some <em>body</em> text.</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('.//em').empty())
        <html><head><title>Some Title</title></head><body>Some <em/>
        text.</body></html>

        :rtype: `Transformer`
        """
        return self.apply(EmptyTransformation())

    def remove(self):
        """Remove selection from the stream.

        Example:

        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body>Some <em>body</em> text.</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('.//em').remove())
        <html><head><title>Some Title</title></head><body>Some
        text.</body></html>

        :rtype: `Transformer`
        """
        return self.apply(RemoveTransformation())

    #{ Direct element operations

    def unwrap(self):
        """Remove outermost enclosing elements from selection.

        Example:

        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body>Some <em>body</em> text.</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('.//em').unwrap())
        <html><head><title>Some Title</title></head><body>Some body
        text.</body></html>

        :rtype: `Transformer`
        """
        return self.apply(UnwrapTransformation())

    def wrap(self, element):
        """Wrap selection in an element.

        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body>Some <em>body</em> text.</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('.//em').wrap('strong'))
        <html><head><title>Some Title</title></head><body>Some
        <strong><em>body</em></strong> text.</body></html>

        :param element: either a tag name (as string) or an `Element` object
        :rtype: `Transformer`
        """
        return self.apply(WrapTransformation(element))

    #{ Content insertion operations

    def replace(self, content):
        """Replace selection with content.

        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body>Some <em>body</em> text.</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('.//title/text()').replace('New Title'))
        <html><head><title>New Title</title></head><body>Some <em>body</em>
        text.</body></html>

        :param content: Either a callable, an iterable of events, or a string
                        to insert.
        :rtype: `Transformer`
        """
        return self.apply(ReplaceTransformation(content))

    def before(self, content):
        """Insert content before selection.

        In this example we insert the word 'emphasised' before the <em> opening
        tag:

        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body>Some <em>body</em> text.</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('.//em').before('emphasised '))
        <html><head><title>Some Title</title></head><body>Some emphasised
        <em>body</em> text.</body></html>

        :param content: Either a callable, an iterable of events, or a string
                        to insert.
        :rtype: `Transformer`
        """
        return self.apply(BeforeTransformation(content))

    def after(self, content):
        """Insert content after selection.

        Here, we insert some text after the </em> closing tag:

        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body>Some <em>body</em> text.</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('.//em').after(' rock'))
        <html><head><title>Some Title</title></head><body>Some <em>body</em>
        rock text.</body></html>

        :param content: Either a callable, an iterable of events, or a string
                        to insert.
        :rtype: `Transformer`
        """
        return self.apply(AfterTransformation(content))

    def prepend(self, content):
        """Insert content after the ENTER event of the selection.

        Inserting some new text at the start of the <body>:

        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body>Some <em>body</em> text.</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('.//body').prepend('Some new body text. '))
        <html><head><title>Some Title</title></head><body>Some new body text.
        Some <em>body</em> text.</body></html>

        :param content: Either a callable, an iterable of events, or a string
                        to insert.
        :rtype: `Transformer`
        """
        return self.apply(PrependTransformation(content))

    def append(self, content):
        """Insert content before the END event of the selection.

        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body>Some <em>body</em> text.</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('.//body').append(' Some new body text.'))
        <html><head><title>Some Title</title></head><body>Some <em>body</em>
        text. Some new body text.</body></html>

        :param content: Either a callable, an iterable of events, or a string
                        to insert.
        :rtype: `Transformer`
        """
        return self.apply(AppendTransformation(content))

    #{ Attribute manipulation

    def attr(self, name, value):
        """Add, replace or delete an attribute on selected elements.

        If `value` evaulates to `None` the attribute will be deleted from the
        element:

        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body>Some <em class="before">body</em> <em>text</em>.</body>'
        ...             '</html>', encoding='utf-8')
        >>> print(html | Transformer('body/em').attr('class', None))
        <html><head><title>Some Title</title></head><body>Some <em>body</em>
        <em>text</em>.</body></html>

        Otherwise the attribute will be set to `value`:

        >>> print(html | Transformer('body/em').attr('class', 'emphasis'))
        <html><head><title>Some Title</title></head><body>Some <em
        class="emphasis">body</em> <em class="emphasis">text</em>.</body></html>

        If `value` is a callable it will be called with the attribute name and
        the `START` event for the matching element. Its return value will then
        be used to set the attribute:

        >>> def print_attr(name, event):
        ...     attrs = event[1][1]
        ...     print(attrs)
        ...     return attrs.get(name)
        >>> print(html | Transformer('body/em').attr('class', print_attr))
        Attrs([(QName('class'), u'before')])
        Attrs()
        <html><head><title>Some Title</title></head><body>Some <em
        class="before">body</em> <em>text</em>.</body></html>

        :param name: the name of the attribute
        :param value: the value that should be set for the attribute.
        :rtype: `Transformer`
        """
        return self.apply(AttrTransformation(name, value))

    #{ Buffer operations

    def copy(self, buffer, accumulate=False):
        """Copy selection into buffer.

        The buffer is replaced by each *contiguous* selection before being passed
        to the next transformation. If accumulate=True, further selections will
        be appended to the buffer rather than replacing it.

        >>> from genshi.builder import tag
        >>> buffer = StreamBuffer()
        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body>Some <em>body</em> text.</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('head/title/text()').copy(buffer)
        ...     .end().select('body').prepend(tag.h1(buffer)))
        <html><head><title>Some Title</title></head><body><h1>Some
        Title</h1>Some <em>body</em> text.</body></html>

        This example illustrates that only a single contiguous selection will
        be buffered:

        >>> print(html | Transformer('head/title/text()').copy(buffer)
        ...     .end().select('body/em').copy(buffer).end().select('body')
        ...     .prepend(tag.h1(buffer)))
        <html><head><title>Some Title</title></head><body><h1>Some
        Title</h1>Some <em>body</em> text.</body></html>
        >>> print(buffer)
        <em>body</em>

        Element attributes can also be copied for later use:

        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body><em>Some</em> <em class="before">body</em>'
        ...             '<em>text</em>.</body></html>',
        ...             encoding='utf-8')
        >>> buffer = StreamBuffer()
        >>> def apply_attr(name, entry):
        ...     return list(buffer)[0][1][1].get('class')
        >>> print(html | Transformer('body/em[@class]/@class').copy(buffer)
        ...     .end().buffer().select('body/em[not(@class)]')
        ...     .attr('class', apply_attr))
        <html><head><title>Some Title</title></head><body><em
        class="before">Some</em> <em class="before">body</em><em
        class="before">text</em>.</body></html>


        :param buffer: the `StreamBuffer` in which the selection should be
                       stored
        :rtype: `Transformer`
        :note: Copy (and cut) copy each individual selected object into the
               buffer before passing to the next transform. For example, the
               XPath ``*|text()`` will select all elements and text, each
               instance of which will be copied to the buffer individually
               before passing to the next transform. This has implications for
               how ``StreamBuffer`` objects can be used, so some
               experimentation may be required.

        """
        return self.apply(CopyTransformation(buffer, accumulate))

    def cut(self, buffer, accumulate=False):
        """Copy selection into buffer and remove the selection from the stream.

        >>> from genshi.builder import tag
        >>> buffer = StreamBuffer()
        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...             '<body>Some <em>body</em> text.</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('.//em/text()').cut(buffer)
        ...     .end().select('.//em').after(tag.h1(buffer)))
        <html><head><title>Some Title</title></head><body>Some
        <em/><h1>body</h1> text.</body></html>

        Specifying accumulate=True, appends all selected intervals onto the
        buffer. Combining this with the .buffer() operation allows us operate
        on all copied events rather than per-segment. See the documentation on
        buffer() for more information.

        :param buffer: the `StreamBuffer` in which the selection should be
                       stored
        :rtype: `Transformer`
        :note: this transformation will buffer the entire input stream
        """
        return self.apply(CutTransformation(buffer, accumulate))

    def buffer(self):
        """Buffer the entire stream (can consume a considerable amount of
        memory).

        Useful in conjunction with copy(accumulate=True) and
        cut(accumulate=True) to ensure that all marked events in the entire
        stream are copied to the buffer before further transformations are
        applied.

        For example, to move all <note> elements inside a <notes> tag at the
        top of the document:

        >>> doc = HTML('<doc><notes></notes><body>Some <note>one</note> '
        ...            'text <note>two</note>.</body></doc>',
        ...             encoding='utf-8')
        >>> buffer = StreamBuffer()
        >>> print(doc | Transformer('body/note').cut(buffer, accumulate=True)
        ...     .end().buffer().select('notes').prepend(buffer))
        <doc><notes><note>one</note><note>two</note></notes><body>Some  text
        .</body></doc>

        """
        return self.apply(list)

    #{ Miscellaneous operations

    def filter(self, filter):
        """Apply a normal stream filter to the selection. The filter is called
        once for each contiguous block of marked events.

        >>> from genshi.filters.html import HTMLSanitizer
        >>> html = HTML('<html><body>Some text<script>alert(document.cookie)'
        ...             '</script> and some more text</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('body/*').filter(HTMLSanitizer()))
        <html><body>Some text and some more text</body></html>

        :param filter: The stream filter to apply.
        :rtype: `Transformer`
        """
        return self.apply(FilterTransformation(filter))

    def map(self, function, kind):
        """Applies a function to the ``data`` element of events of ``kind`` in
        the selection.

        >>> html = HTML('<html><head><title>Some Title</title></head>'
        ...               '<body>Some <em>body</em> text.</body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('head/title').map(unicode.upper, TEXT))
        <html><head><title>SOME TITLE</title></head><body>Some <em>body</em>
        text.</body></html>

        :param function: the function to apply
        :param kind: the kind of event the function should be applied to
        :rtype: `Transformer`
        """
        return self.apply(MapTransformation(function, kind))

    def substitute(self, pattern, replace, count=1):
        """Replace text matching a regular expression.

        Refer to the documentation for ``re.sub()`` for details.

        >>> html = HTML('<html><body>Some text, some more text and '
        ...             '<b>some bold text</b>\\n'
        ...             '<i>some italicised text</i></body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('body/b').substitute('(?i)some', 'SOME'))
        <html><body>Some text, some more text and <b>SOME bold text</b>
        <i>some italicised text</i></body></html>
        >>> tags = tag.html(tag.body('Some text, some more text and\\n',
        ...      Markup('<b>some bold text</b>')))
        >>> print(tags.generate() | Transformer('body').substitute(
        ...     '(?i)some', 'SOME'))
        <html><body>SOME text, some more text and
        <b>SOME bold text</b></body></html>

        :param pattern: A regular expression object or string.
        :param replace: Replacement pattern.
        :param count: Number of replacements to make in each text fragment.
        :rtype: `Transformer`
        """
        return self.apply(SubstituteTransformation(pattern, replace, count))

    def rename(self, name):
        """Rename matching elements.

        >>> html = HTML('<html><body>Some text, some more text and '
        ...             '<b>some bold text</b></body></html>',
        ...             encoding='utf-8')
        >>> print(html | Transformer('body/b').rename('strong'))
        <html><body>Some text, some more text and <strong>some bold text</strong></body></html>
        """
        return self.apply(RenameTransformation(name))

    def trace(self, prefix='', fileobj=None):
        """Print events as they pass through the transform.

        >>> html = HTML('<body>Some <em>test</em> text</body>', encoding='utf-8')
        >>> print(html | Transformer('em').trace())
        (None, ('START', (QName('body'), Attrs()), (None, 1, 0)))
        (None, ('TEXT', u'Some ', (None, 1, 6)))
        ('ENTER', ('START', (QName('em'), Attrs()), (None, 1, 11)))
        ('INSIDE', ('TEXT', u'test', (None, 1, 15)))
        ('EXIT', ('END', QName('em'), (None, 1, 19)))
        (None, ('TEXT', u' text', (None, 1, 24)))
        (None, ('END', QName('body'), (None, 1, 29)))
        <body>Some <em>test</em> text</body>

        :param prefix: a string to prefix each event with in the output
        :param fileobj: the writable file-like object to write to; defaults to
                        the standard output stream
        :rtype: `Transformer`
        """
        return self.apply(TraceTransformation(prefix, fileobj=fileobj))

    # Internal methods

    def _mark(self, stream):
        for event in stream:
            yield OUTSIDE, event

    def _unmark(self, stream):
        for mark, event in stream:
            kind = event[0]
            if not (kind is None or kind is ATTR or kind is BREAK):
                yield event


class SelectTransformation(object):
    """Select and mark events that match an XPath expression."""

    def __init__(self, path):
        """Create selection.

        :param path: an XPath expression (as string) or a `Path` object
        """
        if not isinstance(path, Path):
            path = Path(path)
        self.path = path

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: the marked event stream to filter
        """
        namespaces = {}
        variables = {}
        test = self.path.test()
        stream = iter(stream)
        next = stream.next
        for mark, event in stream:
            if mark is None:
                yield mark, event
                continue
            result = test(event, namespaces, variables)
            # XXX This is effectively genshi.core._ensure() for transform
            # streams.
            if result is True:
                if event[0] is START:
                    yield ENTER, event
                    depth = 1
                    while depth > 0:
                        mark, subevent = next()
                        if subevent[0] is START:
                            depth += 1
                        elif subevent[0] is END:
                            depth -= 1
                        if depth == 0:
                            yield EXIT, subevent
                        else:
                            yield INSIDE, subevent
                        test(subevent, namespaces, variables, updateonly=True)
                else:
                    yield OUTSIDE, event
            elif isinstance(result, Attrs):
                # XXX  Selected *attributes* are given a "kind" of None to
                # indicate they are not really part of the stream.
                yield ATTR, (ATTR, (QName(event[1][0] + '@*'), result), event[2])
                yield None, event
            elif isinstance(result, tuple):
                yield OUTSIDE, result
            elif result:
                # XXX Assume everything else is "text"?
                yield None, (TEXT, unicode(result), (None, -1, -1))
            else:
                yield None, event


class InvertTransformation(object):
    """Invert selection so that marked events become unmarked, and vice versa.

    Specificaly, all input marks are converted to null marks, and all input
    null marks are converted to OUTSIDE marks.
    """

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: the marked event stream to filter
        """
        for mark, event in stream:
            if mark:
                yield None, event
            else:
                yield OUTSIDE, event


class EndTransformation(object):
    """End the current selection."""

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: the marked event stream to filter
        """
        for mark, event in stream:
            yield OUTSIDE, event


class EmptyTransformation(object):
    """Empty selected elements of all content."""

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: the marked event stream to filter
        """
        for mark, event in stream:
            yield mark, event
            if mark is ENTER:
                for mark, event in stream:
                    if mark is EXIT:
                        yield mark, event
                        break


class RemoveTransformation(object):
    """Remove selection from the stream."""

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: the marked event stream to filter
        """
        for mark, event in stream:
            if mark is None:
                yield mark, event


class UnwrapTransformation(object):
    """Remove outtermost enclosing elements from selection."""

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: the marked event stream to filter
        """
        for mark, event in stream:
            if mark not in (ENTER, EXIT):
                yield mark, event


class WrapTransformation(object):
    """Wrap selection in an element."""

    def __init__(self, element):
        if isinstance(element, Element):
            self.element = element
        else:
            self.element = Element(element)

    def __call__(self, stream):
        for mark, event in stream:
            if mark:
                element = list(self.element.generate())
                for prefix in element[:-1]:
                    yield None, prefix
                yield mark, event
                start = mark
                stopped = False
                for mark, event in stream:
                    if start is ENTER and mark is EXIT:
                        yield mark, event
                        stopped = True
                        break
                    if not mark:
                        break
                    yield mark, event
                else:
                    stopped = True
                yield None, element[-1]
                if not stopped:
                    yield mark, event
            else:
                yield mark, event


class TraceTransformation(object):
    """Print events as they pass through the transform."""

    def __init__(self, prefix='', fileobj=None):
        """Trace constructor.

        :param prefix: text to prefix each traced line with.
        :param fileobj: the writable file-like object to write to
        """
        self.prefix = prefix
        self.fileobj = fileobj or sys.stdout

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: the marked event stream to filter
        """
        for event in stream:
            self.fileobj.write('%s%s\n' % (self.prefix, event))
            yield event


class FilterTransformation(object):
    """Apply a normal stream filter to the selection. The filter is called once
    for each selection."""

    def __init__(self, filter):
        """Create the transform.

        :param filter: The stream filter to apply.
        """
        self.filter = filter

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: The marked event stream to filter
        """
        def flush(queue):
            if queue:
                for event in self.filter(queue):
                    yield OUTSIDE, event
                del queue[:]

        queue = []
        for mark, event in stream:
            if mark is ENTER:
                queue.append(event)
                for mark, event in stream:
                    queue.append(event)
                    if mark is EXIT:
                        break
                for queue_event in flush(queue):
                    yield queue_event
            elif mark is OUTSIDE:
                stopped = False
                queue.append(event)
                for mark, event in stream:
                    if mark is not OUTSIDE:
                        break
                    queue.append(event)
                else:
                    stopped = True
                for queue_event in flush(queue):
                    yield queue_event
                if not stopped:
                    yield mark, event
            else:
                yield mark, event
        for queue_event in flush(queue):
            yield queue_event


class MapTransformation(object):
    """Apply a function to the `data` element of events of ``kind`` in the
    selection.
    """

    def __init__(self, function, kind):
        """Create the transform.

        :param function: the function to apply; the function must take one
                         argument, the `data` element of each selected event
        :param kind: the stream event ``kind`` to apply the `function` to
        """
        self.function = function
        self.kind = kind

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: The marked event stream to filter
        """
        for mark, (kind, data, pos) in stream:
            if mark and self.kind in (None, kind):
                yield mark, (kind, self.function(data), pos)
            else:
                yield mark, (kind, data, pos)


class SubstituteTransformation(object):
    """Replace text matching a regular expression.

    Refer to the documentation for ``re.sub()`` for details.
    """
    def __init__(self, pattern, replace, count=0):
        """Create the transform.

        :param pattern: A regular expression object, or string.
        :param replace: Replacement pattern.
        :param count: Number of replacements to make in each text fragment.
        """
        if isinstance(pattern, basestring):
            self.pattern = re.compile(pattern)
        else:
            self.pattern = pattern
        self.count = count
        self.replace = replace

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: The marked event stream to filter
        """
        for mark, (kind, data, pos) in stream:
            if mark is not None and kind is TEXT:
                new_data = self.pattern.sub(self.replace, data, self.count)
                if isinstance(data, Markup):
                    data = Markup(new_data)
                else:
                    data = new_data
            yield mark, (kind, data, pos)


class RenameTransformation(object):
    """Rename matching elements."""
    def __init__(self, name):
        """Create the transform.

        :param name: New element name.
        """
        self.name = QName(name)

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: The marked event stream to filter
        """
        for mark, (kind, data, pos) in stream:
            if mark is ENTER:
                data = self.name, data[1]
            elif mark is EXIT:
                data = self.name
            yield mark, (kind, data, pos)


class InjectorTransformation(object):
    """Abstract base class for transformations that inject content into a
    stream.

    >>> class Top(InjectorTransformation):
    ...     def __call__(self, stream):
    ...         for event in self._inject():
    ...             yield event
    ...         for event in stream:
    ...             yield event
    >>> html = HTML('<body>Some <em>test</em> text</body>', encoding='utf-8')
    >>> print(html | Transformer('.//em').apply(Top('Prefix ')))
    Prefix <body>Some <em>test</em> text</body>
    """
    def __init__(self, content):
        """Create a new injector.

        :param content: An iterable of Genshi stream events, or a string to be
                        injected.
        """
        self.content = content

    def _inject(self):
        content = self.content
        if hasattr(content, '__call__'):
            content = content()
        for event in _ensure(content):
            yield None, event


class ReplaceTransformation(InjectorTransformation):
    """Replace selection with content."""

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: The marked event stream to filter
        """
        stream = PushBackStream(stream)
        for mark, event in stream:
            if mark is not None:
                start = mark
                for subevent in self._inject():
                    yield subevent
                for mark, event in stream:
                    if start is ENTER:
                        if mark is EXIT:
                            break
                    elif mark != start:
                        stream.push((mark, event))
                        break
            else:
                yield mark, event


class BeforeTransformation(InjectorTransformation):
    """Insert content before selection."""

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: The marked event stream to filter
        """
        stream = PushBackStream(stream)
        for mark, event in stream:
            if mark is not None:
                start = mark
                for subevent in self._inject():
                    yield subevent
                yield mark, event
                for mark, event in stream:
                    if mark != start and start is not ENTER:
                        stream.push((mark, event))
                        break
                    yield mark, event
                    if start is ENTER and mark is EXIT:
                        break
            else:
                yield mark, event


class AfterTransformation(InjectorTransformation):
    """Insert content after selection."""

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: The marked event stream to filter
        """
        stream = PushBackStream(stream)
        for mark, event in stream:
            yield mark, event
            if mark:
                start = mark
                for mark, event in stream:
                    if start is not ENTER and mark != start:
                        stream.push((mark, event))
                        break
                    yield mark, event
                    if start is ENTER and mark is EXIT:
                        break
                for subevent in self._inject():
                    yield subevent


class PrependTransformation(InjectorTransformation):
    """Prepend content to the inside of selected elements."""

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: The marked event stream to filter
        """
        for mark, event in stream:
            yield mark, event
            if mark is ENTER:
                for subevent in self._inject():
                    yield subevent


class AppendTransformation(InjectorTransformation):
    """Append content after the content of selected elements."""

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: The marked event stream to filter
        """
        for mark, event in stream:
            yield mark, event
            if mark is ENTER:
                for mark, event in stream:
                    if mark is EXIT:
                        break
                    yield mark, event
                for subevent in self._inject():
                    yield subevent
                yield mark, event


class AttrTransformation(object):
    """Set an attribute on selected elements."""

    def __init__(self, name, value):
        """Construct transform.

        :param name: name of the attribute that should be set
        :param value: the value to set
        """
        self.name = name
        self.value = value

    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: The marked event stream to filter
        """
        callable_value = hasattr(self.value, '__call__')
        for mark, (kind, data, pos) in stream:
            if mark is ENTER:
                if callable_value:
                    value = self.value(self.name, (kind, data, pos))
                else:
                    value = self.value
                if value is None:
                    attrs = data[1] - [QName(self.name)]
                else:
                    attrs = data[1] | [(QName(self.name), value)]
                data = (data[0], attrs)
            yield mark, (kind, data, pos)



class StreamBuffer(Stream):
    """Stream event buffer used for cut and copy transformations."""

    def __init__(self):
        """Create the buffer."""
        Stream.__init__(self, [])

    def append(self, event):
        """Add an event to the buffer.

        :param event: the markup event to add
        """
        self.events.append(event)

    def reset(self):
        """Empty the buffer of events."""
        del self.events[:]


class CopyTransformation(object):
    """Copy selected events into a buffer for later insertion."""

    def __init__(self, buffer, accumulate=False):
        """Create the copy transformation.

        :param buffer: the `StreamBuffer` in which the selection should be
                       stored
        """
        if not accumulate:
            buffer.reset()
        self.buffer = buffer
        self.accumulate = accumulate

    def __call__(self, stream):
        """Apply the transformation to the marked stream.

        :param stream: the marked event stream to filter
        """
        stream = PushBackStream(stream)

        for mark, event in stream:
            if mark:
                if not self.accumulate:
                    self.buffer.reset()
                events = [(mark, event)]
                self.buffer.append(event)
                start = mark
                for mark, event in stream:
                    if start is not ENTER and mark != start:
                        stream.push((mark, event))
                        break
                    events.append((mark, event))
                    self.buffer.append(event)
                    if start is ENTER and mark is EXIT:
                        break
                for i in events:
                    yield i
            else:
                yield mark, event


class CutTransformation(object):
    """Cut selected events into a buffer for later insertion and remove the
    selection.
    """

    def __init__(self, buffer, accumulate=False):
        """Create the cut transformation.

        :param buffer: the `StreamBuffer` in which the selection should be
                       stored
        """
        self.buffer = buffer
        self.accumulate = accumulate


    def __call__(self, stream):
        """Apply the transform filter to the marked stream.

        :param stream: the marked event stream to filter
        """
        attributes = []
        stream = PushBackStream(stream)
        broken = False
        if not self.accumulate:
            self.buffer.reset()
        for mark, event in stream:
            if mark:
                # Send a BREAK event if there was no other event sent between 
                if not self.accumulate:
                    if not broken and self.buffer:
                        yield BREAK, (BREAK, None, None)
                    self.buffer.reset()
                self.buffer.append(event)
                start = mark
                if mark is ATTR:
                    attributes.extend([name for name, _ in event[1][1]])
                for mark, event in stream:
                    if start is mark is ATTR:
                        attributes.extend([name for name, _ in event[1][1]])
                    # Handle non-element contiguous selection
                    if start is not ENTER and mark != start:
                        # Operating on the attributes of a START event
                        if start is ATTR:
                            kind, data, pos = event
                            assert kind is START
                            data = (data[0], data[1] - attributes)
                            attributes = None
                            stream.push((mark, (kind, data, pos)))
                        else:
                            stream.push((mark, event))
                        break
                    self.buffer.append(event)
                    if start is ENTER and mark is EXIT:
                        break
                broken = False
            else:
                broken = True
                yield mark, event
        if not broken and self.buffer:
            yield BREAK, (BREAK, None, None)
