stringview.py

"""stringview.py - a string class where slices are views not copies as
                   with the standard Python strings.

   stringview.pxd - declarations for the cython Python to C compiler
                   to speed up handling of StringViews.

Copyright 2016  by Eckhart Arnold (arnold@badw.de)
                Bavarian Academy of Sciences an Humanities (badw.de)

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
implied.  See the License for the specific language governing
permissions and limitations under the License.

StringView provides string-slicing without copying.
Slicing Python-strings always yields copies of a segment of the original
string. See: https://mail.python.org/pipermail/python-dev/2008-May/079699.html
However, this becomes costly (in terms of space and as a consequence also
time) when parsing longer documents. Unfortunately, Python's `memoryview`
does not work for unicode strings. Hence, the StringView class.
"""


import collections

from DHParser.toolkit import typing
from typing import Optional, Union, Iterable, Tuple


__all__ = ('StringView', 'EMPTY_STRING_VIEW')


def first_char(text, begin: int, end: int) -> int:
    """Returns the index of the first non-whitespace character in string
     `text` within the bounds [begin, end].
    """
    while begin < end and text[begin] in ' \n\t':
        begin += 1
    return begin


def last_char(text, begin: int, end: int) -> int:
    """Returns the index of the first non-whitespace character in string
    `text` within the bounds [begin, end].
    """
    while end > begin and text[end] in ' \n\t':
        end -= 1
    return end


def pack_index(index: int, length: int) -> int:
    """Transforms `index` into a positive index counting from the beginning
    of the string, capping it at the boundaries [0, len].
    Examples:
    >>> pack_index(-1, 5)
    4
    >>> pack_index(6, 5)
    5
    >>> pack_index(-7, 5)
    0
    """
    # assert length >= 0
    index = index if index >= 0 else index + length
    return 0 if index < 0 else length if index > length else index


def real_indices(begin: Optional[int],
                 end: Optional[int],
                 length) -> Tuple[int, int]:   # "length: int" fails with cython!?
    """Returns the tuple of real (i.e. positive) indices from the slice
    indices `begin`,  `end`, assuming a string of size `length`.
    """
    cbegin = 0 if begin is None else begin
    cend = length if end is None else end
    return pack_index(cbegin, length), pack_index(cend, length)


class StringView(collections.abc.Sized):
    """
    A rudimentary StringView class, just enough for the use cases
    in parse.py. The difference between a StringView and the python
    builtin strings is that StringView-objects do slicing without
    copying, i.e. slices are just a view on a section of the sliced
    string.
    """
    __slots__ = ['text', 'begin', 'end', 'len', 'fullstring_flag']

    def __init__(self, text: str, begin: Optional[int] = 0, end: Optional[int] = None) -> None:
        assert isinstance(text, str)
        self.text = text  # type: str
        self.begin, self.end = real_indices(begin, end, len(text))
        self.len = max(self.end - self.begin, 0)  # type: int
        self.fullstring_flag = (self.begin == 0 and self.len == len(self.text))  # type: bool

    def __bool__(self):
        return self.end > self.begin  # and bool(self.text)

    def __len__(self):
        return self.len

    def __str__(self):
        if self.fullstring_flag:  # optimization: avoid slicing/copying
            return self.text
        # since the slice is being copyied now, anyway, the copy might
        # as well be stored in the string view
        self.text = self.text[self.begin:self.end]
        self.begin = 0
        self.len = len(self.text)
        self.end = self.len
        self.fullstring_flag = True
        return self.text

    def __eq__(self, other):
        # PERFORMANCE WARNING: This creates copies of the strings
        return len(other) == len(self) and str(self) == str(other)

    def __hash__(self):
        # PERFORMANCE WARNING: This creates a copy of the string-slice
        return hash(str(self))

    def __add__(self, other):
        if isinstance(other, str):
            return str(self) + other
        else:
            return StringView(str(self) + str(other))

    def __radd__(self, other):
        if isinstance(other, str):
            return other + str(self)
        else:
            return StringView(str(other) + str(self))

    def __getitem__(self, index):
        # assert isinstance(index, slice), "As of now, StringView only allows slicing."
        # assert index.step is None or index.step == 1, \
        #     "Step sizes other than 1 are not yet supported by StringView"
        start, stop = real_indices(index.start, index.stop, self.len)
        return StringView(self.text, self.begin + start, self.begin + stop)

    def count(self, sub: str, start=None, end=None) -> int:
        """Returns the number of non-overlapping occurrences of substring
        `sub` in StringView S[start:end].  Optional arguments start and end
        are interpreted as in slice notation.
        """
        if self.fullstring_flag:
            return self.text.count(sub, start, end)
        elif start is None and end is None:
            return self.text.count(sub, self.begin, self.end)
        else:
            start, end = real_indices(start, end, self.len)
            return self.text.count(sub, self.begin + start, self.begin + end)

    def find(self, sub: str, start=None, end=None) -> int:
        """Returns the lowest index in S where substring `sub` is found,
        such that `sub` is contained within S[start:end].  Optional
        arguments `start` and `end` are interpreted as in slice notation.
        Returns -1 on failure.
        """
        if self.fullstring_flag:
            return self.text.find(sub, start, end)
        elif start is None and end is None:
            return self.text.find(sub, self.begin, self.end) - self.begin
        else:
            start, end = real_indices(start, end, self.len)
            return self.text.find(sub, self.begin + start, self.begin + end) - self.begin

    def rfind(self, sub: str, start=None, end=None) -> int:
        """Returns the highest index in S where substring `sub` is found,
        such that `sub` is contained within S[start:end].  Optional
        arguments `start` and `end` are interpreted as in slice notation.
        Returns -1 on failure.
        """
        if self.fullstring_flag:
            return self.text.rfind(sub, start, end)
        if start is None and end is None:
            return self.text.rfind(sub, self.begin, self.end) - self.begin
        else:
            start, end = real_indices(start, end, self.len)
            return self.text.rfind(sub, self.begin + start, self.begin + end) - self.begin

    def startswith(self,
                   prefix: Union[str, Tuple[str, ...]],
                   start: int = 0,
                   end: Optional[int] = None) -> bool:
        """Return True if S starts with the specified prefix, False otherwise.
        With optional `start`, test S beginning at that position.
        With optional `end`, stop comparing S at that position.
        prefix can also be a tuple of strings to try.
        """
        start += self.begin
        end = self.end if end is None else self.begin + end
        return self.text.startswith(prefix, start, end)

    def match(self, regex):
        """Executes `regex.match` on the StringView object and returns the
        result, which is either a match-object or None.
        """
        return regex.match(self.text, pos=self.begin, endpos=self.end)

    def index(self, absolute_index: int) -> int:
        """
        Converts an index for a string watched by a StringView object
        to an index relative to the string view object, e.g.:
        >>> import re
        >>> sv = StringView('xxIxx')[2:3]
        >>> match = sv.match(re.compile('I'))
        >>> match.end()
        3
        >>> sv.index(match.end())
        1
        """
        return absolute_index - self.begin

    def indices(self, absolute_indices: Iterable[int]) -> Tuple[int, ...]:
        """Converts indices for a string watched by a StringView object
        to indices relative to the string view object. See also: `sv_index()`
        """
        return tuple(index - self.begin for index in absolute_indices)

    def search(self, regex):
        """Executes regex.search on the StringView object and returns the
        result, which is either a match-object or None.
        """
        return regex.search(self.text, pos=self.begin, endpos=self.end)

    def strip(self):
        """Returns a copy of the StringView `self` with leading and trailing
        whitespace removed.
        """
        if self.fullstring_flag:
            return self.text.strip()
        else:
            begin = first_char(self.text, self.begin, self.end)
            end = last_char(self.text, self.begin, self.end)
            return self.text[begin:end]

    def split(self, sep=None):
        """Returns a list of the words in `self`, using `sep` as the
        delimiter string.  If `sep` is not specified or is None, any
        whitespace string is a separator and empty strings are
        removed from the result.
        """
        if self.fullstring_flag:
            return self.text.split(sep)
        else:
            pieces = []
            l = len(sep)
            k = 0
            i = self.find(sep, k)
            while i >= 0:
                pieces.append(self.text[self.begin + k: self.begin + i])
                k = i + l
                i = self.find(sep, k)
            pieces.append(self.text[self.begin + k: self.end])
            return pieces

    def replace(self, old, new):
        """Returns a string where `old` is replaced by `new`."""
        return str(self).replace(old, new)


EMPTY_STRING_VIEW = StringView('')