#!/usr/bin/env python
#
# Copyright 2007 Google Inc.
#
# 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.
#

"""Key range representation and splitting."""


import os

from google.appengine.ext import db


class Error(Exception):
  """Base class for exceptions in this module."""


class KeyRangeError(Error):
  """Error while trying to generate a KeyRange."""


class EmptyQuery(db.Query):
  def get(self):
    return None

  def fetch(self, limit=1000, offset=0):
    return []

  def count(self, limit=1000):
    return 0


class KeyRange(object):
  """Represents a range of keys in the datastore.

  A KeyRange object represents a key range
    (key_start, include_start, key_end, include_end)
  and a scan direction (KeyRange.DESC or KeyRange.ASC).
  """

  DESC = 'DESC'
  ASC = 'ASC'

  def __init__(self,
               key_start=None,
               key_end=None,
               direction=None,
               include_start=True,
               include_end=True):
    """Initialize a KeyRange object.

    Args:
      key_start: The starting key for this range.
      key_end: The ending key for this range.
      direction: The direction of the query for this range.
      include_start: Whether the start key should be included in the range.
      include_end: Whether the end key should be included in the range.
    """
    if direction is None:
      direction = KeyRange.ASC
    assert direction in (KeyRange.ASC, KeyRange.DESC)
    self.direction = direction
    self.key_start = key_start
    self.key_end = key_end
    self.include_start = include_start
    self.include_end = include_end

  def __str__(self):
    if self.include_start:
      left_side = '['
    else:
      left_side = '('
    if self.include_end:
      right_side = ']'
    else:
      right_side = '('
    return '%s%s%s-%s%s' % (self.direction, left_side, repr(self.key_start),
                            repr(self.key_end), right_side)

  def __repr__(self):
    return ('key_range.KeyRange(key_start=%s,key_end=%s,direction=%s,'
            'include_start=%s,include_end=%s)') % (repr(self.key_start),
                                                   repr(self.key_end),
                                                   repr(self.direction),
                                                   repr(self.include_start),
                                                   repr(self.include_end))

  def filter_query(self, query):
    """Add query filter to restrict to this key range.

    Args:
      query: A db.Query instance.
    """
    if self.key_start == self.key_end and not (
        self.include_start or self.include_end):
      return EmptyQuery()
    if self.include_start:
      start_comparator = '>='
    else:
      start_comparator = '>'
    if self.include_end:
      end_comparator = '<='
    else:
      end_comparator = '<'
    if self.key_start and self.key_end:
      query.filter('__key__ %s' % start_comparator, self.key_start)
      query.filter('__key__ %s' % end_comparator, self.key_end)
    elif self.key_start:
      query.filter('__key__ %s' % start_comparator, self.key_start)
    elif self.key_end:
      query.filter('__key__ %s' % end_comparator, self.key_end)

  def make_directed_query(self, kind_class):
    """Construct a query for this key range, including the scan direction.

    Args:
      kind_class: A kind implementation class.

    Returns:
      A db.Query instance.

    Raises:
      Error: if self.direction is not one of KeyRange.ASC, KeyRange.DESC
    """
    if self.direction == KeyRange.ASC:
      direction = ''
    elif self.direction == KeyRange.DESC:
      direction = '-'
    else:
      raise KeyRangeError('KeyRange direction unexpected: %s', self.direction)
    query = db.Query(kind_class)
    query.order('%s__key__' % direction)

    self.filter_query(query)
    return query

  def make_ascending_query(self, kind_class):
    """Construct a query for this key range without setting the scan direction.

    Args:
      kind_class: A kind implementation class.

    Returns:
      A db.Query instance.
    """
    query = db.Query(kind_class)
    query.order('__key__')

    self.filter_query(query)
    return query

  def split_range(self, batch_size=0):
    """Split this key range into a list of at most two ranges.

    This method attempts to split the key range approximately in half.
    Numeric ranges are split in the middle into two equal ranges and
    string ranges are split lexicographically in the middle.  If the
    key range is smaller than batch_size it is left unsplit.

    Note that splitting is done without knowledge of the distribution
    of actual entities in the key range, so there is no guarantee (nor
    any particular reason to believe) that the entities of the range
    are evenly split.

    Args:
      batch_size: The maximum size of a key range that should not be split.

    Returns:
      A list of one or two key ranges covering the same space as this range.
    """
    key_start = self.key_start
    key_end = self.key_end
    include_start = self.include_start
    include_end = self.include_end

    key_pairs = []
    if not key_start:
      key_pairs.append((key_start, include_start, key_end, include_end,
                        KeyRange.ASC))
    elif not key_end:
      key_pairs.append((key_start, include_start, key_end, include_end,
                        KeyRange.DESC))
    else:
      key_split = KeyRange.split_keys(key_start, key_end, batch_size)
      key_pairs.append((key_start, include_start,
                        key_split, True, KeyRange.DESC))
      key_pairs.append((key_split, False,
                        key_end, include_end, KeyRange.ASC))

    ranges = [KeyRange(key_start=start,
                       include_start=include_start,
                       key_end=end,
                       include_end=include_end,
                       direction=direction)
              for (start, include_start, end, include_end, direction)
              in key_pairs]

    return ranges

  def __cmp__(self, other):
    """Compare two key ranges.

    Key ranges with a value of None for key_start or key_end, are always
    considered to have include_start=False or include_end=False, respectively,
    when comparing.  Since None indicates an unbounded side of the range,
    the include specifier is meaningless.  The ordering generated is total
    but somewhat arbitrary.
    """
    if not isinstance(other, KeyRange):
      return 1

    self_list = [self.key_start, self.key_end, self.direction,
                 self.include_start, self.include_end]
    if not self.key_start:
      self_list[3] = False
    if not self.key_end:
      self_list[4] = False

    other_list = [other.key_start,
                  other.key_end,
                  other.direction,
                  other.include_start,
                  other.include_end]
    if not other.key_start:
      other_list[3] = False
    if not other.key_end:
      other_list[4] = False

    return cmp(self_list, other_list)

  @staticmethod
  def bisect_string_range(start, end):
    """Returns a string that is approximately in the middle of the range.

    (start, end) is treated as a string range, and it is assumed
    start <= end in the usual lexicographic string ordering. The output key
    mid is guaranteed to satisfy start <= mid <= end.

    The method proceeds by comparing initial characters of start and
    end.  When the characters are equal, they are appended to the mid
    string.  In the first place that the characters differ, the
    difference characters are averaged and this average is appended to
    the mid string.  If averaging resulted in rounding down, and
    additional character is added to the mid string to make up for the
    rounding down.  This extra step is necessary for correctness in
    the case that the average of the two characters is equal to the
    character in the start string.

    This method makes the assumption that most keys are ascii and it
    attempts to perform splitting within the ascii range when that
    results in a valid split.

    Args:
      start: A string.
      end: A string such that start <= end.

    Returns:
      A string mid such that start <= mid <= end.
    """
    if start == end:
      return start
    start += '\0'
    end += '\0'
    midpoint = []
    expected_max = 127
    for i in xrange(min(len(start), len(end))):
      if start[i] == end[i]:
        midpoint.append(start[i])
      else:
        ord_sum = ord(start[i]) + ord(end[i])
        midpoint.append(unichr(ord_sum / 2))
        if ord_sum % 2:
          if len(start) > i + 1:
            ord_start = ord(start[i+1])
          else:
            ord_start = 0
          if ord_start < expected_max:
            ord_split = (expected_max + ord_start) / 2
          else:
            ord_split = (0xFFFF + ord_start) / 2
          midpoint.append(unichr(ord_split))
        break
    return ''.join(midpoint)

  @staticmethod
  def split_keys(key_start, key_end, batch_size):
    """Return a key that is between key_start and key_end inclusive.

    This method compares components of the ancestor paths of key_start
    and key_end.  The first place in the path that differs is
    approximately split in half.  If the kind components differ, a new
    non-existent kind halfway between the two is used to split the
    space. If the id_or_name components differ, then a new id_or_name
    that is halfway between the two is selected.  If the lower
    id_or_name is numeric and the upper id_or_name is a string, then
    the minumum string key u'\0' is used as the split id_or_name.  The
    key that is returned is the shared portion of the ancestor path
    followed by the generated split component.

    Args:
      key_start: A db.Key instance for the lower end of a range.
      key_end: A db.Key instance for the upper end of a range.
      batch_size: The maximum size of a range that should not be split.

    Returns:
      A db.Key instance, k, such that key_start <= k <= key_end.
    """
    assert key_start.app() == key_end.app()
    path1 = key_start.to_path()
    path2 = key_end.to_path()
    len1 = len(path1)
    len2 = len(path2)
    assert len1 % 2 == 0
    assert len2 % 2 == 0
    out_path = []
    min_path_len = min(len1, len2) / 2
    for i in xrange(min_path_len):
      kind1 = path1[2*i]
      kind2 = path2[2*i]

      if kind1 != kind2:
        split_kind = KeyRange.bisect_string_range(kind1, kind2)
        out_path.append(split_kind)
        out_path.append(unichr(0))
        break

      last = (len1 == len2 == 2*(i + 1))

      id_or_name1 = path1[2*i + 1]
      id_or_name2 = path2[2*i + 1]
      id_or_name_split = KeyRange._split_id_or_name(
          id_or_name1, id_or_name2, batch_size, last)
      if id_or_name1 == id_or_name_split:
        out_path.append(kind1)
        out_path.append(id_or_name1)
      else:
        out_path.append(kind1)
        out_path.append(id_or_name_split)
        break

    return db.Key.from_path(*out_path)

  @staticmethod
  def _split_id_or_name(id_or_name1, id_or_name2, batch_size, maintain_batches):
    """Return an id_or_name that is between id_or_name1 an id_or_name2.

    Attempts to split the range [id_or_name1, id_or_name2] in half,
    unless maintain_batches is true and the size of the range
    [id_or_name1, id_or_name2] is less than or equal to batch_size.

    Args:
      id_or_name1: A number or string or the id_or_name component of a key
      id_or_name2: A number or string or the id_or_name component of a key
      batch_size: The range size that will not be split if maintain_batches
        is true.
      maintain_batches: A boolean for whether to keep small ranges intact.

    Returns:
      An id_or_name such that id_or_name1 <= id_or_name <= id_or_name2.
    """
    if (isinstance(id_or_name1, (int, long)) and
        isinstance(id_or_name2, (int, long))):
      if not maintain_batches or id_or_name2 - id_or_name1 > batch_size:
        return (id_or_name1 + id_or_name2) / 2
      else:
        return id_or_name1
    elif (isinstance(id_or_name1, basestring) and
          isinstance(id_or_name2, basestring)):
      return KeyRange.bisect_string_range(id_or_name1, id_or_name2)
    else:
      assert (isinstance(id_or_name1, (int, long)) and
              isinstance(id_or_name2, basestring))
      return unichr(0)
