source: utilitiespy/tudelft/utilities/immutablelist/SubList.py@ 1021

from typing import TypeVar

from tudelft.utilities.immutablelist.AbstractImmutableList import AbstractImmutableList
from tudelft.utilities.immutablelist.ImmutableList import ImmutableList


def  bitCount(n:int):
        '''
        @return number of 1's in binary representation of number 
        '''
        count = 0
        while (n):
                count += n & 1
                n >>= 1
        return count
   
   
T=TypeVar("T")
class SubList(AbstractImmutableList[T]):
        '''
        Selects a specific sublist of the given list.
        This is a sort of subset, but the original order of the elements is
        maintained. Notice this is not a cheap operation.
        
        @param T the element type of all lists that we receive.
        '''
        
        def __init__(self, lis: ImmutableList[T], selection:int):
                '''
                @param lis      the list to take the subset of. The list size must be
                     ≤ {@link Integer#MAX_VALUE} elements
                @param selection a number. When interpreted binary, eg 1011, the 1's
                     indicate selected elements and 0's non-selected
                     elements. This number should be in the range
                     0..2^(|list|+1).
                '''
                self._list = lis
                self._selection = selection

        def get(self, index:int) -> T:
                return self._list.get(self._getIndex(index))


        def _getIndex(self, n1:int)->int:
                '''
                @param n the index in our list. We use int as int's are used anyway
                         internally in BigInteger for indexing bits.
                @return the index in the original array, which is the index of the nth 1
                        in the selection number. NOTICE this is expensive operation as we
                        have to iterate to get to the required element.
                '''
                n = n1
                for i in range(self._selection.bit_length()):
                        if self._selection >>i & 1:
                                if n == 0:
                                        return i
                                n-=1
                raise IndexError("Index out of bounds:" + str(n1))

        def  size(self) -> int:
                return bitCount(self._selection)

        def getSelection(self) -> int:
                return self._selection

        def complement(self)-> "SubList[T]" :
                '''
                @return list with all elements from list that were not selected now
                        selected, and those that were selected now not selected.
                '''
                # do 111111 XOR the selection.
                sel = (1<<self._list.size()) - 1
                sel = sel ^ self._selection
                return SubList[T](self._list,sel)

        def __hash__(self):
                return hash((self._list, self._selection))
        
        def __eq__(self, other):
                return isinstance(other, self.__class__) \
                        and self._list==other._list\
                        and self._selection==other._selection