referential_array.py

""" Basic class implementation of an array of references for FIT units

The code for the init function is a bit cryptic, so I explain it here in
detail. The instance variables holding the physical array is constructed
using the ctypes library to create a py_object (an object that can hold
a reference to any python object). Note that for each value of length we
have that (length * ctypes.py_object) is a type (e.g., if length=5, it
would be a type called py_object_Array_5). Then (length *
ctypes.py_object)() is equivalent to the initialisation in MIPS of the
space to hold the references.

Note that while I do check the precondition in __init__ (noone else
would), I do not check that of getitem or setitem, since that is already
checked by self.array[index].
"""
__author__ = "Julian Garcia for the __init__ code, Maria Garcia de la Banda for the rest"
__docformat__ = 'reStructuredText'

from ctypes import py_object
from typing import TypeVar, Generic

T = TypeVar('T')

class ArrayR(Generic[T]):
    def __init__(self, length: int) -> None:
        """ Creates an array of references to objects of the given length
        :complexity: O(length) for best/worst case to initialise to None
        :pre: length > 0
        """
        if length <= 0:
            raise ValueError("Array length should be larger than 0.")
        self.array = (length * py_object)() # initialises the space
        self.array[:] =  [None for _ in range(length)]

    def __len__(self) -> int:
        """ Returns the length of the array
        :complexity: O(1)
        """
        return len(self.array)

    def __getitem__(self, index: int) -> T:
        """ Returns the object in position index.
        :complexity: O(1)
        :pre: index in between 0 and length - self.array[] checks it
        """
        return self.array[index]

    def __setitem__(self, index: int, value: T) -> None:
        """ Sets the object in position index to value
        :complexity: O(1)
        :pre: index in between 0 and length - self.array[] checks it
        """
        self.array[index] = value