Added base64 to binary conversion. (#12209)

* Added base64 to binary conversion.

* updating DIRECTORY.md

---------

Co-authored-by: Christian Clauss <cclauss@me.com>
Co-authored-by: cclauss <cclauss@users.noreply.github.com>

DIRECTORY.md

@@ -208,6 +208,7 @@
 
 ## [Conversions](conversions)
   * [Astronomical Length Scale Conversion](conversions/astronomical_length_scale_conversion.py)
+  * [Base64 To Binary](conversions/base64_to_binary.py)
   * [Binary To Decimal](conversions/binary_to_decimal.py)
   * [Binary To Excess3](conversions/binary_to_excess3.py)
   * [Binary To Gray](conversions/binary_to_gray.py)

conversions/base64_to_binary.py

@@ -0,0 +1,112 @@
+"""
+* Author: Siddharth Singh (https://github.com/coolsidd)
+* Description: Convert a binary string to Base64.
+References for better understanding:
+https://en.wikipedia.org/wiki/Base64
+"""
+
+B64_TO_BITS = {
+    "A": "000000",
+    "B": "000001",
+    "C": "000010",
+    "D": "000011",
+    "E": "000100",
+    "F": "000101",
+    "G": "000110",
+    "H": "000111",
+    "I": "001000",
+    "J": "001001",
+    "K": "001010",
+    "L": "001011",
+    "M": "001100",
+    "N": "001101",
+    "O": "001110",
+    "P": "001111",
+    "Q": "010000",
+    "R": "010001",
+    "S": "010010",
+    "T": "010011",
+    "U": "010100",
+    "V": "010101",
+    "W": "010110",
+    "X": "010111",
+    "Y": "011000",
+    "Z": "011001",
+    "a": "011010",
+    "b": "011011",
+    "c": "011100",
+    "d": "011101",
+    "e": "011110",
+    "f": "011111",
+    "g": "100000",
+    "h": "100001",
+    "i": "100010",
+    "j": "100011",
+    "k": "100100",
+    "l": "100101",
+    "m": "100110",
+    "n": "100111",
+    "o": "101000",
+    "p": "101001",
+    "q": "101010",
+    "r": "101011",
+    "s": "101100",
+    "t": "101101",
+    "u": "101110",
+    "v": "101111",
+    "w": "110000",
+    "x": "110001",
+    "y": "110010",
+    "z": "110011",
+    "0": "110100",
+    "1": "110101",
+    "2": "110110",
+    "3": "110111",
+    "4": "111000",
+    "5": "111001",
+    "6": "111010",
+    "7": "111011",
+    "8": "111100",
+    "9": "111101",
+    "+": "111110",
+    "/": "111111",
+}
+
+
+def base64_to_bin(base64_str: str) -> str:
+    """Convert a base64 value to its binary equivalent.
+
+    >>> base64_to_bin("ABC")
+    '000000000001000010'
+    >>> base64_to_bin("    -abc    ")
+    '-011010011011011100'
+    >>> base64_to_bin("    a-bc    ")
+    Traceback (most recent call last):
+      ...
+    ValueError: Invalid base64 string. Invalid char - at pos 1
+    >>> base64_to_bin("")
+    Traceback (most recent call last):
+      ...
+    ValueError: Empty string was passed to the function
+    """
+    b64_str = str(base64_str).strip()
+    if not b64_str:
+        raise ValueError("Empty string was passed to the function")
+    is_negative = b64_str[0] == "-"
+    if is_negative:
+        b64_str = b64_str[1:]
+    bin_str = ""
+    for i, x in enumerate(b64_str):
+        if x not in B64_TO_BITS:
+            exception_message = f"Invalid base64 string. Invalid char {x} at pos {i}"
+            raise ValueError(exception_message)
+        bin_str += B64_TO_BITS[x]
+    if is_negative:
+        bin_str = "-" + bin_str
+    return bin_str
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()