Adding multibit-manipulation algorithm implemented with bitwise operations

bit_manipulation/multibit_manipulation.py

@@ -0,0 +1,235 @@
+"""Bit integer manipulation, both single bit and multi-bit list-like
+   slicing functions ( get, set, insert, remove ) implemented with
+   builtin bitwise operations.
+
+See:
+https://high-python-ext-3-algorithms.readthedocs.io/ko/latest/chapter5.html#insert-bit
+https://en.wikipedia.org/wiki/Bit_manipulation#Bit_manipulation_operations
+https://github.com/billbreit/BitWiseApps
+
+All parameters must be must be int >= 0, referred to as a 'bit integer'.
+
+ bint:int
+   The bit integer to be accessed or returned as modified.
+
+ index:int
+   The offset into the bit position from right,
+         0b010111 -> list [1,1,1,0,1,0]. big-endian -> little-endian
+   For inserts, index is the position to the right of index,
+         index 0 -> right of rightmost bit.
+   For gets, sets and removes, it is the position of the bit itself.
+
+ value:int
+   Either [0,1] for single bit, or bit mask, bit_length(value) <= bitlen.
+
+ bitlen:int
+   The effective mask length, spec. leading zeros
+         ( bitlen 4 value 1 -> 0001 )
+
+The bitwise expressions may look convoluted, but basically, there are
+just three parts: left-hand side, value, right-hand side.
+
+For example, say you want to insert two ones in the middle of 0b101101,
+that is ->  0b10111101.  Index is 2 ( 0 ,1, 2 from the right ) and the
+value is 3 (0b11) with a bit length of 2.
+
+- Shift >> index right to produce 0b101
+- Shift left << bit length to produce 0b10100.
+- OR in the ones producing 0b10111.
+- Left shift << index producing 0b10111000.
+- Using a bit mask (1 << index)-1 -> 0b111, AND with the original bint,
+  ( 0b101101 & 0b111 ) -> 0b101.
+- OR that into the working 0b10111000, that is, ( 0b10111000 | 0b101 )
+  -> 0b10111101.
+
+To remove the center two bits of 0b101101 -> 0b1001, the process is mostly
+the same.
+
+- The initial right shift is index(2) + bit_length(2), taking out the two
+  middle bits and producing 0b10.
+- The left shift of index produces 0b1000.
+- The original bint is ANDed with bitmask 0b11 producing 0b01 which is
+  ORed with 0b1000 yielding the target 0b1001.
+
+It's not so bad once you get the hang of it.
+
+Various bit insert/remove solutions exist using bin() string functions
+and slicing, but this bitwise implementation is significantly faster
+(about 3x) on Python for big ints (2^100).
+
+See https://github.com/billbreit/BitWiseApps/blob/main/dev/time_ops.py
+
+"""
+
+bit_length = int.bit_length
+
+"""The only consistent error checking is for bint < 0 or index < 0 etc.,
+   and for bit_length(value) > bit_len, which can cause silent errors.
+   Anything like int(None) is going to cause a loud error. """
+
+
+def bit_get(bint: int, index: int):
+    """Get value of bit at index in bint.
+
+    >>> bit_get(15, 0)
+    1
+    >>> bit_get(15, 4)
+    0
+    >>> bit_get(0, 4)
+    0
+    >>> bit_get(-1, 2) is None
+    True
+    >>> bit_get(0, -1) is None
+    True
+    """
+
+    return multibit_get(bint, index, 1)
+
+
+def bit_set(bint: int, index: int, value: int = 1):
+    """Set bit at index to value 1 or 0, like set() or unset().
+
+    >>> bit_set(15, 0, 0)
+    14
+    >>> bit_set(15, 4, 1)
+    31
+    >>> bit_set(31, 6, 0)
+    31
+    >>> bit_set(31, 6, 3) is None
+    True
+    """
+
+    if value not in [0, 1]:
+        return None  # error
+
+    return multibit_set(bint, index, 1, value)
+
+
+def bit_insert(bint: int, index: int, value: int = 1):
+    """Insert bit value before index.
+
+    >>> bit_insert(15, 0, 0)
+    30
+    >>> bit_insert(15, 0, 1)
+    31
+    >>> bit_insert(15, 4, 1)
+    31
+    >>> bit_insert(31, 6, 0)
+    31
+    """
+
+    if value not in [0, 1]:
+        return None  # error
+
+    return multibit_insert(bint, index, 1, value)
+
+
+def bit_remove(bint: int, index: int) -> int:
+    """Remove the bit at index from bint.
+
+    >>> bit_remove(15, 0)
+    7
+    >>> bit_remove(15, 1)
+    7
+    >>> bit_remove(31, 4)
+    15
+    >>> bit_remove(31, 6)
+    31
+    """
+
+    return multibit_remove(bint, index, 1)
+
+
+def multibit_get(bint: int, index: int, bit_len: int) -> int:
+    """Get bit_len number of bits starting from index.
+       819 = 1100110011.
+
+    >>> multibit_get(0, 1, 1)
+    0
+    >>> multibit_get(15, 0, 3)
+    7
+    >>> multibit_get(819, 2, 4)
+    12
+    >>> multibit_get(819, 4, 6)
+    51
+    """
+
+    if bint < 0 or index < 0 or bit_len < 0:
+        return None  # error
+
+    return (bint >> index) & ((1 << bit_len) - 1)
+
+
+def multibit_set(bint: int, index: int, bit_len: int, value: int) -> int:
+    """Overlay bint at index with value for bit_len bits.
+
+    >>> multibit_set(0, 1, 1, 0)
+    0
+    >>> multibit_set(15, 0, 2, 0)
+    12
+    >>> multibit_set(22, 0, 1, 1)
+    23
+    >>> multibit_set(22, 2, 1, 0)
+    18
+    >>> multibit_set(22, 2, 1, 3) is None
+    True
+    """
+
+    if bint < 0 or index < 0 or bit_len < 0 or value < 0:
+        return None  # error
+    if bit_length(value) > bit_len:
+        return None
+
+    return ((((bint >> (index + bit_len)) << bit_len) | value) << index) | (
+        bint & (1 << index) - 1
+    )
+
+
+def multibit_insert(bint: int, index: int, bit_len: int, value: int) -> int:
+    """Insert before index-th slot
+
+    >>> multibit_insert(0, 1, 1, 1)
+    2
+    >>> multibit_insert(15, 1, 2, 0)
+    57
+    >>> multibit_insert(22, 0, 1, 1)
+    45
+    >>> multibit_insert(22, 2, 1, 0)
+    42
+    >>> multibit_insert(22, 2, 1, 3) is None
+    True
+    """
+
+    if bint < 0 or index < 0 or bit_len < 0 or value < 0:
+        return None  # error
+    if bit_length(value) > bit_len:
+        return None
+
+    return ((((bint >> index) << bit_len) | value) << index) | bint & ((1 << index) - 1)
+
+
+def multibit_remove(bint: int, index: int, bit_len: int) -> int:
+    """Remove bits in bint from index to index+bit_len.
+
+    >>> multibit_remove(3, 1, 1)
+    1
+    >>> multibit_remove(15, 1, 2)
+    3
+    >>> multibit_remove(22, 0, 1)
+    11
+    >>> multibit_remove(22, 2, 2)
+    6
+    >>> multibit_remove(22, 2, 6)
+    2
+    """
+
+    if bint < 0 or index < 0:
+        return None  # error
+
+    return ((bint >> index + bit_len) << index) | bint & ((1 << index) - 1)
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()