takeskip 0.2.0

Do bit manipulations with take/skip commands

TakeSkip

A Python library for declarative bit manipulation using an intuitive command syntax.

Overview

TakeSkip provides a domain-specific language for selecting, rearranging, and manipulating bits in binary arrays. Instead of writing complex indexing logic, you can express operations using simple commands like t8s4r8 (take 8 bits, skip 4, reverse 8).

Installation

pip install takeskip

Basic Usage

import numpy as np
from takeskip import takeskip

# Create a binary array (8 bits)
bits = np.array([1, 0, 1, 1, 0, 0, 1, 0], dtype=np.uint8)

# Skip first 2 bits, take next 4
result = takeskip("s2t4", bits)
print(result)  # [1, 1, 0, 0]

# Take 4 bits, reverse next 4
result = takeskip("t4r4", bits)
print(result)  # [1, 0, 1, 1, 0, 1, 0, 0]

Command Reference

Basic Operations

Command	Description	Example
t<n>	Take n bits	t8 - take 8 bits
s<n>	Skip n bits	s4 - skip 4 bits
r<n>	Reverse n bits	r8 - reverse 8 bits
i<n>	Invert n bits (0↔1)	i4 - invert 4 bits
ri<n>	Reverse and invert n bits	ri8 - reverse then invert 8 bits
b<n>	Backup pointer n positions	b4 - move back 4 positions

Padding Operations

Command	Description	Example
z<n>	Insert n zeros	z4 - add 4 zeros
n<n>	Insert n ones	n4 - add 4 ones
d<binary>	Insert literal binary data	d101 - add bits 1,0,1

Permutation

Command	Description	Example
p<indices>	Permute using 1-based indices	p1,3,5 - select bits 1, 3, 5
p<range>	Permute using ranges	p1-4 - select bits 1 through 4
p<mixed>	Mix indices and ranges	p1-4,8,6-5 - complex permutation

Note: Permutation indices are 1-based for user convenience. Ranges are inclusive.

Grouping and Repetition

# Repeat a sequence
bits = np.array([1, 0] * 8, dtype=np.uint8)
result = takeskip("(t4s4)3", bits)  # Repeat "take 4, skip 4" three times

# Group complex operations
result = takeskip("(t2r2s1)4", bits)  # Repeat grouped operations

Advanced Examples

Nibble Swap (swap 4-bit chunks)

bits = np.array([1, 1, 1, 1, 0, 0, 0, 0], dtype=np.uint8)
result = takeskip("s4t4b8t4", bits)  # [0, 0, 0, 0, 1, 1, 1, 1]

Extract Every Other Bit

bits = np.array([1, 0, 1, 0, 1, 0, 1, 0], dtype=np.uint8)
result = takeskip("(t1s1)4", bits)  # [1, 1, 1, 1]

Deinterleave Nibbles

bits = np.array([1, 0, 1, 0, 1, 0, 1, 0], dtype=np.uint8)
result = takeskip("(t1s1)4 b8 (s1t1)4", bits)  # [1, 1, 1, 1, 0, 0, 0, 0]

Complex Permutation

bits = np.array([1, 0, 1, 1, 0, 0, 1, 0], dtype=np.uint8)
# Take bits at positions 1,2,3,4 then 8,7,6,5 (1-based)
result = takeskip("p1-4,8-5", bits)  # [1, 0, 1, 1, 0, 1, 0, 0]

Interleave Pattern with Padding

bits = np.array([1, 1, 1, 1, 0, 0, 0, 0], dtype=np.uint8)
result = takeskip("(t1z1)4", bits)
# [1, 0, 1, 0, 1, 0, 1, 0] - interleaved with zeros

Remnant Handling

Control what happens to remaining bits after commands execute:

bits = np.array([1, 0, 1, 1, 0, 0, 1, 0], dtype=np.uint8)

# Default: discard remaining bits
result = takeskip("t4", bits, remnant="remove")  # [1, 0, 1, 1]

# Keep remaining bits
result = takeskip("t4", bits, remnant="keep")  # [1, 0, 1, 1, 0, 0, 1, 0]

# Pad with zeros to original length
result = takeskip("t4", bits, remnant="pad")  # [1, 0, 1, 1, 0, 0, 0, 0]

Multi-dimensional Arrays

TakeSkip operates on the last axis of multi-dimensional arrays:

# 2D array: 3 rows of 8 bits each
bits = np.array([
    [1, 0, 1, 1, 0, 0, 1, 0],
    [0, 1, 0, 1, 1, 1, 0, 1],
    [1, 1, 0, 0, 1, 0, 1, 1],
], dtype=np.uint8)

result = takeskip("s2t4", bits)
# [[1, 1, 0, 0],
#  [0, 1, 1, 1],
#  [0, 0, 1, 0]]

Use Cases

• Data packing/unpacking: Extract fields from packed binary formats
• Protocol parsing: Parse binary protocols with field alignment
• Bit manipulation: Rearrange bits in encryption/encoding operations

Command Chaining

Commands are executed left-to-right with a maintained pointer. The pointer must stay within [0, array_length] after each command; a ValueError is raised if Backup moves before the start or Skip/Take moves past the end.

bits = np.array([1, 0, 1, 1, 0, 0, 1, 0], dtype=np.uint8)

# Pointer starts at 0
# t2: take bits 0-1 -> [1,0], pointer now at 2
# s2: skip bits 2-3, pointer now at 4  
# t4: take bits 4-7 -> [0,0,1,0], pointer now at 8
result = takeskip("t2s2t4", bits)  # [1, 0, 0, 0, 1, 0]

Syntax Notes

• Commands are case-insensitive: T4, t4, and T4 are equivalent
• Whitespace is ignored: t4 s2 r8 = t4s2r8
• Parentheses create groups: (t8s8)4 repeats the sequence 4 times
• Permutation uses 1-based indexing (bit 1 is the first bit)
• Ranges in permutation are inclusive: 1-4 includes bits 1, 2, 3, and 4

API Documentation

Main Function

def takeskip(
    command: str,
    array: npt.NDArray[np.uint8],
    *,
    remnant: Literal["remove", "keep", "pad"] = "remove",
) -> np.ndarray

Parameters:

• command: Command string expressing the operation
• array: Target numpy array (dtype=uint8, values 0 or 1)
• remnant: How to handle remaining bits ("remove", "keep", or "pad")

Returns:

• Numpy array with same dtype, modified according to commands

Raises:

• ValueError: Invalid remnant argument or invalid command syntax

Contributing

When adding new commands:

1. Define command class in commands.py inheriting from Command
2. Add grammar rule in takeskip.lark
3. Add transformer method in parser.py
4. Update documentation

License

takeskip is licensed under the MIT License - see the LICENSE file for details