Add endianness conversion algorithm

- Implements byte order conversions for 16/32/64-bit integers
- Provides bidirectional bytes ↔ int conversions
- Supports both big-endian and little-endian formats
- Includes comprehensive doctests with real-world examples
- Documents use cases: network protocols, cryptography, file formats
- All tests pass: ruff, mypy, doctests

Author: rodrigo.nogueira

conversions/endianness.py

@@ -0,0 +1,329 @@
+"""
+Endianness Conversion Algorithm
+
+This module implements endianness (byte order) conversion utilities for converting
+between big-endian and little-endian representations of multi-byte integers.
+
+Endianness refers to the order of bytes in a multi-byte data type:
+- Big-endian: Most significant byte first (e.g., 0x12345678 → [0x12, 0x34, 0x56, 0x78])
+- Little-endian: Least significant byte first
+  (e.g., 0x12345678 → [0x78, 0x56, 0x34, 0x12])
+
+Common uses:
+- Network protocols (TCP/IP uses big-endian for multi-byte fields)
+- File format parsing (PNG, JPEG headers specify endianness)
+- Cryptographic algorithms (MD5 uses little-endian, SHA-256 uses big-endian)
+- Binary data serialization (Protocol Buffers, MessagePack)
+- Hardware interfacing (ARM is bi-endian, x86 is strictly little-endian)
+- Cross-platform data exchange
+
+Real-world examples:
+- IP addresses are transmitted in big-endian (network byte order)
+- Modern ARM and x86 processors typically operate in little-endian mode
+- Java class files use big-endian format
+- Bitcoin uses little-endian for block hashes
+- USB and PCI protocols use little-endian
+
+References:
+- https://en.wikipedia.org/wiki/Endianness
+- RFC 1700 (Network Byte Order)
+- https://tools.ietf.org/html/rfc1700
+"""
+
+
+def swap_endianness_16(value: int) -> int:
+    """
+    Swap the byte order of a 16-bit integer.
+
+    Args:
+        value: 16-bit integer (0 to 65,535)
+
+    Returns:
+        Integer with swapped byte order
+
+    Raises:
+        ValueError: If value is negative or exceeds 16-bit range
+
+    >>> swap_endianness_16(0x1234)
+    13330
+
+    >>> hex(swap_endianness_16(0x1234))
+    '0x3412'
+
+    >>> swap_endianness_16(0xABCD)
+    52651
+
+    >>> hex(swap_endianness_16(0xABCD))
+    '0xcdab'
+
+    >>> swap_endianness_16(0)
+    0
+
+    >>> swap_endianness_16(0xFFFF)
+    65535
+    """
+    if value < 0 or value > 0xFFFF:
+        msg = f"value must be between 0 and {0xFFFF}"
+        raise ValueError(msg)
+
+    return ((value & 0xFF) << 8) | ((value & 0xFF00) >> 8)
+
+
+def swap_endianness_32(value: int) -> int:
+    """
+    Swap the byte order of a 32-bit integer.
+
+    Args:
+        value: 32-bit integer (0 to 4,294,967,295)
+
+    Returns:
+        Integer with swapped byte order
+
+    Raises:
+        ValueError: If value is negative or exceeds 32-bit range
+
+    >>> swap_endianness_32(0x12345678)
+    2018915346
+
+    >>> hex(swap_endianness_32(0x12345678))
+    '0x78563412'
+
+    >>> swap_endianness_32(0xDEADBEEF)
+    4022250974
+
+    >>> hex(swap_endianness_32(0xDEADBEEF))
+    '0xefbeadde'
+
+    >>> swap_endianness_32(0)
+    0
+
+    >>> swap_endianness_32(0xFFFFFFFF)
+    4294967295
+
+    >>> swap_endianness_32(0x01020304)
+    67305985
+    """
+    if value < 0 or value > 0xFFFFFFFF:
+        msg = f"value must be between 0 and {0xFFFFFFFF}"
+        raise ValueError(msg)
+
+    return (
+        ((value & 0x000000FF) << 24)
+        | ((value & 0x0000FF00) << 8)
+        | ((value & 0x00FF0000) >> 8)
+        | ((value & 0xFF000000) >> 24)
+    )
+
+
+def swap_endianness_64(value: int) -> int:
+    """
+    Swap the byte order of a 64-bit integer.
+
+    Args:
+        value: 64-bit integer (0 to 18,446,744,073,709,551,615)
+
+    Returns:
+        Integer with swapped byte order
+
+    Raises:
+        ValueError: If value is negative or exceeds 64-bit range
+
+    >>> swap_endianness_64(0x0123456789ABCDEF)
+    17279655951921914625
+
+    >>> hex(swap_endianness_64(0x0123456789ABCDEF))
+    '0xefcdab8967452301'
+
+    >>> swap_endianness_64(0xFEDCBA9876543210)
+    1167088121787636990
+
+    >>> hex(swap_endianness_64(0xFEDCBA9876543210))
+    '0x1032547698badcfe'
+
+    >>> swap_endianness_64(0)
+    0
+
+    >>> swap_endianness_64(0xFFFFFFFFFFFFFFFF)
+    18446744073709551615
+    """
+    if value < 0 or value > 0xFFFFFFFFFFFFFFFF:
+        msg = f"value must be between 0 and {0xFFFFFFFFFFFFFFFF}"
+        raise ValueError(msg)
+
+    return (
+        ((value & 0x00000000000000FF) << 56)
+        | ((value & 0x000000000000FF00) << 40)
+        | ((value & 0x0000000000FF0000) << 24)
+        | ((value & 0x00000000FF000000) << 8)
+        | ((value & 0x000000FF00000000) >> 8)
+        | ((value & 0x0000FF0000000000) >> 24)
+        | ((value & 0x00FF000000000000) >> 40)
+        | ((value & 0xFF00000000000000) >> 56)
+    )
+
+
+def bytes_to_int_little(data: bytes) -> int:
+    """
+    Convert bytes to integer using little-endian byte order.
+
+    Args:
+        data: Byte sequence to convert (1-8 bytes)
+
+    Returns:
+        Integer representation in little-endian order
+
+    Raises:
+        TypeError: If data is not bytes
+        ValueError: If data is empty or exceeds 8 bytes
+
+    >>> bytes_to_int_little(b'\\x78\\x56\\x34\\x12')
+    305419896
+
+    >>> hex(bytes_to_int_little(b'\\x78\\x56\\x34\\x12'))
+    '0x12345678'
+
+    >>> bytes_to_int_little(b'\\x01\\x02')
+    513
+
+    >>> bytes_to_int_little(b'\\xff')
+    255
+
+    >>> bytes_to_int_little(b'\\x00\\x00\\x00\\x01')
+    16777216
+    """
+    if not data or len(data) > 8:
+        msg = "data must be between 1 and 8 bytes"
+        raise ValueError(msg)
+
+    return int.from_bytes(data, byteorder="little")
+
+
+def bytes_to_int_big(data: bytes) -> int:
+    """
+    Convert bytes to integer using big-endian byte order.
+
+    Args:
+        data: Byte sequence to convert (1-8 bytes)
+
+    Returns:
+        Integer representation in big-endian order
+
+    Raises:
+        TypeError: If data is not bytes
+        ValueError: If data is empty or exceeds 8 bytes
+
+    >>> bytes_to_int_big(b'\\x12\\x34\\x56\\x78')
+    305419896
+
+    >>> hex(bytes_to_int_big(b'\\x12\\x34\\x56\\x78'))
+    '0x12345678'
+
+    >>> bytes_to_int_big(b'\\x01\\x02')
+    258
+
+    >>> bytes_to_int_big(b'\\xff')
+    255
+
+    >>> bytes_to_int_big(b'\\x00\\x00\\x00\\x01')
+    1
+    """
+    if not data or len(data) > 8:
+        msg = "data must be between 1 and 8 bytes"
+        raise ValueError(msg)
+
+    return int.from_bytes(data, byteorder="big")
+
+
+def int_to_bytes_little(value: int, num_bytes: int) -> bytes:
+    """
+    Convert integer to bytes using little-endian byte order.
+
+    Args:
+        value: Integer to convert (non-negative)
+        num_bytes: Number of bytes in output (1-8)
+
+    Returns:
+        Bytes representation in little-endian order
+
+    Raises:
+        ValueError: If value is negative, num_bytes invalid, or value too large
+
+    >>> int_to_bytes_little(0x12345678, 4)
+    b'xV4\\x12'
+
+    >>> int_to_bytes_little(513, 2)
+    b'\\x01\\x02'
+
+    >>> int_to_bytes_little(255, 1)
+    b'\\xff'
+
+    >>> int_to_bytes_little(16777216, 4)
+    b'\\x00\\x00\\x00\\x01'
+    """
+    if value < 0:
+        msg = "value must be non-negative"
+        raise ValueError(msg)
+
+    if num_bytes < 1 or num_bytes > 8:
+        msg = "num_bytes must be between 1 and 8"
+        raise ValueError(msg)
+
+    if value >= (1 << (num_bytes * 8)):
+        msg = f"value {value} too large for {num_bytes} bytes"
+        raise ValueError(msg)
+
+    return value.to_bytes(num_bytes, byteorder="little")
+
+
+def int_to_bytes_big(value: int, num_bytes: int) -> bytes:
+    """
+    Convert integer to bytes using big-endian byte order.
+
+    Args:
+        value: Integer to convert (non-negative)
+        num_bytes: Number of bytes in output (1-8)
+
+    Returns:
+        Bytes representation in big-endian order
+
+    Raises:
+        ValueError: If value is negative, num_bytes invalid, or value too large
+
+    >>> int_to_bytes_big(0x12345678, 4)
+    b'\\x124Vx'
+
+    >>> int_to_bytes_big(258, 2)
+    b'\\x01\\x02'
+
+    >>> int_to_bytes_big(255, 1)
+    b'\\xff'
+
+    >>> int_to_bytes_big(1, 4)
+    b'\\x00\\x00\\x00\\x01'
+    """
+    if value < 0:
+        msg = "value must be non-negative"
+        raise ValueError(msg)
+
+    if num_bytes < 1 or num_bytes > 8:
+        msg = "num_bytes must be between 1 and 8"
+        raise ValueError(msg)
+
+    if value >= (1 << (num_bytes * 8)):
+        msg = f"value {value} too large for {num_bytes} bytes"
+        raise ValueError(msg)
+
+    return value.to_bytes(num_bytes, byteorder="big")
+
+
+if __name__ == "__main__":
+    import doctest
+
+    _ = doctest.testmod()
+
+    print("Endianness Conversion Examples:")
+    print(f"16-bit: 0x1234 → {hex(swap_endianness_16(0x1234))}")
+    print(f"32-bit: 0x12345678 → {hex(swap_endianness_32(0x12345678))}")
+    print(f"64-bit: 0x0123456789ABCDEF → {hex(swap_endianness_64(0x0123456789ABCDEF))}")
+    print(f"Bytes to int (little): {bytes_to_int_little(b'\x78\x56\x34\x12'):#x}")
+    print(f"Bytes to int (big): {bytes_to_int_big(b'\x12\x34\x56\x78'):#x}")