// SPDX-License-Identifier: GPL-2.0-only

  /*
   * crc4.c - simple crc-4 calculations.

   */
  
  #include <linux/crc4.h>
  #include <linux/module.h>
  
  static const uint8_t crc4_tab[] = {
  	0x0, 0x7, 0xe, 0x9, 0xb, 0xc, 0x5, 0x2,
  	0x1, 0x6, 0xf, 0x8, 0xa, 0xd, 0x4, 0x3,
  };
  
  /**
   * crc4 - calculate the 4-bit crc of a value.

   * @c:    starting crc4

   * @x:    value to checksum
   * @bits: number of bits in @x to checksum
   *
   * Returns the crc4 value of @x, using polynomial 0b10111.
   *
   * The @x value is treated as left-aligned, and bits above @bits are ignored
   * in the crc calculations.
   */
  uint8_t crc4(uint8_t c, uint64_t x, int bits)
  {
  	int i;
  
  	/* mask off anything above the top bit */
  	x &= (1ull << bits) - 1;
  
  	/* Align to 4-bits */
  	bits = (bits + 3) & ~0x3;
  
  	/* Calculate crc4 over four-bit nibbles, starting at the MSbit */
  	for (i = bits - 4; i >= 0; i -= 4)
  		c = crc4_tab[c ^ ((x >> i) & 0xf)];
  
  	return c;
  }
  EXPORT_SYMBOL_GPL(crc4);
  
  MODULE_DESCRIPTION("CRC4 calculations");
  MODULE_LICENSE("GPL");