/*
   * YAFFS: Yet Another Flash File System. A NAND-flash specific file system.
   *

   * Copyright (C) 2002-2011 Aleph One Ltd.

   *   for Toby Churchill Ltd and Brightstar Engineering
   *
   * Created by Charles Manning <charles@aleph1.co.uk>
   *
   * This program is free software; you can redistribute it and/or modify
   * it under the terms of the GNU General Public License version 2 as
   * published by the Free Software Foundation.
   */
  
  /*
   * This code implements the ECC algorithm used in SmartMedia.
   *

   * The ECC comprises 22 bits of parity information and is stuffed into 3 bytes.

   * The two unused bit are set to 1.

   * The ECC can correct single bit errors in a 256-byte page of data. Thus, two
   * such ECC blocks are used on a 512-byte NAND page.

   *
   */

  #include "yportenv.h"
  
  #include "yaffs_ecc.h"

  /* Table generated by gen-ecc.c
   * Using a table means we do not have to calculate p1..p4 and p1'..p4'
   * for each byte of data. These are instead provided in a table in bits7..2.

   * Bit 0 of each entry indicates whether the entry has an odd or even parity,
   * and therefore this bytes influence on the line parity.

   */

  static const unsigned char column_parity_table[] = {
  	0x00, 0x55, 0x59, 0x0c, 0x65, 0x30, 0x3c, 0x69,
  	0x69, 0x3c, 0x30, 0x65, 0x0c, 0x59, 0x55, 0x00,
  	0x95, 0xc0, 0xcc, 0x99, 0xf0, 0xa5, 0xa9, 0xfc,
  	0xfc, 0xa9, 0xa5, 0xf0, 0x99, 0xcc, 0xc0, 0x95,
  	0x99, 0xcc, 0xc0, 0x95, 0xfc, 0xa9, 0xa5, 0xf0,
  	0xf0, 0xa5, 0xa9, 0xfc, 0x95, 0xc0, 0xcc, 0x99,
  	0x0c, 0x59, 0x55, 0x00, 0x69, 0x3c, 0x30, 0x65,
  	0x65, 0x30, 0x3c, 0x69, 0x00, 0x55, 0x59, 0x0c,
  	0xa5, 0xf0, 0xfc, 0xa9, 0xc0, 0x95, 0x99, 0xcc,
  	0xcc, 0x99, 0x95, 0xc0, 0xa9, 0xfc, 0xf0, 0xa5,
  	0x30, 0x65, 0x69, 0x3c, 0x55, 0x00, 0x0c, 0x59,
  	0x59, 0x0c, 0x00, 0x55, 0x3c, 0x69, 0x65, 0x30,
  	0x3c, 0x69, 0x65, 0x30, 0x59, 0x0c, 0x00, 0x55,
  	0x55, 0x00, 0x0c, 0x59, 0x30, 0x65, 0x69, 0x3c,
  	0xa9, 0xfc, 0xf0, 0xa5, 0xcc, 0x99, 0x95, 0xc0,
  	0xc0, 0x95, 0x99, 0xcc, 0xa5, 0xf0, 0xfc, 0xa9,
  	0xa9, 0xfc, 0xf0, 0xa5, 0xcc, 0x99, 0x95, 0xc0,
  	0xc0, 0x95, 0x99, 0xcc, 0xa5, 0xf0, 0xfc, 0xa9,
  	0x3c, 0x69, 0x65, 0x30, 0x59, 0x0c, 0x00, 0x55,
  	0x55, 0x00, 0x0c, 0x59, 0x30, 0x65, 0x69, 0x3c,
  	0x30, 0x65, 0x69, 0x3c, 0x55, 0x00, 0x0c, 0x59,
  	0x59, 0x0c, 0x00, 0x55, 0x3c, 0x69, 0x65, 0x30,
  	0xa5, 0xf0, 0xfc, 0xa9, 0xc0, 0x95, 0x99, 0xcc,
  	0xcc, 0x99, 0x95, 0xc0, 0xa9, 0xfc, 0xf0, 0xa5,
  	0x0c, 0x59, 0x55, 0x00, 0x69, 0x3c, 0x30, 0x65,
  	0x65, 0x30, 0x3c, 0x69, 0x00, 0x55, 0x59, 0x0c,
  	0x99, 0xcc, 0xc0, 0x95, 0xfc, 0xa9, 0xa5, 0xf0,
  	0xf0, 0xa5, 0xa9, 0xfc, 0x95, 0xc0, 0xcc, 0x99,
  	0x95, 0xc0, 0xcc, 0x99, 0xf0, 0xa5, 0xa9, 0xfc,
  	0xfc, 0xa9, 0xa5, 0xf0, 0x99, 0xcc, 0xc0, 0x95,
  	0x00, 0x55, 0x59, 0x0c, 0x65, 0x30, 0x3c, 0x69,
  	0x69, 0x3c, 0x30, 0x65, 0x0c, 0x59, 0x55, 0x00,
  };

  
  /* Calculate the ECC for a 256-byte block of data */

  void yaffs_ecc_calc(const unsigned char *data, unsigned char *ecc)

  {
  	unsigned int i;

  	unsigned char col_parity = 0;
  	unsigned char line_parity = 0;
  	unsigned char line_parity_prime = 0;
  	unsigned char t;
  	unsigned char b;
  
  	for (i = 0; i < 256; i++) {
  		b = column_parity_table[*data++];
  		col_parity ^= b;

  		if (b & 0x01) {	/* odd number of bits in the byte */

  			line_parity ^= i;
  			line_parity_prime ^= ~i;
  		}

  	}
  
  	ecc[2] = (~col_parity) | 0x03;
  
  	t = 0;
  	if (line_parity & 0x80)
  		t |= 0x80;
  	if (line_parity_prime & 0x80)
  		t |= 0x40;
  	if (line_parity & 0x40)
  		t |= 0x20;
  	if (line_parity_prime & 0x40)
  		t |= 0x10;
  	if (line_parity & 0x20)
  		t |= 0x08;
  	if (line_parity_prime & 0x20)
  		t |= 0x04;
  	if (line_parity & 0x10)
  		t |= 0x02;
  	if (line_parity_prime & 0x10)
  		t |= 0x01;
  	ecc[1] = ~t;
  
  	t = 0;
  	if (line_parity & 0x08)
  		t |= 0x80;
  	if (line_parity_prime & 0x08)
  		t |= 0x40;
  	if (line_parity & 0x04)
  		t |= 0x20;
  	if (line_parity_prime & 0x04)
  		t |= 0x10;
  	if (line_parity & 0x02)
  		t |= 0x08;
  	if (line_parity_prime & 0x02)
  		t |= 0x04;
  	if (line_parity & 0x01)
  		t |= 0x02;
  	if (line_parity_prime & 0x01)
  		t |= 0x01;
  	ecc[0] = ~t;

  }

  /* Correct the ECC on a 256 byte block of data */

  int yaffs_ecc_correct(unsigned char *data, unsigned char *read_ecc,
  		      const unsigned char *test_ecc)

  {
  	unsigned char d0, d1, d2;	/* deltas */
  
  	d0 = read_ecc[0] ^ test_ecc[0];
  	d1 = read_ecc[1] ^ test_ecc[1];
  	d2 = read_ecc[2] ^ test_ecc[2];
  
  	if ((d0 | d1 | d2) == 0)

  		return 0;	/* no error */

  
  	if (((d0 ^ (d0 >> 1)) & 0x55) == 0x55 &&
  	    ((d1 ^ (d1 >> 1)) & 0x55) == 0x55 &&
  	    ((d2 ^ (d2 >> 1)) & 0x54) == 0x54) {
  		/* Single bit (recoverable) error in data */
  
  		unsigned byte;
  		unsigned bit;

  		bit = byte = 0;
  
  		if (d1 & 0x80)
  			byte |= 0x80;
  		if (d1 & 0x20)
  			byte |= 0x40;
  		if (d1 & 0x08)
  			byte |= 0x20;
  		if (d1 & 0x02)
  			byte |= 0x10;
  		if (d0 & 0x80)
  			byte |= 0x08;
  		if (d0 & 0x20)
  			byte |= 0x04;
  		if (d0 & 0x08)
  			byte |= 0x02;
  		if (d0 & 0x02)
  			byte |= 0x01;
  
  		if (d2 & 0x80)
  			bit |= 0x04;
  		if (d2 & 0x20)
  			bit |= 0x02;
  		if (d2 & 0x08)
  			bit |= 0x01;
  
  		data[byte] ^= (1 << bit);

  		return 1;	/* Corrected the error */

  	}

  	if ((hweight8(d0) + hweight8(d1) + hweight8(d2)) == 1) {

  		/* Reccoverable error in ecc */
  
  		read_ecc[0] = test_ecc[0];
  		read_ecc[1] = test_ecc[1];
  		read_ecc[2] = test_ecc[2];

  		return 1;	/* Corrected the error */

  	}

  	/* Unrecoverable error */
  
  	return -1;
  
  }

  /*
   * ECCxxxOther does ECC calcs on arbitrary n bytes of data
   */

  void yaffs_ecc_calc_other(const unsigned char *data, unsigned n_bytes,
  			  struct yaffs_ecc_other *ecc_other)

  {
  	unsigned int i;

  	unsigned char col_parity = 0;
  	unsigned line_parity = 0;
  	unsigned line_parity_prime = 0;
  	unsigned char b;

  	for (i = 0; i < n_bytes; i++) {

  		b = column_parity_table[*data++];
  		col_parity ^= b;

  		if (b & 0x01) {

  			/* odd number of bits in the byte */
  			line_parity ^= i;
  			line_parity_prime ^= ~i;
  		}
  
  	}

  	ecc_other->col_parity = (col_parity >> 2) & 0x3f;
  	ecc_other->line_parity = line_parity;
  	ecc_other->line_parity_prime = line_parity_prime;

  }

  int yaffs_ecc_correct_other(unsigned char *data, unsigned n_bytes,
  			    struct yaffs_ecc_other *read_ecc,
  			    const struct yaffs_ecc_other *test_ecc)

  {

  	unsigned char delta_col;	/* column parity delta */
  	unsigned delta_line;	/* line parity delta */
  	unsigned delta_line_prime;	/* line parity delta */

  	unsigned bit;

  	delta_col = read_ecc->col_parity ^ test_ecc->col_parity;
  	delta_line = read_ecc->line_parity ^ test_ecc->line_parity;
  	delta_line_prime =
  	    read_ecc->line_parity_prime ^ test_ecc->line_parity_prime;

  	if ((delta_col | delta_line | delta_line_prime) == 0)
  		return 0;	/* no error */

  	if (delta_line == ~delta_line_prime &&
  	    (((delta_col ^ (delta_col >> 1)) & 0x15) == 0x15)) {

  		/* Single bit (recoverable) error in data */
  
  		bit = 0;

  		if (delta_col & 0x20)

  			bit |= 0x04;

  		if (delta_col & 0x08)

  			bit |= 0x02;

  		if (delta_col & 0x02)

  			bit |= 0x01;

  		if (delta_line >= n_bytes)

  			return -1;

  		data[delta_line] ^= (1 << bit);

  		return 1;	/* corrected */

  	}

  	if ((hweight32(delta_line) +
  	     hweight32(delta_line_prime) +
  	     hweight8(delta_col)) == 1) {

  		/* Reccoverable error in ecc */
  
  		*read_ecc = *test_ecc;

  		return 1;	/* corrected */

  	}
  
  	/* Unrecoverable error */
  
  	return -1;

  }