/* ----------------------------------------------------------------------------
 *         SAM Software Package License
 * ----------------------------------------------------------------------------
 * Copyright (c) 2015, Atmel Corporation
 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 * - Redistributions of source code must retain the above copyright notice,
 * this list of conditions and the disclaimer below.
 *
 * Atmel's name may not be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * DISCLAIMER: THIS SOFTWARE IS PROVIDED BY ATMEL "AS IS" AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT ARE
 * DISCLAIMED. IN NO EVENT SHALL ATMEL BE LIABLE FOR ANY DIRECT, INDIRECT,
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 * ----------------------------------------------------------------------------
 */

/** \file */

/*----------------------------------------------------------------------------
 *        Headers
 *----------------------------------------------------------------------------*/

#include "hamming.h"
#include "trace.h"

/*----------------------------------------------------------------------------
 *         Internal function
 *----------------------------------------------------------------------------*/

/**
 *  Counts and return the number of bits set to '1' in the given byte.
 *  \param byte  Byte to count.
 */
static uint8_t count_bits_in_byte(uint8_t byte)
{
	uint8_t count = 0;

	while (byte > 0) {
		if (byte & 1) {
			count++;
		}
		byte >>= 1;
	}

	return count;
}

/**
 *  Counts and return the number of bits set to '1' in the given hamming code.
 *  \param code Hamming code.
 */
static uint8_t count_bits_in_code256(uint8_t *code)
{
	return count_bits_in_byte(code[0]) +
		count_bits_in_byte(code[1]) +
		count_bits_in_byte(code[2]);
}

/**
 *  Calculates the 22-bit hamming code for a 256-bytes block of data.
 *  \param data Data buffer to calculate code for.
 *  \param code Pointer to a buffer where the code should be stored.
 */
static void compute256(const uint8_t *data, uint8_t *code)
{
	uint32_t i;
	uint8_t column_sum = 0;
	uint8_t even_line_code = 0;
	uint8_t odd_line_code = 0;
	uint8_t even_column_code = 0;
	uint8_t odd_column_code = 0;

	// Xor all bytes together to get the column sum;
	// At the same time, calculate the even and odd line codes
	for (i = 0; i < 256; i++) {
		column_sum ^= data[i];

		// If the xor sum of the byte is 0, then this byte has no incidence on
		// the computed code; so check if the sum is 1.
		if ((count_bits_in_byte(data[i]) & 1) == 1) {
			// Parity groups are formed by forcing a particular index bit to 0
			// (even) or 1 (odd).
			// Example on one byte:
			//
			// bits (dec)  7   6   5   4   3   2   1   0
			//      (bin) 111 110 101 100 011 010 001 000
			//                            '---'---'---'----------.
			//                                                   |
			// groups P4' ooooooooooooooo eeeeeeeeeeeeeee P4     |
			//        P2' ooooooo eeeeeee ooooooo eeeeeee P2     |
			//        P1' ooo eee ooo eee ooo eee ooo eee P1     |
			//                                                   |
			// We can see that:                                  |
			//  - P4  -> bit 2 of index is 0 --------------------'
			//  - P4' -> bit 2 of index is 1.
			//  - P2  -> bit 1 of index if 0.
			//  - etc...
			// We deduce that a bit position has an impact on all even Px if
			// the log2(x)nth bit of its index is 0
			//     ex: log2(4) = 2, bit2 of the index must be 0 (-> 0 1 2 3)
			// and on all odd Px' if the log2(x)nth bit of its index is 1
			//     ex: log2(2) = 1, bit1 of the index must be 1 (-> 0 1 4 5)
			//
			// As such, we calculate all the possible Px and Px' values at the
			// same time in two variables, even_line_code and odd_line_code, such as
			//     even_line_code bits: P128  P64  P32  P16  P8  P4  P2  P1
			//     odd_line_code  bits: P128' P64' P32' P16' P8' P4' P2' P1'
			//
			even_line_code ^= (255 - i);
			odd_line_code ^= i;
		}
	}

	// At this point, we have the line parities, and the column sum. First, We
	// must caculate the parity group values on the column sum.
	for (i = 0; i < 8; i++) {
		if (column_sum & 1) {
			even_column_code ^= (7 - i);
			odd_column_code ^= i;
		}
		column_sum >>= 1;
	}

	// Now, we must interleave the parity values, to obtain the following layout:
	// Code[0] = Line1
	// Code[1] = Line2
	// Code[2] = Column
	// Line = Px' Px P(x-1)- P(x-1) ...
	// Column = P4' P4 P2' P2 P1' P1 PadBit PadBit
	code[0] = 0;
	code[1] = 0;
	code[2] = 0;

	for (i = 0; i < 4; i++) {
		code[0] <<= 2;
		code[1] <<= 2;
		code[2] <<= 2;

		// Line 1
		if ((odd_line_code & 0x80) != 0) {
			code[0] |= 2;
		}

		if ((even_line_code & 0x80) != 0) {
			code[0] |= 1;
		}
		// Line 2
		if ((odd_line_code & 0x08) != 0) {
			code[1] |= 2;
		}

		if ((even_line_code & 0x08) != 0) {
			code[1] |= 1;
		}
		// Column
		if ((odd_column_code & 0x04) != 0) {
			code[2] |= 2;
		}

		if ((even_column_code & 0x04) != 0) {
			code[2] |= 1;
		}

		odd_line_code <<= 1;
		even_line_code <<= 1;
		odd_column_code <<= 1;
		even_column_code <<= 1;
	}

	// Invert codes (linux compatibility)
	code[0] = ~code[0];
	code[1] = ~code[1];
	code[2] = ~code[2];

	trace_debug("Computed code = %02x %02x %02x\n\r",
			(unsigned)code[0],
			(unsigned)code[1],
			(unsigned)code[2]);
}

/**
 *  Verifies and corrects a 256-bytes block of data using the given 22-bits
 *  hamming code.
 *
 *  \param data Data buffer to check.
 *  \param code Hamming code to use for verifying the data.
 *
 *  \return 0 if there is no error, otherwise returns a HAMMING_ERROR code.
 */
static uint8_t verify256(uint8_t *data, const uint8_t *code)
{
	/* Calculate new code */
	uint8_t computed_code[3];
	uint8_t correction_code[3];

	compute256(data, computed_code);

	/* Xor both codes together */
	correction_code[0] = computed_code[0] ^ code[0];
	correction_code[1] = computed_code[1] ^ code[1];
	correction_code[2] = computed_code[2] ^ code[2];

	trace_debug("Correction code = %02x %02x %02x\n\r",
			(unsigned)correction_code[0],
			(unsigned)correction_code[1],
			(unsigned)correction_code[2]);

	// If all bytes are 0, there is no error
	if (correction_code[0] == 0 && correction_code[1] == 0 &&
			correction_code[2] == 0) {
		return 0;
	}

	/* If there is a single bit error, there are 11 bits set to 1 */
	if (count_bits_in_code256(correction_code) == 11) {
		// Get byte and bit indexes
		uint8_t byte = correction_code[0] & 0x80;
		byte |= (correction_code[0] << 1) & 0x40;
		byte |= (correction_code[0] << 2) & 0x20;
		byte |= (correction_code[0] << 3) & 0x10;

		byte |= (correction_code[1] >> 4) & 0x08;
		byte |= (correction_code[1] >> 3) & 0x04;
		byte |= (correction_code[1] >> 2) & 0x02;
		byte |= (correction_code[1] >> 1) & 0x01;

		uint8_t bit = (correction_code[2] >> 5) & 0x04;
		bit |= (correction_code[2] >> 4) & 0x02;
		bit |= (correction_code[2] >> 3) & 0x01;

		/* Correct bit */
		printf("Correcting byte #%d at bit %d\n\r", byte, bit);
		data[byte] ^= (1 << bit);

		return HAMMING_ERROR_SINGLEBIT;
	}

	/* Check if ECC has been corrupted */
	if (count_bits_in_code256(correction_code) == 1) {
		return HAMMING_ERROR_ECC;
	} else {
		/* Otherwise, this is a multi-bit error */
		return HAMMING_ERROR_MULTIPLEBITS;
	}
}

/*----------------------------------------------------------------------------
 *         Exported functions
 *----------------------------------------------------------------------------*/

/**
 *  Computes 3-bytes hamming codes for a data block whose size is multiple of
 *  256 bytes. Each 256 bytes block gets its own code.
 *  \param data Data to compute code for.
 *  \param size Data size in bytes.
 *  \param code Codes buffer.
 */
void hamming_compute_256x(const uint8_t *data, uint32_t size, uint8_t *code)
{
	trace_debug("hamming_compute_256x()\n\r");

	while (size > 0) {
		compute256(data, code);

		data += 256;
		code += 3;
		size -= 256;
	}
}

/**
 *  Verifies 3-bytes hamming codes for a data block whose size is multiple of
 *  256 bytes. Each 256-bytes block is verified with its own code.
 *
 *  \return 0 if the data is correct, HAMMING_ERROR_SINGLEBIT if one or more
 *  block(s) have had a single bit corrected, or either HAMMING_ERROR_ECC
 *  or HAMMING_ERROR_MULTIPLEBITS.
 *
 *  \param data Data buffer to verify.
 *  \param size Size of the data in bytes.
 *  \param code Original codes.
 */
uint8_t hamming_verify_256x(uint8_t *data, uint32_t size, const uint8_t *code)
{
	uint8_t error;
	uint8_t result = 0;

	trace_debug("hamming_verify_256x()\n\r");

	while (size > 0) {
		error = verify256(data, code);

		if (error == HAMMING_ERROR_SINGLEBIT) {
			result = HAMMING_ERROR_SINGLEBIT;
		} else {
			if (error) {
				return error;
			}
		}

		data += 256;
		code += 3;
		size -= 256;
	}

	return result;
}