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Diffstat (limited to 'src/Hamming/Hamming.c')
| -rw-r--r-- | src/Hamming/Hamming.c | 174 |
1 files changed, 174 insertions, 0 deletions
diff --git a/src/Hamming/Hamming.c b/src/Hamming/Hamming.c new file mode 100644 index 0000000..cb1fba3 --- /dev/null +++ b/src/Hamming/Hamming.c @@ -0,0 +1,174 @@ +/* +Hamming Error-Correcting Code (ECC) +Optimized for avr-gcc 4.8.1 in Atmel AVR Studio 6.2 +August 12, 2014 + +You should include Hamming.c, Hamming.h, and only one of the other Hamming files in your project. +The other Hamming files implement the same methods in different ways, that may be better or worse for your needs. + +This was created for LoFi in the TheHackadayPrize contest. +http://hackaday.io/project/1552-LoFi + +Copyright 2014 David Cook +RobotRoom.com + +Licensed under the Apache License, Version 2.0 (the "License"); +you may not use this file except in compliance with the License. +You may obtain a copy of the License at + +http://www.apache.org/licenses/LICENSE-2.0 + +Unless required by applicable law or agreed to in writing, software +distributed under the License is distributed on an "AS IS" BASIS, +WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +See the License for the specific language governing permissions and +limitations under the License. +*/ + +#include <avr/pgmspace.h> + +#include "Hamming.h" + +/****************************/ +/* */ +/* Constants and structures */ +/* */ +/****************************/ + +#ifndef null +#define null ((void*) 0) +#endif + +// If transmitting/writing only, you don't need to include this file. +// If receiving/reading, then this provides the methods to correct bit errors. + +#define UNCORRECTABLE 0xFF +#define ERROR_IN_PARITY 0xFE +#define NO_ERROR 0x00 + + +/****************************/ +/* */ +/* Global variables */ +/* */ +/****************************/ + +// Private table. Faster and more compact than multiple if statements. +static const byte _hammingCorrect128Syndrome[16] PROGMEM = +{ + NO_ERROR, // 0 + ERROR_IN_PARITY, // 1 + ERROR_IN_PARITY, // 2 + 0x01, // 3 + ERROR_IN_PARITY, // 4 + 0x02, // 5 + 0x04, // 6 + 0x08, // 7 + ERROR_IN_PARITY, // 8 + 0x10, // 9 + 0x20, // 10 + 0x40, // 11 + 0x80, // 12 + UNCORRECTABLE, // 13 + UNCORRECTABLE, // 14 + UNCORRECTABLE, // 15 +}; + +/****************************/ +/* */ +/* Private methods */ +/* */ +/****************************/ + +// Give a pointer to a received byte, +// and given a nibble difference in parity (parity ^ calculated parity) +// this will correct the received byte value if possible. +// It returns the number of bits corrected: +// 0 means no errors +// 1 means one corrected error +// 3 means corrections not possible +static byte HammingCorrect128Syndrome(byte* value, byte syndrome) +{ + // Using only the lower nibble (& 0x0F), look up the bit + // to correct in a table + byte correction = pgm_read_byte(&(_hammingCorrect128Syndrome[syndrome & 0x0F])); + + if (correction != NO_ERROR) + { + if (correction == UNCORRECTABLE || value == null) + { + return 3; // Non-recoverable error + } + else + { + if ( correction != ERROR_IN_PARITY) + { + *value ^= correction; + } + + return 1; // 1-bit recoverable error; + } + } + + return 0; // No errors +} + + +/****************************/ +/* */ +/* Public methods */ +/* */ +/****************************/ + +// Given a pointer to a received byte and the received parity (as a lower nibble), +// this calculates what the parity should be and fixes the received value if needed. +// It returns the number of bits corrected: +// 0 means no errors +// 1 means one corrected error +// 3 means corrections not possible +byte HammingCorrect128(byte* value, nibble parity) +{ + byte syndrome; + + if (value == null) + { + return 3; // Non-recoverable error + } + + syndrome = HammingCalculateParity128(*value) ^ parity; + + if (syndrome != 0) + { + return HammingCorrect128Syndrome(value, syndrome); + } + + return 0; // No errors +} + + +// Given a pointer to a first value and a pointer to a second value and +// their combined given parity (lower nibble first parity, upper nibble second parity), +// this calculates what the parity should be and fixes the values if needed. +// It returns the number of bits corrected: +// 0 means no errors +// 1 means one corrected error +// 2 means two corrected errors +// 3 means corrections not possible +byte HammingCorrect2416(byte* first, byte* second, byte parity) +{ + byte syndrome; + + if (first == null || second == null) + { + return 3; // Non-recoverable error + } + + syndrome = HammingCalculateParity2416(*first, *second) ^ parity; + + if (syndrome != 0) + { + return HammingCorrect128Syndrome(first, syndrome) + HammingCorrect128Syndrome(second, syndrome >> 4); + } + + return 0; // No errors +} |