add (untested) Hamming forward error correction code and corresponding class

The system now uses a FECModem instance, which inherits the receive methods
etc. from Modem.

Up next: Make the modem's buffer read methods private and expose them in
error-corrected FECModem methods instead

1 files changed, 49 insertions, 0 deletions

diff --git a/src/fecmodem.cc b/src/fecmodem.cc
new file mode 100644
index 0000000..5323135
--- /dev/null
+++ b/src/fecmodem.cc
@@ -0,0 +1,49 @@
+#include <avr/io.h>
+#include <stdlib.h>
+#include "fecmodem.h"
+
+uint8_t FECModem::parity128(uint8_t byte)
+{
+	return pgm_read_byte(&hammingParityLow[byte & 0x0f]) ^ pgm_read_byte(&hammingParityHigh[byte >> 4]);
+}
+
+uint8_t FECModem::parity2416(uint8_t byte1, uint8_t byte2)
+{
+	return parity128(byte1) | (parity128(byte2) << 4);
+}
+
+uint8_t FECModem::correct128(uint8_t *byte, uint8_t err)
+{
+	uint8_t result = pgm_read_byte(&hammingParityCheck[err & 0x0f]);
+
+	if (result != NO_ERROR) {
+		if (result == UNCORRECTABLE || byte == NULL) {
+			return 3;
+		} else {
+			if (result != ERROR_IN_PARITY) {
+				*byte ^= result;
+			}
+			return 1;
+		}
+	}
+	return 0;
+}
+
+uint8_t FECModem::hamming2416(uint8_t *byte1, uint8_t *byte2, uint8_t parity)
+{
+	uint8_t err;
+
+	if (byte1 == NULL || byte2 == NULL) {
+		return 3;
+	}
+
+	err = parity2416(*byte1, *byte2) ^ parity;
+
+	if (err) {
+		return correct128(byte1, err) + correct128(byte2, err >> 4);
+	}
+
+	return 0;
+}
+
+FECModem modem;