1 files changed, 143 insertions, 0 deletions
diff --git a/src/core/ao_viterbi.c b/src/core/ao_viterbi.c
new file mode 100644
index 00000000..2d441f4b
--- /dev/null
+++ b/src/core/ao_viterbi.c
@@ -0,0 +1,143 @@
+/*
+ * Copyright © 2012 Keith Packard <keithp@keithp.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
+ */
+
+#include <ao_fec.h>
+#include <stdio.h>
+
+/*
+ * 'input' is 8-bits per symbol soft decision data
+ * 'len' is output byte length
+ */
+
+static const uint8_t ao_fec_encode_table[16] = {
+/* next 0  1	  state */
+	0, 3,	/* 000 */
+	1, 2,	/* 001 */
+	3, 0,	/* 010 */
+	2, 1,	/* 011 */
+	3, 0,	/* 100 */
+	2, 1,	/* 101 */
+	0, 3,	/* 110 */
+	1, 2	/* 111 */
+};
+
+struct ao_soft_sym {
+	uint8_t	a, b;
+};
+
+struct ao_soft_sym
+ao_soft_sym(uint8_t bits)
+{
+	struct ao_soft_sym	s;
+
+	s.a = ((bits & 2) >> 1) * 0xff;
+	s.b = (bits & 1) * 0xff;
+	return s;
+}
+
+uint8_t
+ao_next_state(uint8_t state, uint8_t bit)
+{
+	return ((state << 1) | bit) & 0x7;
+}
+
+static inline abs(int x) { return x < 0 ? -x : x; }
+
+int
+ao_cost(struct ao_soft_sym a, struct ao_soft_sym b)
+{
+	return abs(a.a - b.a) + abs(a.b - b.b);
+}
+
+uint8_t
+ao_fec_decode(uint8_t *in, int len, uint8_t *out)
+{
+	int	cost[len/2 + 1][8];
+	uint8_t	prev[len/2 + 1][8];
+	int	c;
+	int	i, b;
+	uint8_t	state = 0, min_state;
+	uint8_t	bits[len/2];
+
+	for (c = 0; c < 8; c++)
+		cost[0][c] = 10000;
+	cost[0][0] = 0;
+
+	for (i = 0; i < len; i += 2) {
+		b = i/2;
+		struct ao_soft_sym s = { .a = in[i], .b = in[i+1] };
+
+		for (state = 0; state < 8; state++)
+			cost[b+1][state] = 10000;
+
+		for (state = 0; state < 8; state++) {
+			struct ao_soft_sym zero = ao_soft_sym(ao_fec_encode_table[state * 2 + 0]);
+			struct ao_soft_sym one = ao_soft_sym(ao_fec_encode_table[state * 2 + 1]);
+			uint8_t	zero_state = ao_next_state(state, 0);
+			uint8_t	one_state = ao_next_state(state, 1);
+			int	zero_cost = ao_cost(s, zero);
+			int	one_cost = ao_cost(s, one);
+
+#if 0
+			printf ("saw %02x %02x expected %02x %02x (%d) or %02x %02x (%d)\n",
+				s.a, s.b, zero.a, zero.b, zero_cost, one.a, one.b, one_cost);
+#endif
+			zero_cost += cost[b][state];
+			one_cost += cost[b][state];
+			if (zero_cost < cost[b+1][zero_state]) {
+				prev[b+1][zero_state] = state;
+				cost[b+1][zero_state] = zero_cost;
+			}
+
+			if (one_cost < cost[b+1][one_state]) {
+				prev[b+1][one_state] = state;
+				cost[b+1][one_state] = one_cost;
+			}
+		}
+
+		printf ("bit %3d symbol %2x %2x:", i/2, s.a, s.b);
+		for (state = 0; state < 8; state++) {
+			printf (" %5d", cost[b+1][state]);
+		}
+		printf ("\n");
+	}
+
+	b = len / 2;
+	c = cost[b][0];
+	min_state = 0;
+	for (state = 1; state < 8; state++) {
+		if (cost[b][state] < c) {
+			c = cost[b][state];
+			min_state = state;
+		}
+	}
+
+	for (b = len/2; b > 0; b--) {
+		bits[b-1] = min_state & 1;
+		min_state = prev[b][min_state];
+	}
+
+	for (i = 0; i < len/2; i += 8) {
+		uint8_t	byte;
+
+		byte = 0;
+		for (b = 0; b < 8; b++)
+			byte = (byte << 1) | bits[i + b];
+		out[i/8] = byte;
+	}
+	return len/16;
+}