remove_dust: don't ever create dust outputs.

This behavior will be specified in BOLT #3.

Closes: #14

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Makefile

@@ -44,6 +44,7 @@ CORE_SRC :=					\
 	opt_bits.c				\
 	permute_tx.c				\
 	protobuf_convert.c			\
+	remove_dust.c				\
 	version.c
 CORE_OBJS := $(CORE_SRC:.c=.o)
 
@@ -152,6 +153,7 @@ CORE_HEADERS := close_tx.h			\
 	overflows.h				\
 	permute_tx.h				\
 	protobuf_convert.h			\
+	remove_dust.h				\
 	state.h					\
 	state_types.h				\
 	version.h

commit_tx.c

@@ -7,6 +7,7 @@
 #include "funding.h"
 #include "overflows.h"
 #include "permute_tx.h"
+#include "remove_dust.h"
 #include <assert.h>
 
 static bool add_htlc(struct bitcoin_tx *tx, size_t n,
@@ -98,5 +99,7 @@ struct bitcoin_tx *create_commit_tx(const tal_t *ctx,
 
 	*map = tal_arr(ctx, int, tx->output_count);
 	permute_outputs(tx->output, tx->output_count, *map);
+	remove_dust(tx, *map);
+
 	return tx;
 }

remove_dust.c

@@ -0,0 +1,28 @@
+#include "remove_dust.h"
+#include <assert.h>
+#include <stdbool.h>
+#include <string.h>
+
+void remove_dust(struct bitcoin_tx *tx, int *map)
+{
+	size_t i, j, num = tx->output_count;
+
+	assert(tal_count(map) == num);
+	/* Do it in map order so we can remove from map, too */
+	for (i = 0; i < num; i++) {
+		assert(map[i] < tx->output_count);
+		if (tx->output[map[i]].amount >= DUST_THRESHOLD)
+			continue;
+
+		/* Eliminate that output from tx */
+		tx->output_count--;
+		memmove(tx->output + map[i], tx->output + map[i] + 1,
+			(tx->output_count-map[i]) * sizeof(*tx->output));
+
+		/* Fixup map. */
+		for (j = 0; j < num; j++)
+			if (map[j] > map[i])
+				map[j]--;
+		map[i] = -1;
+	}
+}

remove_dust.h

@@ -0,0 +1,12 @@
+#ifndef LIGHTNING_REMOVE_DUST_H
+#define LIGHTNING_REMOVE_DUST_H
+#include "config.h"
+#include "bitcoin/tx.h"
+
+/* Remove all dust outputs from tx */
+void remove_dust(struct bitcoin_tx *tx, int *map);
+
+/* Less than this is dust. */
+#define DUST_THRESHOLD 546
+
+#endif /* LIGHTNING_REMOVE_DUST_H */