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>

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;
+	}
+}