watchtower: Add a struct containing the penalty base information

Suggested-by: Rusty Russell <@rustyrussell>

common/Makefile

@@ -47,6 +47,7 @@ COMMON_SRC_NOGEN :=				\
 	common/onion.c				\
 	common/onionreply.c			\
 	common/param.c				\
+	common/penalty_base.c			\
 	common/per_peer_state.c			\
 	common/peer_billboard.c			\
 	common/peer_failed.c			\

common/penalty_base.c

@@ -0,0 +1,37 @@
+#include <assert.h>
+#include <common/penalty_base.h>
+#include <wire/wire.h>
+
+/* txout must be within tx! */
+struct penalty_base *penalty_base_new(const tal_t *ctx,
+				      u64 commitment_num,
+				      const struct bitcoin_tx *tx,
+				      const struct wally_tx_output *txout)
+{
+	struct penalty_base *pbase = tal(ctx, struct penalty_base);
+
+	pbase->commitment_num = commitment_num;
+	bitcoin_txid(tx, &pbase->txid);
+	pbase->outnum = txout - tx->wtx->outputs;
+	assert(pbase->outnum < tx->wtx->num_outputs);
+	pbase->amount.satoshis = txout->satoshi; /* Raw: from wally_tx_output */
+
+	return pbase;
+}
+
+void towire_penalty_base(u8 **pptr, const struct penalty_base *pbase)
+{
+	towire_u64(pptr, pbase->commitment_num);
+	towire_bitcoin_txid(pptr, &pbase->txid);
+	towire_u32(pptr, pbase->outnum);
+	towire_amount_sat(pptr, pbase->amount);
+}
+
+void fromwire_penalty_base(const u8 **pptr, size_t *max,
+			   struct penalty_base *pbase)
+{
+	pbase->commitment_num = fromwire_u64(pptr, max);
+	fromwire_bitcoin_txid(pptr, max, &pbase->txid);
+	pbase->outnum = fromwire_u32(pptr, max);
+	pbase->amount = fromwire_amount_sat(pptr, max);
+}

common/penalty_base.h

@@ -0,0 +1,30 @@
+#ifndef LIGHTNING_COMMON_PENALTY_BASE_H
+#define LIGHTNING_COMMON_PENALTY_BASE_H
+#include "config.h"
+#include <bitcoin/tx.h>
+#include <ccan/short_types/short_types.h>
+#include <common/amount.h>
+
+/* To create a penalty, all we need are these. */
+struct penalty_base {
+	/* The remote commitment index. */
+	u64 commitment_num;
+	/* The remote commitment txid. */
+	struct bitcoin_txid txid;
+	/* The remote commitment's "to-local" output. */
+	u32 outnum;
+	/* The amount of the remote commitment's "to-local" output. */
+	struct amount_sat amount;
+};
+
+/* txout must be within tx! */
+struct penalty_base *penalty_base_new(const tal_t *ctx,
+				      u64 commitment_num,
+				      const struct bitcoin_tx *tx,
+				      const struct wally_tx_output *txout);
+
+void towire_penalty_base(u8 **pptr, const struct penalty_base *pbase);
+void fromwire_penalty_base(const u8 **ptr, size_t *max,
+			   struct penalty_base *pbase);
+
+#endif /* LIGHTNING_COMMON_PENALTY_BASE_H */