initial_commit_tx.h: extract base-weight calculation.

This avoids reproducing it inside full_channel.c.

Not sure it does elements correctly though.

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

channeld/full_channel.c

@@ -1015,28 +1015,7 @@ u32 approx_max_feerate(const struct channel *channel)
 	/* Assume none are trimmed; this gives lower bound on feerate. */
 	num = tal_count(committed) + tal_count(adding) - tal_count(removing);
 
-	/* BOLT #3:
+	weight = commit_tx_base_weight(num, false /* FIXME-anchor */);
-	 *
-	 * commitment_transaction: 125 + 43 * num-htlc-outputs bytes
-	 *	- version: 4 bytes
-	 *	- witness_header <---- part of the witness data
-	 *	- count_tx_in: 1 byte
-	 *	- tx_in: 41 bytes
-	 *		funding_input
-	 *	- count_tx_out: 1 byte
-	 *	- tx_out: 74 + 43 * num-htlc-outputs bytes
-	 *		output_paying_to_remote,
-	 *		output_paying_to_local,
-	 *		....htlc_output's...
-	 *	- lock_time: 4 bytes
-	 */
-	/* Those 74 bytes static output are effectively 2 outputs, one
-	 * `output_paying_to_local` and one `output_paying_to_remote`. So when
-	 * adding the elements overhead we need to add 2 + num htlcs
-	 * outputs. */
-
-	weight = 724 + 172 * num;
-	weight = elements_add_overhead(weight, 1, num + 2);
 
 	/* Available is their view */
 	avail = amount_msat_to_sat_round_down(channel->view[!channel->opener].owed[channel->opener]);

common/htlc_tx.h

@@ -19,6 +19,8 @@ struct ripemd160;
  * proofs per input and 35 bytes per output. In addition the explicit fee
  * output will add 9 bytes and the per output overhead as well.
  */
+/* FIXME: This seems to be a generalization of the logic in initial_commit_tx.h
+ * and should be in bitcoin/tx.h */
 static inline size_t elements_add_overhead(size_t weight, size_t incount,
 					   size_t outcount)
 {

common/initial_commit_tx.h

@@ -21,12 +21,12 @@ struct keyset;
 u64 commit_number_obscurer(const struct pubkey *opener_payment_basepoint,
 			   const struct pubkey *accepter_payment_basepoint);
 
-/* Helper to calculate the base fee if we have this many htlc outputs */
+
-static inline struct amount_sat commit_tx_base_fee(u32 feerate_per_kw,
+/* The base weight of a commitment tx */
-						   size_t num_untrimmed_htlcs,
+static inline size_t commit_tx_base_weight(size_t num_untrimmed_htlcs,
-						   bool option_anchor_outputs)
+					   bool option_anchor_outputs)
 {
-	u64 weight;
+	size_t weight;
 
 	/* BOLT-a12da24dd0102c170365124782b46d9710950ac1 #3:
 	 *
@@ -68,12 +68,17 @@ static inline struct amount_sat commit_tx_base_fee(u32 feerate_per_kw,
 		weight += (32 + 1 + 1 + 1) * 4; /* Elements added fields */
 	}
 
-	/* BOLT #3:
+	return weight;
-	 *
+}
-	 *    3. Multiply `feerate_per_kw` by `weight`, divide by 1000 (rounding
+
-	 *    down).
+/* Helper to calculate the base fee if we have this many htlc outputs */
-	 */
+static inline struct amount_sat commit_tx_base_fee(u32 feerate_per_kw,
-	return amount_tx_fee(feerate_per_kw, weight);
+						   size_t num_untrimmed_htlcs,
+						   bool option_anchor_outputs)
+{
+	return amount_tx_fee(feerate_per_kw,
+			     commit_tx_base_weight(num_untrimmed_htlcs,
+						   option_anchor_outputs));
 }
 
 /**