routing: precalc per-block risk factor.

Saves a little by doing up-front calculation.

# 1M nodes:
$ /gossipd/test/run-bench-find_route 1000000 1 > /tmp/out
=> 42863 msec

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

gossipd/routing.c

@@ -267,7 +267,7 @@ static u64 connection_fee(const struct node_connection *c, u64 msatoshi)
  * in order to prefer shorter routes, all things equal. */
 static u64 risk_fee(u64 amount, u32 delay, double riskfactor)
 {
-	return 1 + amount * delay * riskfactor / BLOCKS_PER_YEAR / 10000;
+	return 1 + amount * delay * riskfactor;
 }
 
 /* We track totals, rather than costs.  That's because the fee depends
@@ -311,6 +311,7 @@ static void bfg_one_edge(struct node *node, size_t edgenum, double riskfactor)
 	}
 }
 
+/* riskfactor is already scaled to per-block amount */
 static struct node_connection *
 find_route(const tal_t *ctx, struct routing_state *rstate,
 	   const struct pubkey *from, const struct pubkey *to, u64 msatoshi,
@@ -803,7 +804,8 @@ struct route_hop *get_route(tal_t *ctx, struct routing_state *rstate,
 	struct node_connection *first_conn;
 
 	first_conn = find_route(ctx, rstate, source, destination, msatoshi,
-				riskfactor, &fee, &route);
+				riskfactor / BLOCKS_PER_YEAR / 10000,
+				&fee, &route);
 
 	if (!first_conn) {
 		return NULL;

gossipd/test/run-bench-find_route.c

@@ -158,6 +158,7 @@ int main(int argc, char *argv[])
 	size_t num_success;
 	struct pubkey me = nodeid(0);
 	bool perfme = false;
+	const double riskfactor = 0.01 / BLOCKS_PER_YEAR / 10000;
 
 	secp256k1_ctx = secp256k1_context_create(SECP256K1_CONTEXT_VERIFY
 						 | SECP256K1_CONTEXT_SIGN);
@@ -192,7 +193,7 @@ int main(int argc, char *argv[])
 
 		nc = find_route(ctx, rstate, &from, &to,
 				pseudorand(100000),
-				0.01,
+				riskfactor,
 				&fee, &route);
 		num_success += (nc != NULL);
 		tal_free(route);

gossipd/test/run-find_route-specific.c

@@ -63,6 +63,7 @@ int main(void)
 	struct pubkey a, b, c;
 	u64 fee;
 	struct node_connection **route;
+	const double riskfactor = 1.0 / BLOCKS_PER_YEAR / 10000;
 
 	secp256k1_ctx = secp256k1_context_create(SECP256K1_CONTEXT_VERIFY
 						 | SECP256K1_CONTEXT_SIGN);
@@ -115,7 +116,7 @@ int main(void)
 	nc->flags = 1;
 	nc->last_timestamp = 1504064344;
 
-	nc = find_route(ctx, rstate, &a, &c, 100000, 1.0, &fee, &route);
+	nc = find_route(ctx, rstate, &a, &c, 100000, riskfactor, &fee, &route);
 	assert(nc);
 	assert(tal_count(route) == 1);
 	assert(pubkey_eq(&route[0]->src->id, &b));

gossipd/test/run-find_route.c

@@ -74,6 +74,7 @@ int main(void)
 	struct privkey tmp;
 	u64 fee;
 	struct node_connection **route;
+	const double riskfactor = 1.0 / BLOCKS_PER_YEAR / 10000;
 
 	secp256k1_ctx = secp256k1_context_create(SECP256K1_CONTEXT_VERIFY
 						 | SECP256K1_CONTEXT_SIGN);
@@ -91,7 +92,7 @@ int main(void)
 	/* A<->B */
 	add_connection(rstate, &a, &b, 1, 1, 1);
 
-	nc = find_route(ctx, rstate, &a, &b, 1000, 1.0, &fee, &route);
+	nc = find_route(ctx, rstate, &a, &b, 1000, riskfactor, &fee, &route);
 	assert(nc);
 	assert(tal_count(route) == 0);
 	assert(fee == 0);
@@ -106,7 +107,7 @@ int main(void)
 	status_trace("C = %s", type_to_string(trc, struct pubkey, &c));
 	add_connection(rstate, &b, &c, 1, 1, 1);
 
-	nc = find_route(ctx, rstate, &a, &c, 1000, 1.0, &fee, &route);
+	nc = find_route(ctx, rstate, &a, &c, 1000, riskfactor, &fee, &route);
 	assert(nc);
 	assert(tal_count(route) == 1);
 	assert(fee == 1);
@@ -121,14 +122,14 @@ int main(void)
 	add_connection(rstate, &d, &c, 0, 2, 1);
 
 	/* Will go via D for small amounts. */
-	nc = find_route(ctx, rstate, &a, &c, 1000, 1.0, &fee, &route);
+	nc = find_route(ctx, rstate, &a, &c, 1000, riskfactor, &fee, &route);
 	assert(nc);
 	assert(tal_count(route) == 1);
 	assert(pubkey_eq(&route[0]->src->id, &d));
 	assert(fee == 0);
 
 	/* Will go via B for large amounts. */
-	nc = find_route(ctx, rstate, &a, &c, 3000000, 1.0, &fee, &route);
+	nc = find_route(ctx, rstate, &a, &c, 3000000, riskfactor, &fee, &route);
 	assert(nc);
 	assert(tal_count(route) == 1);
 	assert(pubkey_eq(&route[0]->src->id, &b));
@@ -136,7 +137,7 @@ int main(void)
 
 	/* Make B->C inactive, force it back via D */
 	get_connection(rstate, &b, &c)->active = false;
-	nc = find_route(ctx, rstate, &a, &c, 3000000, 1.0, &fee, &route);
+	nc = find_route(ctx, rstate, &a, &c, 3000000, riskfactor, &fee, &route);
 	assert(nc);
 	assert(tal_count(route) == 1);
 	assert(pubkey_eq(&route[0]->src->id, &d));