amount: add amount_msat_scale, amount_msat_ratio, amount_{msat,sat}_div

It's not all that rare to do these operations, and requiring annotations
for it is a little painful.

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

common/amount.c

@@ -297,6 +297,18 @@ WARN_UNUSED_RESULT bool amount_msat_add_sat(struct amount_msat *val,
 	return amount_msat_add(val, a, msatb);
 }
 
+WARN_UNUSED_RESULT bool amount_msat_scale(struct amount_msat *val,
+					  struct amount_msat sat,
+					  double scale)
+{
+	double scaled = sat.millisatoshis * scale;
+
+	if (scaled > UINT64_MAX)
+		return false;
+	val->millisatoshis = scaled;
+	return true;
+}
+
 bool amount_sat_eq(struct amount_sat a, struct amount_sat b)
 {
 	return a.satoshis == b.satoshis;
@@ -417,6 +429,23 @@ struct amount_sat amount_sat(u64 satoshis)
 	return sat;
 }
 
+double amount_msat_ratio(struct amount_msat a, struct amount_msat b)
+{
+	return (double)a.millisatoshis / b.millisatoshis;
+}
+
+struct amount_msat amount_msat_div(struct amount_msat msat, u64 div)
+{
+	msat.millisatoshis /= div;
+	return msat;
+}
+
+struct amount_sat amount_sat_div(struct amount_sat sat, u64 div)
+{
+	sat.satoshis /= div;
+	return sat;
+}
+
 bool amount_msat_fee(struct amount_msat *fee,
 		     struct amount_msat amt,
 		     u32 fee_base_msat,

common/amount.h

@@ -80,6 +80,12 @@ WARN_UNUSED_RESULT bool amount_msat_add_sat(struct amount_msat *val,
 WARN_UNUSED_RESULT bool amount_sat_sub_msat(struct amount_msat *val,
 					    struct amount_sat a,
 					    struct amount_msat b);
+WARN_UNUSED_RESULT bool amount_msat_scale(struct amount_msat *val,
+					  struct amount_msat msat,
+					  double scale);
+
+struct amount_msat amount_msat_div(struct amount_msat msat, u64 div);
+struct amount_sat amount_sat_div(struct amount_sat sat, u64 div);
 
 /* Is a == b? */
 bool amount_sat_eq(struct amount_sat a, struct amount_sat b);
@@ -112,6 +118,9 @@ bool amount_msat_less_eq_sat(struct amount_msat msat, struct amount_sat sat);
 /* Is msat == sat? */
 bool amount_msat_eq_sat(struct amount_msat msat, struct amount_sat sat);
 
+/* a / b */
+double amount_msat_ratio(struct amount_msat a, struct amount_msat b);
+
 /* Check whether this asset is actually the main / fee-paying asset of the
  * current chain. */
 bool amount_asset_is_main(struct amount_asset *asset);

gossipd/routing.c

@@ -650,14 +650,13 @@ static WARN_UNUSED_RESULT bool risk_add_fee(struct amount_msat *risk,
 					    u32 delay, double riskfactor,
 					    u64 riskbias)
 {
-	double r;
+	struct amount_msat riskfee;
 
-	/* Won't overflow on add, just lose precision */
-	r = (double)riskbias + riskfactor * delay * msat.millisatoshis + risk->millisatoshis; /* Raw: to double */
-	if (r > (double)UINT64_MAX)
+	if (!amount_msat_scale(&riskfee, msat, riskfactor * delay))
 		return false;
-	risk->millisatoshis = r; /* Raw: from double */
-	return true;
+	if (!amount_msat_add(&riskfee, riskfee, amount_msat(riskbias)))
+		return false;
+	return amount_msat_add(risk, *risk, riskfee);
 }
 
 /* Check that we can fit through this channel's indicated
@@ -1209,7 +1208,7 @@ find_shorter_route(const tal_t *ctx, struct routing_state *rstate,
 	 * per block delay, which is close enough to zero to not break
 	 * this algorithm, but still provide some bias towards
 	 * low-delay routes. */
-	riskfactor = (double)1.0 / msat.millisatoshis; /* Raw: inversion */
+	riskfactor = amount_msat_ratio(AMOUNT_MSAT(1), msat);
 
 	/* First, figure out if a short route is even possible.
 	 * We set the cost function to ignore total, riskbias 1 and riskfactor

lightningd/invoice.c

@@ -542,7 +542,7 @@ static struct route_info **select_inchan(const tal_t *ctx,
 			continue;
 		}
 
-		excess_frac = (double)excess.millisatoshis / capacity.millisatoshis; /* Raw: double fraction */
+		excess_frac = amount_msat_ratio(excess, capacity);
 
 		sample.route = &inchans[i];
 		sample.weight = excess_frac;

openingd/openingd.c

@@ -348,8 +348,7 @@ static bool check_config_bounds(struct state *state,
 /* We always set channel_reserve_satoshis to 1%, rounded down. */
 static void set_reserve(struct state *state)
 {
-	state->localconf.channel_reserve.satoshis  /* Raw: rounding. */
-		= state->funding.satoshis / 100;   /* Raw: rounding. */
+	state->localconf.channel_reserve = amount_sat_div(state->funding, 100);
 
 	/* BOLT #2:
 	 *

plugins/libplugin-pay.c

@@ -2313,8 +2313,8 @@ static void shadow_route_cb(struct shadow_route_data *d,
 
 	/* Allow shadowroutes to consume up to 1/4th of our budget. */
 	d->constraints.cltv_budget = p->constraints.cltv_budget / 4;
-	d->constraints.fee_budget = p->constraints.fee_budget;
-	d->constraints.fee_budget.millisatoshis /= 4; /* Raw: msat division. */
+	d->constraints.fee_budget
+		= amount_msat_div(p->constraints.fee_budget, 4);
 
 	if (pseudorand(2) == 0) {
 		return payment_continue(p);
@@ -2672,8 +2672,11 @@ static void presplit_cb(struct presplit_mod_data *d, struct payment *p)
 
 			/* Now adjust the constraints so we don't multiply them
 			 * when splitting. */
-			multiplier = (double)c->amount.millisatoshis / (double)p->amount.millisatoshis; /* Raw: msat division. */
-			c->constraints.fee_budget.millisatoshis *= multiplier; /* Raw: Multiplication */
+			multiplier = amount_msat_ratio(c->amount, p->amount);
+			if (!amount_msat_scale(&c->constraints.fee_budget,
+					       c->constraints.fee_budget,
+					       multiplier))
+				abort(); /* multiplier < 1! */
 			payment_start(c);
 			count++;
 		}
@@ -2790,12 +2793,16 @@ static void adaptive_splitter_cb(struct adaptive_split_mod_data *d, struct payme
 			a->amount.millisatoshis = mid;  /* Raw: split. */
 			b->amount.millisatoshis -= mid; /* Raw: split. */
 
-			double multiplier = (double)a->amount.millisatoshis / (double)p->amount.millisatoshis; /* Raw: msat division */
+			double multiplier = amount_msat_ratio(a->amount,
+							      p->amount);
 			assert(multiplier >= 0.4 && multiplier < 0.6);
 
 			/* Adjust constraints since we don't want to double our
 			 * fee allowance when we split. */
-			a->constraints.fee_budget.millisatoshis = pconstraints->fee_budget.millisatoshis * multiplier; /* Raw: msat multiplication. */
+			if (!amount_msat_scale(&a->constraints.fee_budget,
+					       pconstraints->fee_budget,
+					       multiplier))
+				abort();
 
 			ok = amount_msat_sub(&b->constraints.fee_budget,
 					     pconstraints->fee_budget,