amount: minor comment and text improvements and remove unused function.

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

common/amount.c

@@ -24,16 +24,6 @@ struct amount_sat amount_msat_to_sat_round_down(struct amount_msat msat)
 	return sat;
 }
 
-/* You may not be able to do this. */
-bool amount_msat_to_sat_exact(struct amount_sat *sat,
-			      const struct amount_msat *msat)
-{
-	if (msat->millisatoshis % MSAT_PER_SAT != 0)
-		return false;
-	sat->satoshis = msat->millisatoshis / MSAT_PER_SAT;
-	return true;
-}
-
 /* Different formatting by amounts: btc, sat and msat */
 const char *fmt_amount_msat_btc(const tal_t *ctx,
 				const struct amount_msat *msat,

common/amount.h

@@ -39,10 +39,6 @@ struct amount_msat {
 WARN_UNUSED_RESULT bool amount_sat_to_msat(struct amount_msat *msat,
 					   struct amount_sat sat);
 
-/* This may require rounding. */
-WARN_UNUSED_RESULT bool amount_msat_to_sat_exact(struct amount_sat *,
-						 const struct amount_msat *);
-
 /* You can always truncate millisatoshis->satoshis. */
 struct amount_sat amount_msat_to_sat_round_down(struct amount_msat msat);
 
@@ -110,16 +106,18 @@ WARN_UNUSED_RESULT bool amount_msat_add_fee(struct amount_msat *amt,
 struct amount_sat amount_tx_fee(u32 fee_per_kw, size_t weight);
 
 /* Different formatting by amounts: btc, sat and msat */
+/* => 1.23456789012btc (11 decimals!) */
 const char *fmt_amount_msat_btc(const tal_t *ctx,
 				const struct amount_msat *msat,
 				bool append_unit);
-/* 1234msat */
+/* => 1234msat */
 const char *fmt_amount_msat(const tal_t *ctx, const struct amount_msat *msat);
 
+/* => 1.23456789btc (8 decimals!) */
 const char *fmt_amount_sat_btc(const tal_t *ctx,
 			       const struct amount_sat *sat,
 			       bool append_unit);
-/* 1234sat */
+/* => 1234sat */
 const char *fmt_amount_sat(const tal_t *ctx, const struct amount_sat *sat);
 
 /* Valid strings:

common/test/run-amount.c

@@ -36,15 +36,17 @@ int main(void)
 	FAIL_MSAT(&msat, "0.00000000");
 	FAIL_MSAT(&msat, "0.00000000000");
 	FAIL_MSAT(&msat, "0.00000000msat");
-	FAIL_MSAT(&msat, "0.00000000000msat");
+	FAIL_MSAT(&msat, "-1");
 
 	PASS_MSAT(&msat, "0msat", 0);
 	PASS_MSAT(&msat, "1msat", 1);
 	PASS_MSAT(&msat, "2100000000000000000msat", 2100000000000000000ULL);
+	FAIL_MSAT(&msat, "-1msat");
 
 	PASS_MSAT(&msat, "0sat", 0);
 	PASS_MSAT(&msat, "1sat", 1000);
 	PASS_MSAT(&msat, "2100000000000000sat", 2100000000000000000ULL);
+	FAIL_MSAT(&msat, "-1sat");
 
 	PASS_MSAT(&msat, "0.00000000btc", 0);
 	PASS_MSAT(&msat, "0.00000000000btc", 0);
@@ -55,6 +57,8 @@ int main(void)
 	FAIL_MSAT(&msat, "1btc");
 	FAIL_MSAT(&msat, "1.0000000btc");
 	FAIL_MSAT(&msat, "1.000000000btc");
+	FAIL_MSAT(&msat, "-1.23456789btc");
+	FAIL_MSAT(&msat, "-1.23456789012btc");
 
 	/* Overflowingly big. */
 	FAIL_MSAT(&msat, "21000000000000000000000000.00000000btc");
@@ -71,10 +75,12 @@ int main(void)
 	FAIL_SAT(&sat, "0.00000000000");
 	FAIL_SAT(&sat, "0.00000000sat");
 	FAIL_SAT(&sat, "0.00000000000msat");
+	FAIL_SAT(&sat, "-1");
 
 	PASS_SAT(&sat, "0sat", 0);
 	PASS_SAT(&sat, "1sat", 1);
 	PASS_SAT(&sat, "2100000000000000sat", 2100000000000000ULL);
+	FAIL_SAT(&sat, "-1sat");
 
 	PASS_SAT(&sat, "1000msat", 1);
 	PASS_SAT(&sat, "1000000msat", 1000);
@@ -82,6 +88,7 @@ int main(void)
 	FAIL_SAT(&sat, "0msat");
 	FAIL_SAT(&sat, "100msat");
 	FAIL_SAT(&sat, "2000000000000000999msat");
+	FAIL_SAT(&sat, "-1000msat");
 
 	PASS_SAT(&sat, "0.00000000btc", 0);
 	FAIL_SAT(&sat, "0.00000000000btc");
@@ -92,6 +99,7 @@ int main(void)
 	FAIL_SAT(&sat, "1btc");
 	FAIL_SAT(&sat, "1.0000000btc");
 	FAIL_SAT(&sat, "1.000000000btc");
+	FAIL_SAT(&sat, "-1.23456789btc");
 
 	/* Overflowingly big. */
 	FAIL_SAT(&sat, "21000000000000000000000000.00000000btc");