Add Amount.MulF64.

ok @davecgh, @jcvernaleo

amount.go

@@ -53,6 +53,17 @@ func (u AmountUnit) String() string {
 // to as a `Satoshi').  A single Amount is equal to 1e-8 of a bitcoin.
 type Amount int64
 
+// round converts a floating point number, which may or may not be representable
+// as an integer, to the Amount integer type by rounding to the nearest integer.
+// This is performed by adding or subtracting 0.5 depending on the sign, and
+// relying on integer truncation to round the value to the nearest Amount.
+func round(f float64) Amount {
+	if f < 0 {
+		return Amount(f - 0.5)
+	}
+	return Amount(f + 0.5)
+}
+
 // NewAmount creates an Amount from a floating point value representing
 // some value in bitcoin.  NewAmount errors if f is NaN or +-Infinity, but
 // does not check that the amount is within the total amount of bitcoin
@@ -69,16 +80,7 @@ func NewAmount(f float64) (Amount, error) {
 		return 0, errors.New("invalid bitcoin amount")
 	}
 
-	a := f * float64(SatoshiPerBitcoin)
-
-	// Depending on the sign, add or subtract 0.5 and rely on integer
-	// truncation to correctly round the value up or down.
-	if a < 0 {
-		a = a - 0.5
-	} else {
-		a = a + 0.5
-	}
-	return Amount(a), nil
+	return round(f * satoshiPerBitcoin), nil
 }
 
 // ToUnit converts a monetary amount counted in bitcoin base units to a
@@ -100,3 +102,11 @@ func (a Amount) Format(u AmountUnit) string {
 func (a Amount) String() string {
 	return a.Format(AmountBTC)
 }
+
+// MulF64 multiplies an Amount by a floating point value.  While this is not
+// an operation that must typically be done by a full node or wallet, it is
+// useful for services that build on top of bitcoin (for example, calculating
+// a fee by multiplying by a percentage).
+func (a Amount) MulF64(f float64) Amount {
+	return round(float64(a) * f)
+}

amount_test.go

@@ -192,3 +192,110 @@ func TestAmountUnitConversions(t *testing.T) {
 		}
 	}
 }
+
+func TestAmountMulF64(t *testing.T) {
+	tests := []struct {
+		name string
+		amt  Amount
+		mul  float64
+		res  Amount
+	}{
+		{
+			name: "Multiply 0.1 BTC by 2",
+			amt:  100e5, // 0.1 BTC
+			mul:  2,
+			res:  200e5, // 0.2 BTC
+		},
+		{
+			name: "Multiply 0.2 BTC by 0.02",
+			amt:  200e5, // 0.2 BTC
+			mul:  1.02,
+			res:  204e5, // 0.204 BTC
+		},
+		{
+			name: "Multiply 0.1 BTC by -2",
+			amt:  100e5, // 0.1 BTC
+			mul:  -2,
+			res:  -200e5, // -0.2 BTC
+		},
+		{
+			name: "Multiply 0.2 BTC by -0.02",
+			amt:  200e5, // 0.2 BTC
+			mul:  -1.02,
+			res:  -204e5, // -0.204 BTC
+		},
+		{
+			name: "Multiply -0.1 BTC by 2",
+			amt:  -100e5, // -0.1 BTC
+			mul:  2,
+			res:  -200e5, // -0.2 BTC
+		},
+		{
+			name: "Multiply -0.2 BTC by 0.02",
+			amt:  -200e5, // -0.2 BTC
+			mul:  1.02,
+			res:  -204e5, // -0.204 BTC
+		},
+		{
+			name: "Multiply -0.1 BTC by -2",
+			amt:  -100e5, // -0.1 BTC
+			mul:  -2,
+			res:  200e5, // 0.2 BTC
+		},
+		{
+			name: "Multiply -0.2 BTC by -0.02",
+			amt:  -200e5, // -0.2 BTC
+			mul:  -1.02,
+			res:  204e5, // 0.204 BTC
+		},
+		{
+			name: "Round down",
+			amt:  49, // 49 Satoshis
+			mul:  0.01,
+			res:  0,
+		},
+		{
+			name: "Round up",
+			amt:  50, // 50 Satoshis
+			mul:  0.01,
+			res:  1, // 1 Satoshi
+		},
+		{
+			name: "Multiply by 0.",
+			amt:  1e8, // 1 BTC
+			mul:  0,
+			res:  0, // 0 BTC
+		},
+		{
+			name: "Multiply 1 by 0.5.",
+			amt:  1, // 1 Satoshi
+			mul:  0.5,
+			res:  1, // 1 Satoshi
+		},
+		{
+			name: "Multiply 100 by 66%.",
+			amt:  100, // 100 Satoshis
+			mul:  0.66,
+			res:  66, // 66 Satoshis
+		},
+		{
+			name: "Multiply 100 by 66.6%.",
+			amt:  100, // 100 Satoshis
+			mul:  0.666,
+			res:  67, // 67 Satoshis
+		},
+		{
+			name: "Multiply 100 by 2/3.",
+			amt:  100, // 100 Satoshis
+			mul:  2.0 / 3,
+			res:  67, // 67 Satoshis
+		},
+	}
+
+	for _, test := range tests {
+		a := test.amt.MulF64(test.mul)
+		if a != test.res {
+			t.Errorf("%v: expected %v got %v", test.name, test.res, a)
+		}
+	}
+}

const.go

@@ -5,11 +5,16 @@
 package btcutil
 
 const (
+	// satoshiPerBitcoin is the untyped version of SatoshiPerBitcoin.
+	//
+	// TODO(jrick): Switch the exported consts below to be untyped.
+	satoshiPerBitcoin = 1e8
+
 	// SatoshiPerBitcent is the number of satoshi in one bitcoin cent.
 	SatoshiPerBitcent int64 = 1e6
 
 	// SatoshiPerBitcoin is the number of satoshi in one bitcoin (1 BTC).
-	SatoshiPerBitcoin int64 = 1e8
+	SatoshiPerBitcoin int64 = satoshiPerBitcoin
 
 	// MaxSatoshi is the maximum transaction amount allowed in satoshi.
 	MaxSatoshi int64 = 21e6 * SatoshiPerBitcoin