btcutil: format BTC amounts with trailing zeroes

so that sat amounts can be read without counting zeroes

before:
 350sat = 0.0000035 BTC
3500sat = 0.000035 BTC

after:
 350sat = 0.00000350 BTC
3500sat = 0.00003500 BTC

fixes #1995

btcutil/amount.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"math"
 	"strconv"
+	"strings"
 )
 
 // AmountUnit describes a method of converting an Amount to something
@@ -101,11 +102,24 @@ func (a Amount) ToBTC() float64 {
 
 // Format formats a monetary amount counted in bitcoin base units as a
 // string for a given unit.  The conversion will succeed for any unit,
-// however, known units will be formated with an appended label describing
+// however, known units will be formatted with an appended label describing
 // the units with SI notation, or "Satoshi" for the base unit.
 func (a Amount) Format(u AmountUnit) string {
 	units := " " + u.String()
-	return strconv.FormatFloat(a.ToUnit(u), 'f', -int(u+8), 64) + units
+	bitcoin := a.ToUnit(u)
+	formatted := strconv.FormatFloat(bitcoin, 'f', -int(u+8), 64)
+
+	// when formatting full BTC, add trailing zeroes for numbers with decimal
+	// point to ease reading of sat amount
+	if u == AmountBTC {
+		decimalPoint := "."
+		before, after, containsDecimalPoint := strings.Cut(formatted, decimalPoint)
+		if containsDecimalPoint {
+			trailingZeroes := strings.Repeat("0", 8-len(after))
+			return before + decimalPoint + after + trailingZeroes + units
+		}
+	}
+	return formatted + units
 }
 
 // String is the equivalent of calling Format with AmountBTC.

btcutil/amount_test.go

@@ -136,8 +136,22 @@ func TestAmountUnitConversions(t *testing.T) {
 			name:      "BTC",
 			amount:    44433322211100,
 			unit:      AmountBTC,
-			converted: 444333.22211100,
-			s:         "444333.222111 BTC",
+			converted: 444333.222111,
+			s:         "444333.22211100 BTC",
+		},
+		{
+			name:      "a thousand satoshi as BTC",
+			amount:    1000,
+			unit:      AmountBTC,
+			converted: 0.00001,
+			s:         "0.00001000 BTC",
+		},
+		{
+			name:      "a single satoshi as BTC",
+			amount:    1,
+			unit:      AmountBTC,
+			converted: 0.00000001,
+			s:         "0.00000001 BTC",
 		},
 		{
 			name:      "mBTC",

btcutil/example_test.go

@@ -20,7 +20,7 @@ func ExampleAmount() {
 	// Output:
 	// Zero Satoshi: 0 BTC
 	// 100,000,000 Satoshis: 1 BTC
-	// 100,000 Satoshis: 0.001 BTC
+	// 100,000 Satoshis: 0.00100000 BTC
 }
 
 func ExampleNewAmount() {
@@ -69,7 +69,7 @@ func ExampleAmount_unitConversions() {
 
 	// Output:
 	// Satoshi to kBTC: 444.333222111 kBTC
-	// Satoshi to BTC: 444333.222111 BTC
+	// Satoshi to BTC: 444333.22211100 BTC
 	// Satoshi to MilliBTC: 444333222.111 mBTC
 	// Satoshi to MicroBTC: 444333222111 μBTC
 	// Satoshi to Satoshi: 44433322211100 Satoshi