fix edge cases formatting with fixed precision (#74)

Also bump to Go 1.21.

Author: anzdaddy

.github/workflows/go.yml

@@ -7,10 +7,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-    - name: Set up Go 1.20
+    - name: Set up Go 1.21
       uses: actions/setup-go@v1
       with:
-        go-version: 1.20
+        go-version: 1.21
       id: go
 
     - name: Check out code into the Go module directory

decimal64fmt.go

@@ -22,11 +22,25 @@ func appendFrac64(buf []byte, n, limit uint64) []byte {
 
 var zeros = []byte("0000000000000000")
 
+func appendZeros(buf []byte, n int) []byte {
+	if n <= 0 {
+		return buf
+	}
+	l := len(zeros)
+	for ; n > l; n -= l {
+		buf = append(buf, zeros...)
+	}
+	return append(buf, zeros[:n]...)
+}
+
 func appendFrac64Prec(buf []byte, n uint64, prec int) []byte {
+	if prec < 0 {
+		return buf
+	}
 	// Add a digit in front so strconv.AppendUint doesn't trim leading zeros.
 	n += 10 * decimal64Base
 	if prec < 16 {
-		unit := powersOf10[16-prec]
+		unit := powersOf10[max(0, 16-prec)]
 		rem := n % unit
 		n /= unit
 		if rem > unit/2 || rem == unit/2 && n%2 == 1 {
@@ -40,14 +54,7 @@ func appendFrac64Prec(buf []byte, n uint64, prec int) []byte {
 	buf = strconv.AppendUint(buf[:buflen-1], n, 10)
 	buf[buflen-1] = prefix
 
-	for ; prec >= 2*decimal64Digits; prec -= decimal64Digits {
-		buf = append(buf, zeros...)
-	}
-	if prec > decimal64Digits {
-		buf = append(buf, zeros[:prec-decimal64Digits]...)
-	}
-
-	return buf
+	return appendZeros(buf, prec-decimal64Digits)
 }
 
 func appendUint64(buf []byte, n, limit uint64) []byte {
@@ -132,7 +139,7 @@ formatBlock:
 			buf = append(buf, '.')
 			if exponent < 0 {
 				p += exponent
-				buf = append(buf, zeros[:-exponent]...)
+				buf = appendZeros(buf, min(-exponent, prec))
 			}
 			buf = appendFrac64Prec(buf, frac, p)
 			if prec == -1 {

decimal64fmt_test.go

@@ -157,6 +157,51 @@ func TestDecimal64FormatPrecEdgeCases(t *testing.T) {
 	}
 }
 
+func TestDecimal64FormatPrecEdgeCases2(t *testing.T) {
+	t.Parallel()
+
+	test := func(expected string, input Decimal64, prec int) {
+		t.Helper()
+		data := input.Append(nil, 'f', prec)
+		assert.Equal(t, expected, string(data))
+	}
+
+	test("0.062", MustParse64("0.0625"), 3)
+	test("10000.0000000000", MustParse64("1e4"), 10)
+	test("10000000000.0000000000", MustParse64("1e10"), 10)
+	test("100000000000.0000000000", MustParse64("1e11"), 10)
+	// test("0.0000000000", MustParse64("1e50"), 10)
+	test("0.0001000000", MustParse64("1e-4"), 10)
+	test("0.0000000001", MustParse64("1e-10"), 10)
+	test("0.0000000000", MustParse64("1e-11"), 10)
+	test("0.0000000000", MustParse64("1e-20"), 10)
+	test("0.0000000000", MustParse64("1e-30"), 10)
+	test("0.000000000000000000000000000001", MustParse64("1e-30"), 30)
+	test("0.0000000000", Zero64, 10)
+	test("-0.0000000000", Zero64.NextMinus(), 10)
+	test("0.0000000000", Zero64.NextPlus(), 10)
+	test("inf", Infinity64, 10)
+	test("9999999999999999000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.0000000000", Infinity64.NextMinus(), 10)
+	test("inf", Infinity64.NextPlus(), 10)
+
+	test("-10000.0000000000", MustParse64("-1e4"), 10)
+	test("-10000000000.0000000000", MustParse64("-1e10"), 10)
+	test("-100000000000.0000000000", MustParse64("-1e11"), 10)
+	// test("-0.0000000000", MustParse64("-1e50"), 10)
+	test("-0.0001000000", MustParse64("-1e-4"), 10)
+	test("-0.0000000001", MustParse64("-1e-10"), 10)
+	test("-0.0000000000", MustParse64("-1e-11"), 10)
+	test("-0.0000000000", MustParse64("-1e-20"), 10)
+	test("-0.0000000000", MustParse64("-1e-30"), 10)
+	test("-0.000000000000000000000000000001", MustParse64("-1e-30"), 30)
+	test("-0.0000000000", NegZero64, 10)
+	test("-0.0000000000", Zero64.NextMinus(), 10)
+	test("0.0000000000", Zero64.NextPlus(), 10)
+	test("-inf", NegInfinity64, 10)
+	test("-inf", NegInfinity64.NextMinus(), 10)
+	test("-9999999999999999000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.0000000000", NegInfinity64.NextPlus(), 10)
+}
+
 func TestDecimal64FormatPrecEdgeCasesHalfAway(t *testing.T) {
 	t.Parallel()
 

go.mod

@@ -1,6 +1,6 @@
 module github.com/anz-bank/decimal
 
-go 1.20
+go 1.21
 
 require github.com/stretchr/testify v1.2.2