Exact representation in NewFromFloat (#78)

* Additional (and some breaking) tests for NewFromFloatWithExponent

* Addressing tests for NewFromFloatWithExponent

* Naming cosmetic correction

* removing unused code

* Improving FromFloatWithExponent

* Tests for exact float representation added

* Exact float representation in FromFloat

* Adding breaking test for NewFromFloat

* Fast path in FromFloat is unreliable, fixing it

* Addressing special meaning of zero exponent in float64

* NewFromFloatWithExponent: subnormals support

* NewFromFloatWithExponent: just a few additional test cases

* NewFromFloat: documentation update

* NewFromFloatWithExponent: optimization and some documentation

* NewFromFloatWithExponent: optimizations

* NewFromFloatWithExponent: optimizations

2 files changed, 74 insertions, 49 deletions

diff --git a/decimal.go b/decimal.go
index cd15c30..fba0987 100644
--- a/decimal.go
+++ b/decimal.go
@@ -160,24 +160,12 @@ func NewFromString(value string) (Decimal, error) {
 //     NewFromFloat(123.45678901234567).String() // output: "123.4567890123456"
 //     NewFromFloat(.00000000000000001).String() // output: "0.00000000000000001"
 //
+// NOTE: some float64 numbers can take up about 300 bytes of memory in decimal representation.
+// Consider using NewFromFloatWithExponent if space is more important than precision.
+//
 // NOTE: this will panic on NaN, +/-inf
 func NewFromFloat(value float64) Decimal {
-	floor := math.Floor(value)
-
-	// fast path, where float is an int
-	if floor == value && value <= math.MaxInt64 && value >= math.MinInt64 {
-		return New(int64(value), 0)
-	}
-
-	// slow path: float is a decimal
-	// HACK(vadim): do this the slow hacky way for now because the logic to
-	// convert a base-2 float to base-10 properly is not trivial
-	str := strconv.FormatFloat(value, 'f', -1, 64)
-	dec, err := NewFromString(str)
-	if err != nil {
-		panic(err)
-	}
-	return dec
+	return NewFromFloatWithExponent(value, math.MinInt32)
 }
 
 // NewFromFloatWithExponent converts a float64 to Decimal, with an arbitrary
diff --git a/decimal_test.go b/decimal_test.go
index fa57083..33e344c 100644
--- a/decimal_test.go
+++ b/decimal_test.go
@@ -14,28 +14,35 @@ import (
 	"time"
 )
 
-var testTable = map[float64]string{
-	3.141592653589793:   "3.141592653589793",
-	3:                   "3",
-	1234567890123456:    "1234567890123456",
-	1234567890123456000: "1234567890123456000",
-	1234.567890123456:   "1234.567890123456",
-	.1234567890123456:   "0.1234567890123456",
-	0:                   "0",
-	.1111111111111110:   "0.111111111111111",
-	.1111111111111111:   "0.1111111111111111",
-	.1111111111111119:   "0.1111111111111119",
-	.000000000000000001: "0.000000000000000001",
-	.000000000000000002: "0.000000000000000002",
-	.000000000000000003: "0.000000000000000003",
-	.000000000000000005: "0.000000000000000005",
-	.000000000000000008: "0.000000000000000008",
-	.1000000000000001:   "0.1000000000000001",
-	.1000000000000002:   "0.1000000000000002",
-	.1000000000000003:   "0.1000000000000003",
-	.1000000000000005:   "0.1000000000000005",
-	.1000000000000008:   "0.1000000000000008",
-	1e25:                "10000000000000000000000000",
+type testEnt struct {
+	float float64
+	short string
+	exact string
+}
+
+var testTable = []*testEnt{
+	{3.141592653589793, "3.141592653589793", ""},
+	{3, "3", ""},
+	{1234567890123456, "1234567890123456", ""},
+	{1234567890123456000, "1234567890123456000", ""},
+	{1234.567890123456, "1234.567890123456", ""},
+	{.1234567890123456, "0.1234567890123456", ""},
+	{0, "0", ""},
+	{.1111111111111110, "0.111111111111111", ""},
+	{.1111111111111111, "0.1111111111111111", ""},
+	{.1111111111111119, "0.1111111111111119", ""},
+	{.000000000000000001, "0.000000000000000001", ""},
+	{.000000000000000002, "0.000000000000000002", ""},
+	{.000000000000000003, "0.000000000000000003", ""},
+	{.000000000000000005, "0.000000000000000005", ""},
+	{.000000000000000008, "0.000000000000000008", ""},
+	{.1000000000000001, "0.1000000000000001", ""},
+	{.1000000000000002, "0.1000000000000002", ""},
+	{.1000000000000003, "0.1000000000000003", ""},
+	{.1000000000000005, "0.1000000000000005", ""},
+	{.1000000000000008, "0.1000000000000008", ""},
+	{1e25, "10000000000000000000000000", ""},
+	{math.MaxInt64, strconv.FormatInt(math.MaxInt64, 10), ""},
 }
 
 var testTableScientificNotation = map[string]string{
@@ -54,12 +61,23 @@ var testTableScientificNotation = map[string]string{
 }
 
 func init() {
+	for _, s := range testTable {
+		s.exact = strconv.FormatFloat(s.float, 'f', 300, 64)
+		if strings.ContainsRune(s.exact, '.') {
+			s.exact = strings.TrimRight(s.exact, "0")
+			s.exact = strings.TrimRight(s.exact, ".")
+		}
+	}
+
 	// add negatives
-	for f, s := range testTable {
-		if f > 0 {
-			testTable[-f] = "-" + s
+	withNeg := testTable[:]
+	for _, s := range testTable {
+		if s.float > 0 {
+			withNeg = append(withNeg, &testEnt{-s.float, "-" + s.short, "-" + s.exact})
 		}
 	}
+	testTable = withNeg
+
 	for e, s := range testTableScientificNotation {
 		if string(e[0]) != "-" && s != "0" {
 			testTableScientificNotation["-"+e] = "-" + s
@@ -68,8 +86,9 @@ func init() {
 }
 
 func TestNewFromFloat(t *testing.T) {
-	for f, s := range testTable {
-		d := NewFromFloat(f)
+	for _, x := range testTable {
+		s := x.exact
+		d := NewFromFloat(x.float)
 		if d.String() != s {
 			t.Errorf("expected %s, got %s (%s, %d)",
 				s, d.String(),
@@ -92,7 +111,20 @@ func TestNewFromFloat(t *testing.T) {
 }
 
 func TestNewFromString(t *testing.T) {
-	for _, s := range testTable {
+	for _, x := range testTable {
+		s := x.short
+		d, err := NewFromString(s)
+		if err != nil {
+			t.Errorf("error while parsing %s", s)
+		} else if d.String() != s {
+			t.Errorf("expected %s, got %s (%s, %d)",
+				s, d.String(),
+				d.value.String(), d.exp)
+		}
+	}
+
+	for _, x := range testTable {
+		s := x.exact
 		d, err := NewFromString(s)
 		if err != nil {
 			t.Errorf("error while parsing %s", s)
@@ -289,7 +321,8 @@ func TestNewFromBigIntWithExponent(t *testing.T) {
 }
 
 func TestJSON(t *testing.T) {
-	for _, s := range testTable {
+	for _, x := range testTable {
+		s := x.short
 		var doc struct {
 			Amount Decimal `json:"amount"`
 		}
@@ -358,7 +391,8 @@ func TestBadJSON(t *testing.T) {
 }
 
 func TestNullDecimalJSON(t *testing.T) {
-	for _, s := range testTable {
+	for _, x := range testTable {
+		s := x.short
 		var doc struct {
 			Amount NullDecimal `json:"amount"`
 		}
@@ -450,7 +484,8 @@ func TestNullDecimalBadJSON(t *testing.T) {
 }
 
 func TestXML(t *testing.T) {
-	for _, s := range testTable {
+	for _, x := range testTable {
+		s := x.short
 		var doc struct {
 			XMLName xml.Name `xml:"account"`
 			Amount  Decimal  `xml:"amount"`
@@ -2035,7 +2070,8 @@ func TestNullDecimal_Value(t *testing.T) {
 }
 
 func TestBinary(t *testing.T) {
-	for x := range testTable {
+	for _, y := range testTable {
+		x := y.float
 
 		// Create the decimal
 		d1 := NewFromFloat(x)
@@ -2070,7 +2106,8 @@ func slicesEqual(a, b []byte) bool {
 }
 
 func TestGobEncode(t *testing.T) {
-	for x := range testTable {
+	for _, y := range testTable {
+		x := y.float
 		d1 := NewFromFloat(x)
 
 		b1, err := d1.GobEncode()