Merge 49883997d8 into d00399e161

README.md

@@ -8,6 +8,8 @@ Arbitrary-precision fixed-point decimal numbers in go.
 
 _Note:_ Decimal library can "only" represent numbers with a maximum of 2^31 digits after the decimal point.
 
+_Note:_ The library does not remove trailing zeros when parsing decimals from strings, e.g., `1.18` does not internally equal `1.1800` when parsing with this library, see example in usage
+
 ## Features
 
  * The zero-value is 0, and is safe to use without initialization
@@ -59,6 +61,32 @@ func main() {
 }
 ```
 
+Conservation of precision with trailing zeros. This conservation is, however,
+not translated backwards when printing the decimal as string.
+
+This is important to know.
+
+```go
+package main
+
+import (
+	"fmt"
+	"github.com/shopspring/decimal"
+)
+
+func main() {
+	noZeros := "1.18"
+	trailingZeros := "1.1800"
+
+	d1 := decimal.RequireFromString(noZeros)
+	d2 := decimal.RequireFromString(trailingZeros)
+
+	fmt.Println(d1.Coefficient(), d1.Exponent()) 	// 118 -2
+	fmt.Println(d2.Coefficient(), d2.Exponent()) 	// 118 -4
+	fmt.Println(d1)					// 1.18
+	fmt.Println(d2)					// 1.18
+```
+
 ## Documentation
 
 http://godoc.org/github.com/shopspring/decimal

decimal.go

@@ -124,6 +124,26 @@ func NewFromBigInt(value *big.Int, exp int32) Decimal {
 	}
 }
 
+// NewFromBigRat returns a new Decimal from a big.Rat. The numerator and
+// denominator are divided and rounded to the given precision.
+//
+// Example:
+//
+//     d1 := NewFromBigRat(big.NewRat(0, 1), 0)    // output: "0"
+//     d2 := NewFromBigRat(big.NewRat(4, 5), 1)    // output: "0.8"
+//     d3 := NewFromBigRat(big.NewRat(1000, 3), 3) // output: "333.333"
+//     d4 := NewFromBigRat(big.NewRat(2, 7), 4)    // output: "0.2857"
+//
+func NewFromBigRat(value *big.Rat, precision int32) Decimal {
+	return Decimal{
+		value: new(big.Int).Set(value.Num()),
+		exp:   0,
+	}.DivRound(Decimal{
+		value: new(big.Int).Set(value.Denom()),
+		exp:   0,
+	}, precision)
+}
+
 // NewFromString returns a new Decimal from a string representation.
 // Trailing zeroes are not trimmed.
 //

decimal_test.go

@@ -556,6 +556,51 @@ func TestNewFromBigIntWithExponent(t *testing.T) {
 	}
 }
 
+func TestNewFromBigRat(t *testing.T) {
+	mustParseRat := func(val string) *big.Rat {
+		num, _ := new(big.Rat).SetString(val)
+		return num
+	}
+
+	type Inp struct {
+		val  *big.Rat
+		prec int32
+	}
+
+	tests := map[Inp]string{
+		Inp{big.NewRat(0, 1), 16}:                                                     "0",
+		Inp{big.NewRat(4, 5), 16}:                                                     "0.8",
+		Inp{big.NewRat(10, 2), 16}:                                                    "5",
+		Inp{big.NewRat(1023427554493, 43432632), 16}:                                  "23563.5628642767953828", // rounded
+		Inp{big.NewRat(1, 434324545566634), 16}:                                       "0.0000000000000023",
+		Inp{big.NewRat(1, 3), 16}:                                                     "0.3333333333333333",
+		Inp{big.NewRat(2, 3), 2}:                                                      "0.67",               // rounded
+		Inp{big.NewRat(2, 3), 16}:                                                     "0.6666666666666667", // rounded
+		Inp{big.NewRat(10000, 3), 16}:                                                 "3333.3333333333333333",
+		Inp{mustParseRat("30702832066636633479"), 16}:                                 "30702832066636633479",
+		Inp{mustParseRat("487028320159896636679.1827512895753"), 16}:                  "487028320159896636679.1827512895753",
+		Inp{mustParseRat("127028320612589896636633479.173582751289575278357832"), -2}: "127028320612589896636633500",                  // rounded
+		Inp{mustParseRat("127028320612589896636633479.173582751289575278357832"), 16}: "127028320612589896636633479.1735827512895753", // rounded
+		Inp{mustParseRat("127028320612589896636633479.173582751289575278357832"), 32}: "127028320612589896636633479.173582751289575278357832",
+	}
+
+	// add negatives
+	for p, s := range tests {
+		if p.val.Cmp(new(big.Rat)) > 0 {
+			tests[Inp{p.val.Neg(p.val), p.prec}] = "-" + s
+		}
+	}
+
+	for input, s := range tests {
+		d := NewFromBigRat(input.val, input.prec)
+		if d.String() != s {
+			t.Errorf("expected %s, got %s (%s, %d)",
+				s, d.String(),
+				d.value.String(), d.exp)
+		}
+	}
+}
+
 func TestCopy(t *testing.T) {
 	origin := New(1, 0)
 	cpy := origin.Copy()