decimal: add decomposer interface

For golang/go#30870

.travis.yml

@@ -1,9 +1,8 @@
 language: go
 
 go:
-  - 1.2
-  - 1.3
-  - 1.4
+  - 1.2.2
+  - 1.13
   - tip
 
 install:

decomposer.go

@@ -0,0 +1,47 @@
+package decimal
+
+import (
+	"fmt"
+	"math/big"
+)
+
+// Decompose returns the internal decimal state into parts.
+// If the provided buf has sufficient capacity, buf may be returned as the coefficient with
+// the value set and length set as appropriate.
+func (d Decimal) Decompose(buf []byte) (form byte, negative bool, coefficient []byte, exponent int32) {
+	negative = d.value.Sign() < 0
+	exponent = d.exp
+	coefficient = d.value.Bytes()
+	return
+}
+
+const (
+	decomposeFinite   = 0
+	decomposeInfinite = 1
+	decomposeNaN      = 2
+)
+
+// Compose sets the internal decimal value from parts. If the value cannot be
+// represented then an error should be returned.
+func (d *Decimal) Compose(form byte, negative bool, coefficient []byte, exponent int32) error {
+	switch form {
+	default:
+		return fmt.Errorf("unknown form: %v", form)
+	case decomposeFinite:
+		// Set rest of finite form below.
+	case decomposeInfinite:
+		return fmt.Errorf("Infinite form not supported")
+	case decomposeNaN:
+		return fmt.Errorf("NaN form not supported")
+	}
+	// Finite form.
+	if d.value == nil {
+		d.value = &big.Int{}
+	}
+	d.value.SetBytes(coefficient)
+	if negative && d.value.Sign() >= 0 {
+		d.value.Neg(d.value)
+	}
+	d.exp = exponent
+	return nil
+}

decomposer_test.go

@@ -0,0 +1,90 @@
+package decimal
+
+import (
+	"testing"
+)
+
+func TestDecomposerRoundTrip(t *testing.T) {
+	list := []struct {
+		N string // Name.
+		S string // String value.
+		E bool   // Expect an error.
+	}{
+		{N: "Normal-1", S: "123.456"},
+		{N: "Normal-2", S: "-123.456"},
+		{N: "Large-1", S: "9937443000"},
+		{N: "Large-2", S: "-9937443000"},
+		{N: "AllDecimal-1", S: "0.04828239821"},
+		{N: "AllDecimal-2", S: "-0.04828239821"},
+		{N: "LargeAndDecimal-1", S: "4378273024.234239278"},
+		{N: "LargeAndDecimal-2", S: "-4378273024.234239278"},
+		{N: "Zero", S: "0"},
+		{N: "One-1", S: "1"},
+		{N: "One-2", S: "-1"},
+		{N: "LargerThenFloat-1", S: "1234567890123456842"},
+		{N: "LargerThenFloat-2", S: "-1234567890123456842"},
+	}
+	for _, item := range list {
+		d, err := NewFromString(item.S)
+		if err != nil {
+			t.Fatal(err)
+		}
+		set := &Decimal{}
+		err = set.Compose(d.Decompose(nil))
+		if err == nil && item.E {
+			t.Fatal("expected error, got <nil>")
+		}
+		if err != nil && !item.E {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if set.Cmp(d) != 0 {
+			t.Fatalf("values incorrect, got %v want %v (%s)", set, d, item.S)
+		}
+		if set.String() != item.S {
+			t.Fatalf("string value incorrect, got %q want %q", set.String(), item.S)
+		}
+	}
+}
+
+func TestDecomposerCompose(t *testing.T) {
+	list := []struct {
+		N string // Name.
+		S string // String value.
+
+		Form byte // Form
+		Neg  bool
+		Coef []byte // Coefficent
+		Exp  int32
+
+		Err bool // Expect an error.
+	}{
+		{N: "Zero", S: "0", Coef: nil, Exp: 0},
+		{N: "Normal-1", S: "123.456", Coef: []byte{0x01, 0xE2, 0x40}, Exp: -3},
+		{N: "Neg-1", S: "-123.456", Neg: true, Coef: []byte{0x01, 0xE2, 0x40}, Exp: -3},
+		{N: "PosExp-1", S: "123456000", Coef: []byte{0x01, 0xE2, 0x40}, Exp: 3},
+		{N: "PosExp-2", S: "-123456000", Neg: true, Coef: []byte{0x01, 0xE2, 0x40}, Exp: 3},
+		{N: "AllDec-1", S: "0.123456", Coef: []byte{0x01, 0xE2, 0x40}, Exp: -6},
+		{N: "AllDec-2", S: "-0.123456", Neg: true, Coef: []byte{0x01, 0xE2, 0x40}, Exp: -6},
+		{N: "NaN-1", S: "NaN", Form: 2, Err: true},
+		{N: "NaN-2", S: "-NaN", Form: 2, Neg: true, Err: true},
+		{N: "Infinity-1", S: "Infinity", Form: 1, Err: true},
+		{N: "Infinity-2", S: "-Infinity", Form: 1, Neg: true, Err: true},
+	}
+
+	for _, item := range list {
+		d := &Decimal{}
+		err := d.Compose(item.Form, item.Neg, item.Coef, item.Exp)
+		if err != nil && !item.Err {
+			t.Fatalf("unexpected error, got %v", err)
+		}
+		if item.Err {
+			if err == nil {
+				t.Fatal("expected error, got <nil>")
+			}
+			return
+		}
+		if s := d.String(); s != item.S {
+			t.Fatalf("unexpected value, got %q want %q", s, item.S)
+		}
+	}
+}