apd: add Size method

Revives cockroachdb#97.

I disagree with the rationale behind closing that PR. This logic belongs
in this package more than it belongs in CockroachDB code.

bigint.go

@@ -247,6 +247,20 @@ func (z *BigInt) updateInnerFromUint(val uint, neg bool) {
 	}
 }
 
+const (
+	bigIntSize     = unsafe.Sizeof(BigInt{})
+	mathBigIntSize = unsafe.Sizeof(big.Int{})
+	mathWordSize   = unsafe.Sizeof(big.Word(0))
+)
+
+// Size returns the total memory footprint of z in bytes.
+func (z *BigInt) Size() uintptr {
+	if z.isInline() {
+		return bigIntSize
+	}
+	return bigIntSize + mathBigIntSize + uintptr(cap(z._inner.Bits()))*mathWordSize
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 //                    inline arithmetic for small values                     //
 ///////////////////////////////////////////////////////////////////////////////

decimal.go

@@ -17,6 +17,7 @@ package apd
 import (
 	"strconv"
 	"strings"
+	"unsafe"
 
 	"database/sql/driver"
 	"github.com/pkg/errors"
@@ -737,6 +738,13 @@ func (d *Decimal) Reduce(x *Decimal) (*Decimal, int) {
 	return d, nd
 }
 
+const decimalSize = unsafe.Sizeof(Decimal{})
+
+// Size returns the total memory footprint of d in bytes.
+func (d *Decimal) Size() uintptr {
+	return decimalSize - bigIntSize + d.Coeff.Size()
+}
+
 // Value implements the database/sql/driver.Valuer interface. It converts d to a
 // string.
 func (d Decimal) Value() (driver.Value, error) {

decimal_test.go

@@ -801,6 +801,41 @@ func TestSizeof(t *testing.T) {
 	}
 }
 
+// TestSize tests the Size method on BigInt and Decimal. Unlike Sizeof, which
+// returns the shallow size of the structs, the Size method reports the total
+// memory footprint of each struct and all referenced objects.
+func TestSize(t *testing.T) {
+	var d Decimal
+	if e, s := uintptr(32), d.Size(); e != s {
+		t.Errorf("(*Decimal).Size() != %d: %d", e, s)
+	}
+	if e, s := uintptr(24), d.Coeff.Size(); e != s {
+		t.Errorf("(*BigInt).Size() != %d: %d", e, s)
+	}
+	// Set to an inlinable value.
+	d.SetInt64(1234)
+	if e, s := uintptr(32), d.Size(); e != s {
+		t.Errorf("(*Decimal).Size() != %d: %d", e, s)
+	}
+	if e, s := uintptr(24), d.Coeff.Size(); e != s {
+		t.Errorf("(*BigInt).Size() != %d: %d", e, s)
+	}
+	// Set to a non-inlinable value.
+	if _, _, err := d.SetString("123456789123456789123456789.123456789123456789"); err != nil {
+		t.Fatal(err)
+	}
+	if d.Coeff.isInline() {
+		// Sanity-check, in case inlineWords changes.
+		t.Fatal("BigInt inlined large value. Did inlineWords change?")
+	}
+	if e, s := uintptr(120), d.Size(); e != s {
+		t.Errorf("(*Decimal).Size() != %d: %d", e, s)
+	}
+	if e, s := uintptr(112), d.Coeff.Size(); e != s {
+		t.Errorf("(*BigInt).Size() != %d: %d", e, s)
+	}
+}
+
 func TestJSONEncoding(t *testing.T) {
 	var encodingTests = []string{
 		"0",