Inital commit

element.go

@@ -0,0 +1,9 @@
+package ebml
+
+import "io"
+
+type Element struct {
+	ID   uint32
+	Size uint64
+	Data io.Reader
+}

encoder.go

@@ -0,0 +1,17 @@
+package ebml
+
+import (
+	"bufio"
+	"io"
+)
+
+// An Encoder writes EBML data to an output stream.
+type Encoder struct {
+	*bufio.Writer
+}
+
+// NewEncoder returns a new encoder that writes to w.
+func NewEncoder(w io.Writer) *Encoder {
+	return &Encoder{Writer: bufio.NewWriter(w)}
+}
+

id.go

@@ -0,0 +1,44 @@
+package ebml
+
+func (e *Encoder) EncodeID(x uint32) (err error) {
+	var buf []byte
+	switch {
+	case x == 0:
+		_, err = e.Write([]byte{byte(0)})
+		return err
+
+	case x < 127:
+		b := byte(x)
+		buf = []byte{b | 0x80}
+
+	case x < 16383:
+		buf = make([]byte, 2)
+		buf[1] = byte(x)
+		x >>= 8
+		buf[0] = byte(x) | 0x40
+
+	case x < 2097151:
+		buf = make([]byte, 3)
+		buf[2] = byte(x)
+		x >>= 8
+		buf[1] = byte(x)
+		x >>= 8
+		buf[0] = byte(x) | 0x20
+
+	case x < 268435455:
+		buf = make([]byte, 4)
+		buf[3] = byte(x)
+		x >>= 8
+		buf[2] = byte(x)
+		x >>= 8
+		buf[1] = byte(x)
+		x >>= 8
+		buf[0] = byte(x) | 0x10
+
+	default:
+		panic("element ID overflow")
+	}
+
+	_, err = e.Write(buf)
+	return
+}

id_test.go

@@ -0,0 +1,48 @@
+package ebml
+
+import (
+	"bytes"
+	"io/ioutil"
+	"testing"
+)
+
+var goldenIDs = []struct {
+	in  uint32
+	out []byte
+}{
+	{0, []byte{0}},
+	{1, []byte{129}},
+	{127, []byte{64, 127}},
+	{128, []byte{64, 128}},
+	{256, []byte{65, 0}},
+	{16382, []byte{127, 254}},
+	{16383, []byte{32, 63, 255}},
+	{16384, []byte{32, 64, 0}},
+	{16385, []byte{32, 64, 1}},
+	{32768, []byte{32, 128, 0}},
+	{2097150, []byte{63, 255, 254}},
+	{2097151, []byte{16, 31, 255, 255}},
+	{2097152, []byte{16, 32, 0, 0}},
+	{268435453, []byte{31, 255, 255, 253}},
+	{268435454, []byte{31, 255, 255, 254}},
+}
+
+func TestGoldenIDs(t *testing.T) {
+	for _, g := range goldenIDs {
+		buf := new(bytes.Buffer)
+		enc := NewEncoder(buf)
+		enc.EncodeID(g.in)
+		enc.Flush()
+		got := buf.Bytes()
+		if !bytes.Equal(got, g.out) {
+			t.Errorf("failed to marshal ID %d, wanted %v, got %v", g.in, g.out, got)
+		}
+	}
+}
+
+func BenchmarkEncodeID(b *testing.B) {
+	enc := NewEncoder(ioutil.Discard)
+	for i := 0; i < b.N; i++ {
+		enc.EncodeID(uint32(i))
+	}
+}

size.go

@@ -0,0 +1,85 @@
+package ebml
+
+const (
+	o1 = 1<<7 - 2
+	o2 = 1<<14 - 2
+	o3 = 1<<21 - 2
+	o4 = 1<<28 - 2
+	o5 = 1<<35 - 2
+	o6 = 1<<42 - 2
+	o7 = 1<<49 - 2
+	o8 = 1<<56 - 2
+)
+
+func (e *Encoder) EncodeSize(x uint64) (err error) {
+	var buf []byte
+	switch {
+	case x == 0:
+		_, err = e.Write([]byte{byte(0)})
+		return err
+
+	case x < o1:
+		b := byte(x)
+		buf = []byte{b | 0x80}
+
+	case x < o2:
+		buf = make([]byte, 2)
+		buf[1] = byte(x)
+		x >>= 8
+		buf[0] = byte(x) | 0x40
+
+	case x < o3:
+		buf = make([]byte, 3)
+		buf[2] = byte(x)
+		x >>= 8
+		buf[1] = byte(x)
+		x >>= 8
+		buf[0] = byte(x) | 0x20
+
+	case x < o4:
+		buf = make([]byte, 4)
+		for i := 3; i > 0; i-- {
+			buf[i] = byte(x)
+			x >>= 8
+		}
+		buf[0] = byte(x) | 0x10
+
+	case x < o5:
+		buf = make([]byte, 5)
+		for i := 4; i > 0; i-- {
+			buf[i] = byte(x)
+			x >>= 8
+		}
+		buf[0] = byte(x) | 0x08
+
+	case x < o6:
+		buf = make([]byte, 6)
+		for i := 5; i > 0; i-- {
+			buf[i] = byte(x)
+			x >>= 8
+		}
+		buf[0] = byte(x) | 0x04
+
+	case x < o7:
+		buf = make([]byte, 7)
+		for i := 6; i > 0; i-- {
+			buf[i] = byte(x)
+			x >>= 8
+		}
+		buf[0] = byte(x) | 0x02
+
+	case x < o8:
+		buf = make([]byte, 8)
+		for i := 7; i > 0; i-- {
+			buf[i] = byte(x)
+			x >>= 8
+		}
+		buf[0] = 0x01
+
+	default:
+		panic("element size overflow")
+	}
+
+	_, err = e.Write(buf)
+	return
+}

size_test.go

@@ -0,0 +1,51 @@
+package ebml
+
+import (
+	"bytes"
+	"io/ioutil"
+	"testing"
+)
+
+var goldenSizes = []struct {
+	in  uint64
+	out []byte
+}{
+	{0, []byte{0}},
+	{1, []byte{129}},
+	{127, []byte{64, 127}},
+	{128, []byte{64, 128}},
+	{256, []byte{65, 0}},
+	{16382, []byte{32, 63, 254}},
+	{16383, []byte{32, 63, 255}},
+	{16384, []byte{32, 64, 0}},
+	{16385, []byte{32, 64, 1}},
+	{32768, []byte{32, 128, 0}},
+	{2097150, []byte{16, 31, 255, 254}},
+	{2097151, []byte{16, 31, 255, 255}},
+	{2097152, []byte{16, 32, 0, 0}},
+	{268435454, []byte{8, 15, 255, 255, 254}},
+	{268435455, []byte{8, 15, 255, 255, 255}},
+	// http://en.wikipedia.org/wiki/Big_Endian#Examples_of_storing_the_value_0A0B0C0Dh_in_memory
+	{0x0A0B0C0D, []byte{16 | 0x0A, 0x0B, 0x0C, 0x0D}},
+	{1<<56 - 3, []byte{1, 255, 255, 255, 255, 255, 255, 253}},
+}
+
+func TestGoldenSizes(t *testing.T) {
+	for _, g := range goldenSizes {
+		buf := new(bytes.Buffer)
+		enc := NewEncoder(buf)
+		enc.EncodeSize(g.in)
+		enc.Flush()
+		got := buf.Bytes()
+		if !bytes.Equal(got, g.out) {
+			t.Errorf("failed to marshal size %d, wanted %v, got %v", g.in, g.out, got)
+		}
+	}
+}
+
+func BenchmarkEncodeSize(b *testing.B) {
+	enc := NewEncoder(ioutil.Discard)
+	for i := 0; i < b.N; i++ {
+		enc.EncodeSize(uint64(i))
+	}
+}