add component/foreach helpers

This adds a `Component` helper type and a `ForEach` helper method.

The first attempt used an interface but interfaces imply allocation. We really
can't afford to allocate here.

codec.go

@@ -96,54 +96,58 @@ func validateBytes(b []byte) (err error) {
 	return nil
 }
 
-func bytesToString(b []byte) (ret string, err error) {
-	s := ""
+func readComponent(b []byte) (int, Component, error) {
+	var offset int
+	code, n, err := ReadVarintCode(b)
+	if err != nil {
+		return 0, Component{}, err
+	}
+	offset += n
 
-	for len(b) > 0 {
-		code, n, err := ReadVarintCode(b)
-		if err != nil {
-			return "", err
-		}
-
-		b = b[n:]
-		p := ProtocolWithCode(code)
-		if p.Code == 0 {
-			return "", fmt.Errorf("no protocol with code %d", code)
-		}
-		s += "/" + p.Name
-
-		if p.Size == 0 {
-			continue
-		}
-
-		n, size, err := sizeForAddr(p, b)
-		if err != nil {
-			return "", err
-		}
-
-		b = b[n:]
-
-		if len(b) < size || size < 0 {
-			return "", fmt.Errorf("invalid value for size")
-		}
-
-		if p.Transcoder == nil {
-			return "", fmt.Errorf("no transcoder for %s protocol", p.Name)
-		}
-		a, err := p.Transcoder.BytesToString(b[:size])
-		if err != nil {
-			return "", err
-		}
-		if p.Path && len(a) > 0 && a[0] == '/' {
-			a = a[1:]
-		}
-		if len(a) > 0 {
-			s += "/" + a
-		}
-		b = b[size:]
+	p := ProtocolWithCode(code)
+	if p.Code == 0 {
+		return 0, Component{}, fmt.Errorf("no protocol with code %d", code)
 	}
 
-	return s, nil
+	if p.Size == 0 {
+		return offset, Component{
+			bytes:    b[:offset],
+			offset:   offset,
+			protocol: p,
+		}, nil
+	}
+
+	n, size, err := sizeForAddr(p, b[offset:])
+	if err != nil {
+		return 0, Component{}, err
+	}
+
+	offset += n
+
+	if len(b[offset:]) < size || size < 0 {
+		return 0, Component{}, fmt.Errorf("invalid value for size")
+	}
+
+	return offset + size, Component{
+		bytes:    b[:offset+size],
+		protocol: p,
+		offset:   offset,
+	}, nil
+}
+
+func bytesToString(b []byte) (ret string, err error) {
+	var buf strings.Builder
+
+	for len(b) > 0 {
+		n, c, err := readComponent(b)
+		if err != nil {
+			return "", err
+		}
+		b = b[n:]
+		c.writeTo(&buf)
+	}
+
+	return buf.String(), nil
 }
 
 func sizeForAddr(p Protocol, b []byte) (skip, size int, err error) {

component.go

@@ -0,0 +1,149 @@
+package multiaddr
+
+import (
+	"bytes"
+	"encoding/binary"
+	"fmt"
+	"strings"
+)
+
+// Component is a single multiaddr Component.
+type Component struct {
+	bytes    []byte
+	protocol Protocol
+	offset   int
+}
+
+func (c *Component) Bytes() []byte {
+	return c.bytes
+}
+
+func (c *Component) Equal(o Multiaddr) bool {
+	return bytes.Equal(c.bytes, o.Bytes())
+}
+
+func (c *Component) Protocols() []Protocol {
+	return []Protocol{c.protocol}
+}
+
+func (c *Component) Decapsulate(o Multiaddr) Multiaddr {
+	if c.Equal(o) {
+		return nil
+	}
+	return c
+}
+
+func (c *Component) Encapsulate(o Multiaddr) Multiaddr {
+	m := multiaddr{bytes: c.bytes}
+	return m.Encapsulate(o)
+}
+
+func (c *Component) ValueForProtocol(code int) (string, error) {
+	if c.protocol.Code != code {
+		return "", ErrProtocolNotFound
+	}
+	return c.Value(), nil
+}
+
+func (c *Component) Protocol() Protocol {
+	return c.protocol
+}
+
+func (c *Component) RawValue() []byte {
+	return c.bytes[c.offset:]
+}
+
+func (c *Component) Value() string {
+	if c.protocol.Transcoder == nil {
+		return ""
+	}
+	value, err := c.protocol.Transcoder.BytesToString(c.bytes[c.offset:])
+	if err != nil {
+		// This Component must have been checked.
+		panic(err)
+	}
+	return value
+}
+
+func (c *Component) String() string {
+	var b strings.Builder
+	c.writeTo(&b)
+	return b.String()
+}
+
+// writeTo is an efficient, private function for string-formatting a multiaddr.
+// Trust me, we tend to allocate a lot when doing this.
+func (c *Component) writeTo(b *strings.Builder) {
+	b.WriteByte('/')
+	b.WriteString(c.protocol.Name)
+	value := c.Value()
+	if len(value) == 0 {
+		return
+	}
+	if !(c.protocol.Path && value[0] == '/') {
+		b.WriteByte('/')
+	}
+	b.WriteString(value)
+}
+
+// NewComponent constructs a new multiaddr component
+func NewComponent(protocol, value string) (*Component, error) {
+	p := ProtocolWithName(protocol)
+	if p.Code == 0 {
+		return nil, fmt.Errorf("unsupported protocol: %s", protocol)
+	}
+	if p.Transcoder != nil {
+		bts, err := p.Transcoder.StringToBytes(value)
+		if err != nil {
+			return nil, err
+		}
+		return newComponent(p, bts), nil
+	} else if value != "" {
+		return nil, fmt.Errorf("protocol %s doesn't take a value", p.Name)
+	}
+	return newComponent(p, nil), nil
+	// TODO: handle path /?
+}
+
+func newComponent(protocol Protocol, bvalue []byte) *Component {
+	size := len(bvalue)
+	size += len(protocol.VCode)
+	if protocol.Size < 0 {
+		size += VarintSize(len(bvalue))
+	}
+	maddr := make([]byte, size)
+	var offset int
+	offset += copy(maddr[offset:], protocol.VCode)
+	if protocol.Size < 0 {
+		offset += binary.PutUvarint(maddr[offset:], uint64(len(bvalue)))
+	}
+	copy(maddr[offset:], bvalue)
+
+	// For debugging
+	if len(maddr) != offset+len(bvalue) {
+		panic("incorrect length")
+	}
+
+	return &Component{
+		bytes:    maddr,
+		protocol: protocol,
+		offset:   offset,
+	}
+}
+
+// ForEach walks over the multiaddr, component by component.
+//
+// This function iterates over components *by value* to avoid allocating.
+func ForEach(m Multiaddr, cb func(c Component) bool) {
+	b := m.Bytes()
+	for len(b) > 0 {
+		n, c, err := readComponent(b)
+		if err != nil {
+			panic(err)
+		}
+		if !cb(c) {
+			return
+		}
+		b = b[n:]
+	}
+}

interface.go

@@ -43,5 +43,8 @@ type Multiaddr interface {
 	Decapsulate(Multiaddr) Multiaddr
 
 	// ValueForProtocol returns the value (if any) following the specified protocol
+	//
+	// Note: protocols can appear multiple times in a single multiaddr.
+	// Consider using `ForEach` to walk over the addr manually.
 	ValueForProtocol(code int) (string, error)
 }

multiaddr.go

@@ -127,16 +127,15 @@ func (m multiaddr) Decapsulate(o Multiaddr) Multiaddr {
 
 var ErrProtocolNotFound = fmt.Errorf("protocol not found in multiaddr")
 
-func (m multiaddr) ValueForProtocol(code int) (string, error) {
-	for _, sub := range Split(m) {
-		p := sub.Protocols()[0]
-		if p.Code == code {
-			if p.Size == 0 {
-				return "", nil
-			}
-			return strings.SplitN(sub.String(), "/", 3)[2], nil
+func (m multiaddr) ValueForProtocol(code int) (value string, err error) {
+	err = ErrProtocolNotFound
+	ForEach(m, func(c Component) bool {
+		if c.Protocol().Code == code {
+			value = c.Value()
+			err = nil
+			return false
 		}
-	}
-
-	return "", ErrProtocolNotFound
+		return true
+	})
+	return
 }

util.go

@@ -4,15 +4,11 @@ import "fmt"
 
 // Split returns the sub-address portions of a multiaddr.
 func Split(m Multiaddr) []Multiaddr {
-	split, err := bytesSplit(m.Bytes())
-	if err != nil {
-		panic(fmt.Errorf("invalid multiaddr %s", m.String()))
-	}
-
-	addrs := make([]Multiaddr, len(split))
-	for i, addr := range split {
-		addrs[i] = multiaddr{bytes: addr}
-	}
+	var addrs []Multiaddr
+	ForEach(m, func(c Component) bool {
+		addrs = append(addrs, &c)
+		return true
+	})
 	return addrs
 }