Add pkg/discovery for nodes discovery

Absorb Swarm's discovery package in order to provide a common node
discovery mechanism to be used by both Swarm and networking code.

Signed-off-by: Arnaud Porterie <arnaud.porterie@docker.com>

discovery/entry.go

@@ -0,0 +1,97 @@
+package discovery
+
+import (
+	"fmt"
+	"net"
+)
+
+// NewEntry creates a new entry.
+func NewEntry(url string) (*Entry, error) {
+	host, port, err := net.SplitHostPort(url)
+	if err != nil {
+		return nil, err
+	}
+	return &Entry{host, port}, nil
+}
+
+// An Entry represents a host.
+type Entry struct {
+	Host string
+	Port string
+}
+
+// Equals returns true if cmp contains the same data.
+func (e *Entry) Equals(cmp *Entry) bool {
+	return e.Host == cmp.Host && e.Port == cmp.Port
+}
+
+// String returns the string form of an entry.
+func (e *Entry) String() string {
+	return fmt.Sprintf("%s:%s", e.Host, e.Port)
+}
+
+// Entries is a list of *Entry with some helpers.
+type Entries []*Entry
+
+// Equals returns true if cmp contains the same data.
+func (e Entries) Equals(cmp Entries) bool {
+	// Check if the file has really changed.
+	if len(e) != len(cmp) {
+		return false
+	}
+	for i := range e {
+		if !e[i].Equals(cmp[i]) {
+			return false
+		}
+	}
+	return true
+}
+
+// Contains returns true if the Entries contain a given Entry.
+func (e Entries) Contains(entry *Entry) bool {
+	for _, curr := range e {
+		if curr.Equals(entry) {
+			return true
+		}
+	}
+	return false
+}
+
+// Diff compares two entries and returns the added and removed entries.
+func (e Entries) Diff(cmp Entries) (Entries, Entries) {
+	added := Entries{}
+	for _, entry := range cmp {
+		if !e.Contains(entry) {
+			added = append(added, entry)
+		}
+	}
+
+	removed := Entries{}
+	for _, entry := range e {
+		if !cmp.Contains(entry) {
+			removed = append(removed, entry)
+		}
+	}
+
+	return added, removed
+}
+
+// CreateEntries returns an array of entries based on the given addresses.
+func CreateEntries(addrs []string) (Entries, error) {
+	entries := Entries{}
+	if addrs == nil {
+		return entries, nil
+	}
+
+	for _, addr := range addrs {
+		if len(addr) == 0 {
+			continue
+		}
+		entry, err := NewEntry(addr)
+		if err != nil {
+			return nil, err
+		}
+		entries = append(entries, entry)
+	}
+	return entries, nil
+}