start FindSuccessor query

chord/httpnode/client.go

@@ -0,0 +1,62 @@
+package httpnode
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net/http"
+	"net/netip"
+	"net/url"
+	"path"
+
+	"github.com/go-json-experiment/json"
+
+	"git.sunturtle.xyz/zephyr/chord/chord"
+)
+
+type Client struct {
+	// HTTP is the client used to make requests.
+	HTTP http.Client
+	// APIBase is the path under which the Chord API is served.
+	APIBase string
+}
+
+// FindSuccessor asks s to find the first peer in the network preceding id.
+func (cl *Client) FindSuccessor(ctx context.Context, s chord.Peer, id chord.ID) (chord.Peer, error) {
+	_, addr := s.Values()
+	if !addr.IsValid() {
+		return chord.Peer{}, errors.New("FindSuccessor with invalid peer")
+	}
+	url := url.URL{
+		Scheme:   "http",
+		Host:     addr.String(),
+		Path:     path.Join("/", cl.APIBase, "succ"),
+		RawQuery: url.Values{"s": {id.String()}}.Encode(),
+	}
+	req, err := http.NewRequestWithContext(ctx, "GET", url.String(), nil)
+	if err != nil {
+		return chord.Peer{}, err
+	}
+	resp, err := cl.HTTP.Do(req)
+	if err != nil {
+		return chord.Peer{}, err
+	}
+	var r response[netip.AddrPort]
+	if err := json.UnmarshalRead(resp.Body, &r); err != nil {
+		return chord.Peer{}, err
+	}
+	if r.Error != "" {
+		return chord.Peer{}, fmt.Errorf("%s (%s)", r.Error, resp.Status)
+	}
+	return chord.Address(*r.Data), nil
+}
+
+// Notify tells s we believe n to be its predecessor.
+func (cl *Client) Notify(ctx context.Context, n *chord.Node, s chord.Peer) error {
+	panic("not implemented") // TODO: Implement
+}
+
+// Neighbors requests a peer's beliefs about its own neighbors.
+func (cl *Client) Neighbors(ctx context.Context, p chord.Peer) (pred chord.Peer, succ []chord.Peer, err error) {
+	panic("not implemented") // TODO: Implement
+}

chord/httpnode/httpnode.go

@@ -0,0 +1,15 @@
+// Package httpnode provides an implementation of a Chord client and server
+// using JSON over HTTP.
+package httpnode
+
+import "net/netip"
+
+type response[T any] struct {
+	Data  *T     `json:"data,omitzero"`
+	Error string `json:"error,omitzero"`
+}
+
+type neighbors struct {
+	Succ []netip.AddrPort `json:"succ"`
+	Pred netip.AddrPort   `json:"pred"`
+}

chord/id.go

@@ -3,6 +3,7 @@ package chord
 import (
 	"bytes"
 	"crypto/sha1"
+	"encoding/hex"
 	"net/netip"
 )
 
@@ -42,3 +43,9 @@ func contains(left, right, x ID) bool {
 		return true
 	}
 }
+
+func (id ID) String() string {
+	b := make([]byte, 0, len(ID{})*2)
+	b = hex.AppendEncode(b, id[:])
+	return string(b)
+}

chord/topology.go

@@ -79,6 +79,13 @@ func Address(addr netip.AddrPort) Peer {
 // which was valid.
 func (p Peer) IsValid() bool { return p.addr.IsValid() && p.addr.Port() != 0 }
 
+// Values returns the peer's ID and address.
+func (p Peer) Values() (ID, netip.AddrPort) {
+	// We do this instead of exporting Peer's fields to ensure those fields
+	// are never mutable. If p.IsValid() then p.id == addrID(p.addr) always.
+	return p.id, p.addr
+}
+
 // Create creates a new Chord network using the given address as the initial node.
 func Create(addr netip.AddrPort) (*Node, error) {
 	if !addr.IsValid() {

go.mod

@@ -2,4 +2,7 @@ module git.sunturtle.xyz/zephyr/chord
 
 go 1.24.1
 
-require github.com/urfave/cli/v3 v3.0.0-beta1
+require (
+	github.com/go-json-experiment/json v0.0.0-20250223041408-d3c622f1b874
+	github.com/urfave/cli/v3 v3.0.0-beta1
+)

go.sum

@@ -1,5 +1,7 @@
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/go-json-experiment/json v0.0.0-20250223041408-d3c622f1b874 h1:F8d1AJ6M9UQCavhwmO6ZsrYLfG8zVFWfEfMS2MXPkSY=
+github.com/go-json-experiment/json v0.0.0-20250223041408-d3c622f1b874/go.mod h1:TiCD2a1pcmjd7YnhGH0f/zKNcCD06B029pHhzV23c2M=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=