refactor!: working WD-4 user discovery

pkg/webfinger/webfinger.go

@@ -1,13 +1,19 @@
 package webfinger
 
 import (
+	"context"
+	"encoding/json"
 	"errors"
+	"fmt"
+	"github.com/lysand-org/versia-go/pkg/protoretry"
+	"net/http"
 	"net/url"
 	"strings"
 )
 
 var (
 	ErrInvalidSyntax = errors.New("must follow the format \"acct:<ID|Username>@<DOMAIN>\"")
+	ErrUserNotFound  = errors.New("user could not be found")
 )
 
 func ParseResource(res string) (*UserID, error) {
@@ -48,7 +54,7 @@ type Response struct {
 
 type Link struct {
 	Relation string `json:"rel"`
-	Type     any    `json:"type"`
+	Type     string `json:"type"`
 	Link     string `json:"href"`
 }
 
@@ -70,3 +76,47 @@ func (u User) WebFingerResource() Response {
 		},
 	}
 }
+
+func Discover(c *protoretry.Client, ctx context.Context, baseURI, username string) (*User, error) {
+	u := &User{UserID: UserID{ID: username, Domain: baseURI}}
+
+	body, resp, err := c.GET(ctx, &url.URL{
+		Scheme:   "https",
+		Host:     u.UserID.Domain,
+		Path:     "/.well-known/webfinger",
+		RawQuery: url.Values{"resource": []string{"acct:" + u.String()}}.Encode(),
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	if resp.StatusCode == http.StatusNotFound {
+		return nil, ErrUserNotFound
+	} else if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("unexpected status code: %d", resp.StatusCode)
+	}
+
+	var respBody Response
+	if err := json.Unmarshal(body, &respBody); err != nil {
+		return nil, err
+	}
+
+	if respBody.Error != nil {
+		return nil, fmt.Errorf("webfinger error: %s", *respBody.Error)
+	}
+
+	for _, link := range respBody.Links {
+		if link.Relation == "self" {
+			if u.URI, err = url.Parse(link.Link); err != nil {
+				return nil, err
+			}
+		} else if link.Relation == "avatar" {
+			u.AvatarMIMEType = link.Type
+			if u.Avatar, err = url.Parse(link.Link); err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	return u, nil
+}