webhook: Add several abstracted constructors

api/webhook/webhook.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"mime/multipart"
 	"net/url"
+	"regexp"
 	"strconv"
 
 	"github.com/pkg/errors"
@@ -22,6 +23,23 @@ import (
 // TODO: if there's ever an Arikawa v3, then a new Client abstraction could be
 // made that wraps around Session being an interface. Just a food for thought.
 
+var webhookURLRe = regexp.MustCompile(`https://discord(?:app)?.com/api/webhooks/(\d+)/(.+)`)
+
+// ParseURL parses the given Discord webhook URL.
+func ParseURL(webhookURL string) (id discord.WebhookID, token string, err error) {
+	matches := webhookURLRe.FindStringSubmatch(webhookURL)
+	if matches == nil {
+		return 0, "", errors.New("invalid webhook URL")
+	}
+
+	idInt, err := strconv.ParseUint(matches[1], 10, 64)
+	if err != nil {
+		return 0, "", errors.Wrap(err, "failed to parse webhook ID")
+	}
+
+	return discord.WebhookID(idInt), matches[2], nil
+}
+
 // Session keeps a single webhook session. It is referenced by other webhook
 // clients using the same session.
 type Session struct {
@@ -45,6 +63,11 @@ func (s *Session) OnResponse(r httpdriver.Request, resp httpdriver.Response) err
 	return s.Limiter.Release(r.GetPath(), httpdriver.OptHeader(resp))
 }
 
+// Client creates a new Webhook API client from the session.
+func (s *Session) Client() *Client {
+	return &Client{httputil.NewClient(), s}
+}
+
 // Client is the client used to interact with a webhook.
 type Client struct {
 	// Client is the httputil.Client used to call Discord's API.
@@ -78,6 +101,16 @@ func NewCustom(id discord.WebhookID, token string, hcl *httputil.Client) *Client
 	}
 }
 
+// NewFromURL creates a new webhook client using the passed webhook URL. It
+// uses its own rate limiter.
+func NewFromURL(url string) (*Client, error) {
+	id, token, err := ParseURL(url)
+	if err != nil {
+		return nil, err
+	}
+	return New(id, token), nil
+}
+
 // FromAPI creates a new client that shares the same internal HTTP client with
 // the one in the API's. This is often useful for bots that need webhook
 // interaction, since the rate limiter is shared.