[templating] Add usernameForID function

Signed-off-by: Knut Ahlers <knut@ahlers.me>

functions_twitch.go

@@ -103,6 +103,15 @@ func init() {
 			FakedOutput: "3 hours, 56 minutes",
 		},
 	})
+
+	tplFuncs.Register("usernameForID", plugins.GenericTemplateFunctionGetter(tplTwitchUsernameForID), plugins.TemplateFuncDocumentation{
+		Description: "Returns the currente login name of an user-id",
+		Syntax:      "usernameForID <user-id>",
+		Example: &plugins.TemplateFuncDocumentationExample{
+			Template:    `{{ usernameForID "12826" }}`,
+			FakedOutput: "twitch",
+		},
+	})
 }
 
 func tplTwitchDisplayName(username string, v ...string) (string, error) {
@@ -225,3 +234,7 @@ func tplTwitchStreamUptime(username string) (time.Duration, error) {
 	}
 	return time.Since(si.StartedAt), nil
 }
+
+func tplTwitchUsernameForID(id string) (string, error) {
+	return twitchClient.GetUsernameForID(context.Background(), id)
+}

pkg/twitch/users.go

@@ -159,6 +159,39 @@ func (c *Client) GetIDForUsername(username string) (string, error) {
 	return payload.Data[0].ID, nil
 }
 
+// GetUsernameForID retrieves the login name (not the display name)
+// for the given user ID
+func (c *Client) GetUsernameForID(ctx context.Context, id string) (string, error) {
+	cacheKey := []string{"usernameForID", id}
+	if d := c.apiCache.Get(cacheKey); d != nil {
+		return d.(string), nil
+	}
+
+	var payload struct {
+		Data []User `json:"data"`
+	}
+
+	if err := c.Request(ClientRequestOpts{
+		AuthType: AuthTypeAppAccessToken,
+		Context:  ctx,
+		Method:   http.MethodGet,
+		OKStatus: http.StatusOK,
+		Out:      &payload,
+		URL:      fmt.Sprintf("https://api.twitch.tv/helix/users?id=%s", id),
+	}); err != nil {
+		return "", errors.Wrap(err, "request channel info")
+	}
+
+	if l := len(payload.Data); l != 1 {
+		return "", errors.Errorf("unexpected number of users returned: %d", l)
+	}
+
+	// The username for an ID will not change (often), cache for a long time
+	c.apiCache.Set(cacheKey, timeDay, payload.Data[0].Login)
+
+	return payload.Data[0].Login, nil
+}
+
 func (c *Client) GetUserInformation(user string) (*User, error) {
 	user = strings.TrimLeft(user, "#@")