msgconv: initial impl of ToMatrix, porting convertDiscordTextMessage

pkg/msgconv/embed.go

@@ -0,0 +1,97 @@
+// mautrix-discord - A Matrix-Discord puppeting bridge.
+// Copyright (C) 2024 Tulir Asokan
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package msgconv
+
+import (
+	"regexp"
+
+	"github.com/bwmarrin/discordgo"
+)
+
+type BridgeEmbedType int
+
+const (
+	EmbedUnknown BridgeEmbedType = iota
+	EmbedRich
+	EmbedLinkPreview
+	EmbedVideo
+)
+
+const discordLinkPattern = `https?://[^<\p{Zs}\x{feff}]*[^"'),.:;\]\p{Zs}\x{feff}]`
+
+// Discord links start with http:// or https://, contain at least two characters afterwards,
+// don't contain < or whitespace anywhere, and don't end with "'),.:;]
+//
+// Zero-width whitespace is mostly in the Format category and is allowed, except \uFEFF isn't for some reason
+var discordLinkRegex = regexp.MustCompile(discordLinkPattern)
+var discordLinkRegexFull = regexp.MustCompile("^" + discordLinkPattern + "$")
+
+func isActuallyLinkPreview(embed *discordgo.MessageEmbed) bool {
+	// Sending YouTube links creates a video embed, but we want to bridge it as a URL preview,
+	// so this is a hacky way to detect those.
+	return embed.Video != nil && embed.Video.ProxyURL == ""
+}
+
+// isPlainGifMessage returns whether a Discord message consists entirely of a
+// link to a GIF-like animated image. A single embed must also be present on the
+// message.
+//
+// This helps replicate Discord first-party client behavior, where the link is
+// hidden when these same conditions are fulfilled.
+func isPlainGifMessage(msg *discordgo.Message) bool {
+	if len(msg.Embeds) != 1 {
+		return false
+	}
+	embed := msg.Embeds[0]
+	isGifVideo := embed.Type == discordgo.EmbedTypeGifv && embed.Video != nil
+	isGifImage := embed.Type == discordgo.EmbedTypeImage && embed.Image == nil && embed.Thumbnail != nil && embed.Title == ""
+	contentIsOnlyURL := msg.Content == embed.URL || discordLinkRegexFull.MatchString(msg.Content)
+	return contentIsOnlyURL && (isGifVideo || isGifImage)
+}
+
+// getEmbedType determines how a Discord embed should be bridged to Matrix by
+// returning a BridgeEmbedType.
+func getEmbedType(msg *discordgo.Message, embed *discordgo.MessageEmbed) BridgeEmbedType {
+	switch embed.Type {
+	case discordgo.EmbedTypeLink, discordgo.EmbedTypeArticle:
+		return EmbedLinkPreview
+	case discordgo.EmbedTypeVideo:
+		if isActuallyLinkPreview(embed) {
+			return EmbedLinkPreview
+		}
+		return EmbedVideo
+	case discordgo.EmbedTypeGifv:
+		return EmbedVideo
+	case discordgo.EmbedTypeImage:
+		if msg != nil && isPlainGifMessage(msg) {
+			return EmbedVideo
+		} else if embed.Image == nil && embed.Thumbnail != nil {
+			return EmbedLinkPreview
+		}
+		return EmbedRich
+	case discordgo.EmbedTypeRich:
+		return EmbedRich
+	default:
+		return EmbedUnknown
+	}
+}
+
+var hackyReplyPattern = regexp.MustCompile(`^\*\*\[Replying to]\(https://discord.com/channels/(\d+)/(\d+)/(\d+)\)`)
+
+func isReplyEmbed(embed *discordgo.MessageEmbed) bool {
+	return hackyReplyPattern.MatchString(embed.Description)
+}