From 686b1995fa13cd69a518480d1f31731879ddda7c Mon Sep 17 00:00:00 2001
From: Tulir Asokan <tulir@maunium.net>
Date: Sun, 29 May 2022 14:13:34 +0300
Subject: [PATCH] Add initial Discord->Matrix markdown rendering

---
 go.mod    |  4 ++--
 go.sum    |  4 ++--
 portal.go | 32 +++++++++++++++++++++-----------
 3 files changed, 25 insertions(+), 15 deletions(-)

diff --git a/go.mod b/go.mod
index 0ecc19c..4bcb659 100644
--- a/go.mod
+++ b/go.mod
@@ -9,8 +9,9 @@ require (
 	github.com/lib/pq v1.10.6
 	github.com/mattn/go-sqlite3 v1.14.13
 	github.com/skip2/go-qrcode v0.0.0-20200617195104-da1b6568686e
+	github.com/yuin/goldmark v1.4.12
 	maunium.net/go/maulogger/v2 v2.3.2
-	maunium.net/go/mautrix v0.11.1-0.20220528212725-21d71dc1a1d5
+	maunium.net/go/mautrix v0.11.1-0.20220529105558-04789fc73b32
 )
 
 require (
@@ -19,7 +20,6 @@ require (
 	github.com/tidwall/match v1.1.1 // indirect
 	github.com/tidwall/pretty v1.2.0 // indirect
 	github.com/tidwall/sjson v1.2.4 // indirect
-	github.com/yuin/goldmark v1.4.12 // indirect
 	golang.org/x/crypto v0.0.0-20220513210258-46612604a0f9 // indirect
 	golang.org/x/net v0.0.0-20220513224357-95641704303c // indirect
 	golang.org/x/sys v0.0.0-20220422013727-9388b58f7150 // indirect
diff --git a/go.sum b/go.sum
index b4e8b79..299c495 100644
--- a/go.sum
+++ b/go.sum
@@ -58,5 +58,5 @@ maunium.net/go/mauflag v1.0.0 h1:YiaRc0tEI3toYtJMRIfjP+jklH45uDHtT80nUamyD4M=
 maunium.net/go/mauflag v1.0.0/go.mod h1:nLivPOpTpHnpzEh8jEdSL9UqO9+/KBJFmNRlwKfkPeA=
 maunium.net/go/maulogger/v2 v2.3.2 h1:1XmIYmMd3PoQfp9J+PaHhpt80zpfmMqaShzUTC7FwY0=
 maunium.net/go/maulogger/v2 v2.3.2/go.mod h1:TYWy7wKwz/tIXTpsx8G3mZseIRiC5DoMxSZazOHy68A=
-maunium.net/go/mautrix v0.11.1-0.20220528212725-21d71dc1a1d5 h1:HUIEwZ+zgXUhghtUhmcOidcxYBbx+W+/C7MAuhc15o8=
-maunium.net/go/mautrix v0.11.1-0.20220528212725-21d71dc1a1d5/go.mod h1:CiKpMhAx5QZFHK03jpWb0iKI3sGU8x6+LfsOjDrcO8I=
+maunium.net/go/mautrix v0.11.1-0.20220529105558-04789fc73b32 h1:uAYLJ44nuGvR84XAUYGVrwFRhix30CTKgDIIyVHPD6U=
+maunium.net/go/mautrix v0.11.1-0.20220529105558-04789fc73b32/go.mod h1:CiKpMhAx5QZFHK03jpWb0iKI3sGU8x6+LfsOjDrcO8I=
diff --git a/portal.go b/portal.go
index 6de6287..d133bef 100644
--- a/portal.go
+++ b/portal.go
@@ -3,11 +3,16 @@ package main
 import (
 	"bytes"
 	"fmt"
+	"regexp"
 	"strconv"
 	"strings"
 	"sync"
 	"time"
 
+	"github.com/yuin/goldmark"
+
+	"maunium.net/go/mautrix/format"
+	"maunium.net/go/mautrix/format/mdext"
 	"maunium.net/go/mautrix/util/variationselector"
 
 	"github.com/bwmarrin/discordgo"
@@ -538,6 +543,17 @@ func (portal *Portal) handleDiscordAttachment(intent *appservice.IntentAPI, msgI
 	}
 }
 
+var mdRenderer = goldmark.New(format.Extensions, format.HTMLOptions,
+	goldmark.WithExtensions(mdext.EscapeHTML, mdext.SimpleSpoiler, mdext.DiscordUnderline))
+var escapeFixer = regexp.MustCompile(`\\(__[^_]|\*\*[^*])`)
+
+func renderDiscordMarkdown(text string) event.MessageEventContent {
+	text = escapeFixer.ReplaceAllStringFunc(text, func(s string) string {
+		return s[:2] + `\` + s[2:]
+	})
+	return format.RenderMarkdownCustom(text, mdRenderer)
+}
+
 func (portal *Portal) handleDiscordMessageCreate(user *User, msg *discordgo.Message, thread *Thread) {
 	if portal.MXID == "" {
 		portal.log.Warnln("handle message called without a valid portal")
@@ -593,11 +609,8 @@ func (portal *Portal) handleDiscordMessageCreate(user *User, msg *discordgo.Mess
 
 	ts, _ := discordgo.SnowflakeTimestamp(msg.ID)
 	if msg.Content != "" {
-		content := &event.MessageEventContent{
-			Body:      msg.Content,
-			MsgType:   event.MsgText,
-			RelatesTo: threadRelation.Copy(),
-		}
+		content := renderDiscordMarkdown(msg.Content)
+		content.RelatesTo = threadRelation.Copy()
 
 		if msg.MessageReference != nil {
 			//key := database.PortalKey{msg.MessageReference.ChannelID, user.ID}
@@ -610,7 +623,7 @@ func (portal *Portal) handleDiscordMessageCreate(user *User, msg *discordgo.Mess
 			}
 		}
 
-		resp, err := portal.sendMatrixMessage(intent, event.EventMessage, content, nil, ts.UnixMilli())
+		resp, err := portal.sendMatrixMessage(intent, event.EventMessage, &content, nil, ts.UnixMilli())
 		if err != nil {
 			portal.log.Warnfln("failed to send message %q to matrix: %v", msg.ID, err)
 			return
@@ -709,10 +722,7 @@ func (portal *Portal) handleDiscordMessageUpdate(user *User, msg *discordgo.Mess
 		return
 	}
 
-	content := &event.MessageEventContent{
-		Body:    msg.Content,
-		MsgType: event.MsgText,
-	}
+	content := renderDiscordMarkdown(msg.Content)
 	content.SetEdit(existing.MXID)
 
 	var editTS int64
@@ -720,7 +730,7 @@ func (portal *Portal) handleDiscordMessageUpdate(user *User, msg *discordgo.Mess
 		editTS = msg.EditedTimestamp.UnixMilli()
 	}
 	// TODO figure out some way to deduplicate outgoing edits
-	_, err := portal.sendMatrixMessage(intent, event.EventMessage, content, nil, editTS)
+	_, err := portal.sendMatrixMessage(intent, event.EventMessage, &content, nil, editTS)
 	if err != nil {
 		portal.log.Warnfln("failed to send message %q to matrix: %v", msg.ID, err)