Apply rot13 to discord spoilers in unformatted matrix message body

src/discordmessageparser.ts

@@ -274,7 +274,7 @@ export class DiscordMessageParser {
         // matrix spoilers are still in MSC stage
         // see https://github.com/matrix-org/matrix-doc/pull/2010
         if (!html) {
-            return `(Spoiler: ${node.content})`;
+            return `(Spoiler (rot13): ${Util.Rot13(node.content)})`;
         }
         return `<span data-mx-spoiler>${node.content}</span>`;
     }

src/util.ts

@@ -30,6 +30,22 @@ export class Util {
         return htmlColor;
     }
 
+    public static Rot13(plaintext: string): string {
+        const UPPER_Z = 90;
+        const LOWER_Z = 122;
+        const CAESAR_SHIFT = 13;
+        const ALPHABET_LENGTH = 26;
+        return plaintext.replace(/[a-zA-Z]/g, (c) => {
+            const code = c.charCodeAt(0);
+            let shiftedCode = code + CAESAR_SHIFT;
+            if ((code <= UPPER_Z && shiftedCode > UPPER_Z)
+                || (code <= LOWER_Z && shiftedCode > LOWER_Z)) {
+                shiftedCode = shiftedCode - ALPHABET_LENGTH;
+            }
+            return String.fromCharCode(shiftedCode);
+        });
+    }
+
     public static async AsyncForEach(arr, callback) {
         for (let i = 0; i < arr.length; i++) {
             await callback(arr[i], i, arr);

test/test_discordmessageparser.ts

@@ -350,7 +350,7 @@ describe("DiscordMessageParser", () => {
             const mp = new DiscordMessageParser();
             const msg = getMessage("||foxies||");
             const result = await mp.FormatMessage(defaultOpts, msg);
-            expect(result.body).is.equal("(Spoiler: foxies)");
+            expect(result.body).is.equal("(Spoiler (rot13): sbkvrf)");
             expect(result.formattedBody).is.equal("<span data-mx-spoiler>foxies</span>");
         });
         it("processes unknown emoji correctly", async () => {