Add crosspost detection

.env.example

@@ -19,5 +19,11 @@ DISCORD_PATREON_CHANNEL =
 # Discord role ID to add to users when they first join
 DISCORD_MEMBER_ROLE =
 
+# Discord channel IDs which should be checked for crossposting. Comma separated
+DISCORD_CROSSPOST_CHECK_CHANNELS = 
+
+# (OPTIONAL) Number of historical messages to check for crossposts. Default: 10
+# DISCORD_CROSSPOST_HISTORY_MESSAGES = 10
+
 # Sensitivity for the command trigger - defaults to 0.5
 SIMILARITY_SENSITIVITY =

modules/crosspost/index.js

@@ -0,0 +1,38 @@
+const { MessageEmbed } = require('discord.js');
+const store = require("./store");
+
+module.exports = function (client) {
+    client.on('message', async message => {
+        // Ignore bots, DMs, etc
+        if (!message.author || !message.channel || !message.guild) return;
+
+        // Ensure we have crosspost checks enabled for this channel before doing anything
+        if (!store.channelEnabled(message.channel)) return;
+
+        // Check to make sure the message is longer than 50 characters so that small messages like ok or thanks don't get caught
+        if (message.content.length < 50) return;
+
+        // Check if the user has already recently posted a very similar message
+        let previousMessage = store.findMatch(message);
+
+        if (previousMessage) {
+            // Remove the message from the cache so a double-warning isn't possible
+            store.removeMessage(previousMessage);
+            // Reply with warning message
+            await message.reply(
+                new MessageEmbed()
+                    .setTitle(`<:crosspost:999440431521742928> It looks like you already posted that...`)
+                    .setColor(`#94df03`)
+                    .setURL(`https://discord.com/channels/${previousMessage.guild.id}/${previousMessage.channel.id}/${previousMessage.id}`)
+                    .setDescription(
+                        `<@${message.author.id}> sent the same message to <#${previousMessage.channel.id}> <t:${(message.createdTimestamp / 1000).toFixed(0)}:R>.
+
+                        It's best to ask your question in just a single channel otherwise it can cause confusion between those trying to help!`
+                    )
+            );
+        } else {
+            // Else, log the message in case the user crossposts it in future
+            store.addMessage(message);
+        }
+    });
+};

modules/crosspost/store.js

@@ -0,0 +1,38 @@
+let recentMessages = [];
+
+crosspostMessageStore = {
+    channelEnabled(channel) {
+        // If anything is empty or undefined then return false
+        if (!process.env.DISCORD_CROSSPOST_CHECK_CHANNELS || !channel) return false;
+        return process.env.DISCORD_CROSSPOST_CHECK_CHANNELS.split(",").includes(channel.id);
+    },
+
+    addMessage(message) {
+        recentMessages.push(message);
+        // Ensure we have the right amount of messages still in the cache
+        module.exports.cleanup();
+    },
+
+    cleanup() {
+        recentMessages = recentMessages.slice(0 - parseInt(process.env.DISCORD_CROSSPOST_HISTORY_MESSAGES ?? 10));
+    },
+
+    removeMessage(message) {
+        recentMessages = recentMessages.filter(m => !m.equals(message));
+    },
+
+    findMatch(message) {
+        for (const possibleMatch of recentMessages) {
+            // Ensure match has the same author
+            if (!possibleMatch.author || !possibleMatch.author.equals(message.author)) continue;
+            // Ensure both messages have a non-empty content
+            if (!possibleMatch.content || !message.content || possibleMatch.content === "" || message.content === "") continue;
+            // Ensure they are in different channels
+            if (!possibleMatch.channel || !message.channel || message.channel.equals(possibleMatch.channel)) continue;
+            // If the content is the same, then this is a match - return it
+            if (possibleMatch.content.trim() === message.content.trim()) return possibleMatch;
+        }
+    }
+};
+
+module.exports = crosspostMessageStore;