Add Discord webhook adapter (#196)

* Add Discord webhook adapter

lib/notification/adapter/discord_webhook.js

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+import fetch from 'node-fetch';
+import { getJob } from '../../services/storage/jobStorage.js';
+import { markdown2Html } from '../../services/markdown.js';
+import { normalizeImageUrl } from '../../utils.js';
+
+/**
+ * Generates an idempotent decimal color code. The input string-based color code is
+ * generated using the djb2 hash algorithm.
+ *
+ * @param {string} str - Input string as color code base
+ * @returns {number} Generated decimal color code (0 - 16777215)
+ */
+const generateColorFromString = (str) => {
+  let hash = 5381;  // initial value
+  const input = String(str);
+
+  for (let i = 0; i < input.length; i++) {
+    // hash * 33 + charCode
+    hash = ((hash << 5) + hash) + input.charCodeAt(i);
+    // Ensure the hash is 32 bit
+    hash |= 0; 
+  }
+
+  let positiveHash = hash >>> 0; 
+  const maxColorValue = 16777215; 
+  const colorDecimal = positiveHash % maxColorValue;
+
+  return colorDecimal;
+};
+
+/**
+ * Creates an embed per listing
+ * (-> see https://birdie0.github.io/discord-webhooks-guide/structure/embeds.html).
+ *
+ * @param {string} jobKey - Key of job (used to set embed color)
+ * @param {object} listing - Object holding listing details
+ * @returns {object} Discord webhook embed
+ */
+const buildEmbed = (jobKey, listing) => {
+  const maxTitleLength = 252;  // Max embed title length is 256 characters
+  let title = String(listing.title ?? 'N/A');
+  if (title.length > maxTitleLength) {
+    title = title.substring(0, maxTitleLength) + '...'; 
+  }
+
+  const fields = [
+    {
+      name: 'Price',
+      value: String(listing.price ?? 'n/a'),
+      inline: true,
+    },
+    {
+      name: 'Size',
+      value: listing?.size?.replace(/2m/g, 'm²') ?? 'n/a',
+      inline: true,
+    },
+    {
+      name: 'Address',
+      value: String(listing.address ?? 'n/a'),
+      inline: true,
+    },
+  ]
+
+  const embed = {
+    title: title,
+    color: generateColorFromString(jobKey),
+    url: listing.link,
+    fields: fields,
+  }
+
+  if (listing.image) {
+    embed.image = {
+      url: normalizeImageUrl(listing.image),
+    };
+  }
+
+  return embed
+};
+
+export const send = ({ serviceName, newListings, notificationConfig, jobKey }) => {
+  const adapter = notificationConfig.find((adapter) => adapter.id === config.id);
+  const webhookUrl = adapter?.fields?.webhookUrl;
+  if (!webhookUrl || newListings.length == 0) return Promise.resolve([]);
+
+  const job = getJob(jobKey);
+  const jobName = job?.name || jobKey;
+
+  const embeds = newListings.map((listing) => buildEmbed(jobKey, listing));
+
+  const maxEmbedsPerMessage = 10;  // Discord only allows up to 10 embeds
+  const webhookPromises = [];
+
+  for (let i = 0; i < embeds.length; i += maxEmbedsPerMessage) {
+    // Send multiple Discord messages with up to 10 embeds per message
+    const embedChunk = embeds.slice(i, i + maxEmbedsPerMessage);
+    
+    const content = i === 0 ? `*${jobName}:* ${serviceName} found **${newListings.length}** new listings.` : '';
+    const body = JSON.stringify({
+        content: content,
+        embeds: embedChunk,
+    })
+
+    const fetchPromise = fetch(webhookUrl, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body,
+    })
+    .catch(error => {
+      console.error(`Error sending Discord webhook for chunk starting at ${i}:`, error);
+      return Promise.reject(new Error(`Webhook failed: ${error.message}`));
+    });
+
+    webhookPromises.push(fetchPromise);
+  }
+
+  return Promise.allSettled(webhookPromises);
+};
+
+export const config = {
+  id: 'discord_webhook',
+  name: 'Discord Webhook',
+  readme: markdown2Html('lib/notification/adapter/discord_webhook.md'),
+  description: 'Fredy will send new listings to the Discord channel of your choice.',
+  fields: {
+    webhookUrl: {
+      type: 'text',
+      label: 'Webhook URL',
+      description: 'The URL of the Discord webhook to send messages to.',
+    },
+  },
+};

lib/notification/adapter/discord_webhook.md

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+### Discord Adapter
+
+To use the [Discord](https://discord.com/) Adapter, you need to create a webhook on the Discord channel of your choice. You can follow the instructions of _Making A Webhook_ on [this support website](https://support.discord.com/hc/en-us/articles/228383668-Intro-to-Webhooks).
+Once you have created a webhook, copy and paste the webhook URL.