Dialog: Provide hints for the sender name

The dialog form has a simple text input for the sender's name.
This can lead to typos or mistakes if the GM wants to send a message
using the same sender name as a previous message.

Add a new function that goes through all the players' histories and
compiles a list of all unique senders, which we can then use to build
a list of hints that will be used to show suggestions when the GM fills
the sender name in the form.

This is currently quite ineficcient as it re-does all the work for every
single dialog.
A better way would be to cache it or store it explicitly somewhere when
we send a message, but this would require a way for the GM to edit it.

scripts/send_dialog_form.mjs

@@ -3,6 +3,10 @@
  * so they can be used in the main module.
  */
 
+import { get_existing_senders } from "./utils.mjs";
+
+const sender_hints_id = "sender_hints"
+
 /**
  * @typedef {Object} MessageFormResponse
  * @property {string[]} recipients The list of player IDs the message will be sent to
@@ -16,6 +20,23 @@
  * @property {MessageFormResponse} form_response The response form the dialog form
  */
 
+/**
+ * Generate the HTML for a <datalist> whose options are the provided senders.
+ * @param {String} hints_id The ID to use for the <datalist> providing the hints
+ * @param {String[]} senders The list of existing senders, or an empty string if invalid.
+ */
+function generate_sender_hints(hints_id, senders) {
+    // Both arguments need to be usable to generate something useful.
+    if (!hints_id || !senders || senders.length === 0) {
+        return "";
+    }
+    let sender_hints_html = `<datalist id="${hints_id}">`
+    for (let sender of senders) {
+        sender_hints_html += `<option value="${sender}">`;
+    }
+    return sender_hints_html + "</datalist>";
+}
+
 /**
  * Given that "required" does not seem to have much impact, check that everything
  * has been properly filled.
@@ -88,6 +109,22 @@ export function prepare_send_message_html(existing_values = null, chain = false)
         localize: true
     })
 
+    /*
+     * If we can find previous senders in player's histories, create a datalist
+     * with them so that the names can appear as autocompletion when filling the form.
+     */
+    /*
+     * TODO: This is very silly : it re-computes everything for every form that is opened.
+     *  Find a way to cache it or store the senders list it explicitly rather than deduce it
+     *  from the history.
+     */
+    let existing_senders = get_existing_senders();
+    if (existing_senders.length > 0) {
+        sender_input.setAttribute("list", sender_hints_id)
+        // This keeps the datalist withing the input group as proper HTML.
+        sender_group.insertAdjacentHTML("beforeend", generate_sender_hints(sender_hints_id, existing_senders))
+    }
+
     const message_input = foundry.applications.elements.HTMLProseMirrorElement.create({
         name: "message",
         required: true,

scripts/utils.mjs

@@ -50,3 +50,28 @@ export async function create_history_journal(user) {
         return user.setFlag(module_id, history_flag, journal.id).then(() => journal);
     });
 }
+
+/**
+ * Go through all the user's histories to build a list of existing senders,
+ * so that they can be displayed or re-used.
+ * The final list is sorted and does not contain duplicates, as one sender
+ * can send messages to multiple players at once.
+ * @returns {String[]} A sorted array of the sender names, if any
+ */
+export function get_existing_senders() {
+    /** @type {String[]} */
+    let senders = []
+    for (let user of game.users.players) {
+        /** @type {JournalEntry} */
+        let history = game.journal.get(user.getFlag(module_id, history_flag))
+        if (!history) {
+            continue;
+        }
+        for (let page of history.pages) {
+            if (!senders.includes(page.name)) {
+                senders.push(page.name);
+            }
+        }
+    }
+    return senders.sort();
+}