FIX: stabilize diff algorithm for streaming (#1358)

Previous to this fix we would diff the entire body of text, this
could lead to situations where a diff presented to a user was
wildly off matching areas in the text that should not have been
tested.

New algorithm only checks a portion of the string, this ensures
that during streaming there is no chance for wild mistakes

assets/javascripts/discourse/lib/diff-streamer.gjs

@@ -4,6 +4,8 @@ import loadJSDiff from "discourse/lib/load-js-diff";
 import { parseAsync } from "discourse/lib/text";
 
 const DEFAULT_CHAR_TYPING_DELAY = 30;
+const STREAMING_DIFF_TRUNCATE_THRESHOLD = 0.1;
+const STREAMING_DIFF_TRUNCATE_BUFFER = 10;
 
 export default class DiffStreamer {
   @tracked isStreaming = false;
@@ -122,6 +124,53 @@ export default class DiffStreamer {
     return maybeUnfinishedImage || maybeUnfinishedLink;
   }
 
+  // this is public to make testing easier
+  // is makes it easier to do a "streaming diff" where we want to ensure diff
+  // is focused on the beginning of the text instead of taking the entire body
+  // into account.
+  // This ensures that we do not make mistakes and present wildly different diffs
+  // to what we would stablize on at the end of the stream.
+  streamingDiff(original, suggestion) {
+    const maxDiffLength = Math.floor(
+      suggestion.length +
+        suggestion.length * STREAMING_DIFF_TRUNCATE_THRESHOLD +
+        STREAMING_DIFF_TRUNCATE_BUFFER
+    );
+    const head = original.slice(0, maxDiffLength);
+    const tail = original.slice(maxDiffLength);
+
+    const diffArray = this.jsDiff.diffWordsWithSpace(head, suggestion);
+
+    if (tail.length > 0) {
+      // if last in the array is added, and previous is removed then flip them
+      let last = diffArray[diffArray.length - 1];
+      let secondLast = diffArray[diffArray.length - 2];
+
+      if (last.added && secondLast.removed) {
+        diffArray.pop();
+        diffArray.pop();
+        diffArray.push(last);
+        diffArray.push(secondLast);
+
+        last = secondLast;
+        secondLast = diffArray[diffArray.length - 2];
+      }
+
+      if (!last.removed) {
+        last = {
+          added: false,
+          removed: true,
+          value: "",
+        };
+        diffArray.push(last);
+      }
+
+      last.value = last.value + tail;
+    }
+
+    return diffArray;
+  }
+
   async #streamNextChar() {
     if (this.currentWordIndex < this.words.length) {
       const currentToken = this.words[this.currentWordIndex];
@@ -134,7 +183,7 @@ export default class DiffStreamer {
         this.currentWordIndex++;
         this.currentCharIndex = 0;
 
-        const originalDiff = this.jsDiff.diffWordsWithSpace(
+        const originalDiff = this.streamingDiff(
           this.selectedText,
           this.suggestion
         );

test/javascripts/unit/lib/diff-streamer-test.js

@@ -0,0 +1,67 @@
+import { module, test } from "qunit";
+import DiffStreamer from "discourse/plugins/discourse-ai/discourse/lib/diff-streamer";
+
+module("Unit | Lib | diff-streamer", function () {
+  test("streamingDiff correctly handles trivial cases", async function (assert) {
+    const originalText = "helo world";
+    const targetText = "hello world";
+
+    const diffStreamer = new DiffStreamer(this.originalTextContent);
+    await diffStreamer.loadJSDiff();
+
+    const diffResult = diffStreamer.streamingDiff(originalText, targetText);
+
+    const expectedDiff = [
+      { count: 1, added: false, removed: true, value: "helo" },
+      { count: 1, added: true, removed: false, value: "hello" },
+      { count: 2, added: false, removed: false, value: " world" },
+    ];
+
+    assert.deepEqual(
+      diffResult,
+      expectedDiff,
+      "Diff result should match the expected structure"
+    );
+  });
+
+  test("streamingDiff correctly consolidates and handles diff drift", async function (assert) {
+    const originalText =
+      "This is todone, but I want to can why.\n\nWe\n\nSEO Tags supports a `canonical_url` override. I tried a few possibilities there. The one I wanted to work, ex: `https://www.discourse.org/de`, appended an extra `/de` on the URL, only in the deploy b";
+    const targetText = "This is to-done";
+
+    const diffStreamer = new DiffStreamer(this.originalTextContent);
+    await diffStreamer.loadJSDiff();
+
+    const diffResult = diffStreamer.streamingDiff(originalText, targetText);
+
+    // Verify the diff result is an array with the expected structure
+    assert.true(Array.isArray(diffResult), "Diff result should be an array");
+    assert.strictEqual(
+      diffResult.length,
+      3,
+      "Expecting exactly three parts in the diff result"
+    );
+
+    assert.strictEqual(
+      diffResult[0].value,
+      "This is ",
+      "First part should be unchanged"
+    );
+
+    assert.strictEqual(
+      diffResult[1].value,
+      "to-done",
+      "Second part should be an insertion"
+    );
+
+    assert.true(diffResult[1].added, "Second part should be an insertion");
+
+    assert.strictEqual(
+      diffResult[2].value.length,
+      235,
+      "Third part should include all text"
+    );
+
+    assert.true(diffResult[2].removed, "Third part should be a removal");
+  });
+});