[Obs AI Assistant] Regenerate from last user message (elastic#174911)

## Summary

Changes the Regenerate functionality to look back to the last written
message from the user and use that as the cut of point for where to
generate the next messages.

x-pack/plugins/observability_ai_assistant/public/components/chat/chat_body.test.tsx

@@ -0,0 +1,148 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+import { type Message } from '../../../common/types';
+import { reverseToLastUserMessage } from './chat_body';
+
+describe('<ChatBody>', () => {
+  describe('reverseToLastUserMessage', () => {
+    const firstUserMessage = {
+      message: {
+        role: 'user',
+        content: 'Give me a list of my APM services',
+      },
+    };
+    const firstUserMessageIndex = 1;
+
+    const secondUserMessage = {
+      message: {
+        role: 'user',
+        content: 'Can you tell me about the synth-go-0 service',
+      },
+    };
+    const secondUserMessageIndex = 7;
+
+    const messages = [
+      {
+        message: {
+          role: 'system',
+          content: "You're the best around",
+        },
+      },
+      firstUserMessage,
+      {
+        message: {
+          role: 'assistant',
+          function_call: {
+            name: 'recall',
+            arguments: '{"queries":[],"contexts":[]}',
+            trigger: 'assistant',
+          },
+          content: '',
+        },
+      },
+      {
+        message: {
+          role: 'user',
+          name: 'recall',
+          content: '[]',
+        },
+      },
+      {
+        message: {
+          role: 'assistant',
+          function_call: {
+            name: 'get_apm_services_list',
+            arguments: '{\n  "start": "now-1h",\n  "end": "now"\n}',
+            trigger: 'assistant',
+          },
+          content: '',
+        },
+      },
+      {
+        message: {
+          role: 'user',
+          name: 'get_apm_services_list',
+          content: '[{"service.name":"synth-go-0"}]',
+        },
+      },
+      {
+        message: {
+          role: 'assistant',
+          function_call: {
+            name: '',
+            arguments: '',
+            trigger: 'assistant',
+          },
+          content: 'Here is a list of your APM services:\n\n1. synth-go-0\n',
+        },
+      },
+      secondUserMessage,
+      {
+        message: {
+          role: 'assistant',
+          function_call: {
+            name: 'recall',
+            arguments: '{"queries":[],"contexts":[]}',
+            trigger: 'assistant',
+          },
+          content: '',
+        },
+      },
+      {
+        message: {
+          role: 'user',
+          name: 'recall',
+          content: '[]',
+        },
+      },
+      {
+        message: {
+          role: 'assistant',
+          function_call: {
+            name: 'get_apm_service_summary',
+            arguments:
+              '{\n  "service.name": "synth-go-0",\n  "start": "now-1h",\n  "end": "now"\n}',
+            trigger: 'assistant',
+          },
+          content: '',
+        },
+      },
+      {
+        message: {
+          role: 'user',
+          name: 'get_apm_service_summary',
+          content: '{"service.name":"synth-go-0"}',
+        },
+      },
+      {
+        message: {
+          role: 'assistant',
+          function_call: {
+            name: '',
+            arguments: '',
+            trigger: 'assistant',
+          },
+          content: 'The service named "synth-go-0" is really neat.',
+        },
+      },
+    ] as unknown as Message[];
+
+    it('goes back to the last written user message when regenerating from the end of the conversation', () => {
+      const nextMessages = reverseToLastUserMessage(messages, messages.at(-1)!);
+      expect(nextMessages).toEqual(messages.slice(0, secondUserMessageIndex + 1));
+    });
+
+    it('goes back to the last written user message when regenerating from the middle of the conversation', () => {
+      const nextMessages = reverseToLastUserMessage(
+        messages,
+        messages.at(secondUserMessageIndex - 1)!
+      );
+      expect(nextMessages).toEqual(messages.slice(0, firstUserMessageIndex + 1));
+    });
+  });
+});

x-pack/plugins/observability_ai_assistant/public/components/chat/chat_body.tsx

@@ -20,6 +20,7 @@ import {
 import type { AuthenticatedUser } from '@kbn/security-plugin/common';
 import { euiThemeVars } from '@kbn/ui-theme';
 import { i18n } from '@kbn/i18n';
+import { findLastIndex } from 'lodash';
 import { ChatState } from '../../hooks/use_chat';
 import { useConversation } from '../../hooks/use_conversation';
 import { useLicense } from '../../hooks/use_license';
@@ -263,8 +264,7 @@ export function ChatBody({
                   }}
                   onFeedback={handleFeedback}
                   onRegenerate={(message) => {
-                    const indexOf = messages.indexOf(message);
-                    next(messages.slice(0, indexOf));
+                    next(reverseToLastUserMessage(messages, message));
                   }}
                   onSendTelemetry={(eventWithPayload) =>
                     sendEvent(chatService.analytics, eventWithPayload)
@@ -410,3 +410,19 @@ export function ChatBody({
     </EuiFlexGroup>
   );
 }
+
+// Exported for testing only
+export function reverseToLastUserMessage(messages: Message[], message: Message) {
+  // Drop messages after and including the one marked for regeneration
+  const indexOf = messages.indexOf(message);
+  const previousMessages = messages.slice(0, indexOf);
+
+  // Go back to the last written user message to fully regenerate function calls
+  const lastUserMessageIndex = findLastIndex(
+    previousMessages,
+    (aMessage: Message) => aMessage.message.role === 'user' && !aMessage.message.name
+  );
+  const nextMessages = previousMessages.slice(0, lastUserMessageIndex + 1);
+
+  return nextMessages;
+}