Inital commit

tools/ui/tests/client/update-message-in-place.svelte.test.ts

@@ -0,0 +1,64 @@
+// Pins the contract that makes streaming cheap: updateMessageAtIndex mutates the
+// existing message object instead of replacing it.
+//
+// Replacing it (`{ ...old, ...updates }`) changes the array slot, which
+// invalidates every consumer that merely walks the list - ChatMessages'
+// `displayMessages` rebuilds entries for EVERY message in the conversation. That
+// made per-token cost scale with conversation length (1.26ms at 1 prior message
+// -> 3.07ms at 40). Mutating in place keeps it flat.
+
+import { describe, it, expect } from 'vitest';
+import { conversationsStore } from '$lib/stores/conversations.svelte';
+import type { DatabaseMessage } from '$lib/types';
+import { MessageRole } from '$lib/enums';
+
+function makeMessage(id: string): DatabaseMessage {
+	return {
+		id,
+		convId: 'c1',
+		type: 'text',
+		timestamp: 0,
+		role: MessageRole.ASSISTANT,
+		content: '',
+		parent: null,
+		children: []
+	} as DatabaseMessage;
+}
+
+describe('conversationsStore.updateMessageAtIndex', () => {
+	it('mutates in place, preserving object identity', () => {
+		conversationsStore.activeMessages = [makeMessage('a'), makeMessage('b')];
+		const before = conversationsStore.activeMessages[1];
+
+		conversationsStore.updateMessageAtIndex(1, { content: 'hello' });
+
+		expect(conversationsStore.activeMessages[1].content).toBe('hello');
+		expect(conversationsStore.activeMessages[1]).toBe(before);
+
+		conversationsStore.activeMessages = [];
+	});
+
+	it('leaves other messages and unrelated fields untouched', () => {
+		conversationsStore.activeMessages = [makeMessage('a'), makeMessage('b')];
+		const untouched = conversationsStore.activeMessages[0];
+
+		conversationsStore.updateMessageAtIndex(1, { content: 'x', model: 'm1' });
+
+		expect(conversationsStore.activeMessages[0]).toBe(untouched);
+		expect(conversationsStore.activeMessages[0].content).toBe('');
+		expect(conversationsStore.activeMessages[1].model).toBe('m1');
+		expect(conversationsStore.activeMessages[1].id).toBe('b');
+
+		conversationsStore.activeMessages = [];
+	});
+
+	it('is a no-op for an index of -1 or out of range', () => {
+		conversationsStore.activeMessages = [makeMessage('a')];
+
+		expect(() => conversationsStore.updateMessageAtIndex(-1, { content: 'x' })).not.toThrow();
+		expect(() => conversationsStore.updateMessageAtIndex(9, { content: 'x' })).not.toThrow();
+		expect(conversationsStore.activeMessages[0].content).toBe('');
+
+		conversationsStore.activeMessages = [];
+	});
+});