src/lib/buildPrompt.ts

import {
	PUBLIC_ASSISTANT_MESSAGE_TOKEN,
	PUBLIC_MAX_INPUT_TOKENS,
	PUBLIC_SEP_TOKEN,
	PUBLIC_USER_MESSAGE_TOKEN,
} from "$env/static/public";
import type { Message } from "./types/Message";

/**
 * Convert [{user: "assistant", content: "hi"}, {user: "user", content: "hello"}] to:
 *
 * <|assistant|>hi<|endoftext|><|prompter|>hello<|endoftext|><|assistant|>
 */
export function buildPrompt(messages: Message[]): string {
	const prompt =
		messages
			.map(
				(m) =>
					(m.from === "user"
						? PUBLIC_USER_MESSAGE_TOKEN + m.content
						: PUBLIC_ASSISTANT_MESSAGE_TOKEN + m.content) +
					(m.content.endsWith(PUBLIC_SEP_TOKEN) ? "" : PUBLIC_SEP_TOKEN)
			)
			.join("") + PUBLIC_ASSISTANT_MESSAGE_TOKEN;

	// Not super precise, but it's truncated in the model's backend anyway
	return prompt.split(" ").slice(-parseInt(PUBLIC_MAX_INPUT_TOKENS)).join(" ");
}