import { readFileSync } from "node:fs";
import { join } from "node:path";

/** File name of the bundle-managed system prompt cover inside the agent directory. */
export const COVER_FILE_NAME = "SYSTEM_COVER.md";

/**
 * Matches the newline-delimited boundary before the first top-level prompt section.
 *
 * Pi renders the system prompt as `<preamble>` followed by sections wrapped in tags of
 * the same name (for example `<tools>`). Nested markup such as
 * `<project_instructions path="...">` carries attributes and therefore never matches a
 * bare tag line, so the first match is the end of the preamble.
 */
const FIRST_SECTION_BOUNDARY = /\n<[a-z][a-z0-9_-]*>\n/;

/** Resolve the cover path inside a given agent directory. */
export function resolveCoverPath(agentDir: string): string {
	return join(agentDir, COVER_FILE_NAME);
}

/**
 * Read the bundle-managed cover file.
 *
 * Returns `undefined` when the file is absent, unreadable, or blank so the caller can
 * leave Pi's own default prompt untouched instead of forcing an empty preamble.
 */
export function readCoverFile(coverPath: string): string | undefined {
	let raw: string;
	try {
		raw = readFileSync(coverPath, "utf8");
	} catch {
		return undefined;
	}
	const trimmed = raw.replace(/\s+$/, "");
	return trimmed.length > 0 ? trimmed : undefined;
}

/**
 * Replace the rendered prompt's preamble with `cover`, keeping everything after the first
 * top-level section byte-for-byte identical.
 *
 * Pi builds the preamble at the top of the rendered prompt, so swapping that prefix is
 * what preserves section order: options-based replacement (`customPrompt`) would drop
 * `tools`/`rules`/`docs` and rebuild them at the end, reordering the prompt. Splicing the
 * rendered text keeps the existing order untouched.
 *
 * A rendered prompt without any top-level section degenerates to the cover alone.
 */
export function spliceCover(rendered: string, cover: string): string {
	const trimmed = cover.replace(/\s+$/, "");
	const boundary = rendered.search(FIRST_SECTION_BOUNDARY);
	if (boundary < 0) {
		return trimmed;
	}
	return trimmed + rendered.slice(boundary);
}