import { readFileSync, statSync } from 'node:fs';

const MAX_SNIPPET_FILE_BYTES = 1024 * 1024 * 2;
const SKIPPED_PATH_MARKERS = [
	'.min.js',
	'.min.css',
	'.map',
	'node_modules',
	'.next',
	'dist/',
	'build/',
	'__pycache__',
] as const;

/**
 * Extracts a numbered code snippet around a 1-indexed vulnerable line.
 *
 * The returned snippet spans the range `[line-6, line+5]` in 0-indexed
 * coordinates, with the vulnerable line marked by `→` and all other lines
 * marked by two spaces. Returns `null` for unreadable/missing files,
 * oversized files (>2MB), known generated/minified paths, binary files,
 * or out-of-range line numbers.
 */
export function extractSnippet(filePath: string, line: number): string | null {
	if (!Number.isInteger(line) || line < 1) {
		return null;
	}

	if (shouldSkipSnippetPath(filePath)) {
		return null;
	}

	try {
		const stats = statSync(filePath);
		if (!stats.isFile() || stats.size > MAX_SNIPPET_FILE_BYTES) {
			return null;
		}

		const fileBuffer = readFileSync(filePath);
		if (fileBuffer.includes(0)) {
			return null;
		}

		const lines = fileBuffer.toString('utf8').split(/\r?\n/);
		if (line > lines.length) {
			return null;
		}

		const startIndex = Math.max(0, line - 6);
		const endExclusive = Math.min(lines.length, line + 5);
		const lineNumberWidth = String(endExclusive).length;

		const formatted = lines
			.slice(startIndex, endExclusive)
			.map((content, offset) => {
				const lineNumber = startIndex + offset + 1;
				const marker = lineNumber === line ? '→' : '  ';
				return `${marker} ${String(lineNumber).padStart(lineNumberWidth, ' ')} | ${content}`;
			});

		return formatted.join('\n');
	} catch {
		return null;
	}
}

function shouldSkipSnippetPath(filePath: string): boolean {
	const normalizedPath = filePath.replace(/\\/g, '/').toLowerCase();
	return SKIPPED_PATH_MARKERS.some((marker) => normalizedPath.includes(marker));
}