/**
 * @license
 * Copyright 2025 Google LLC
 * SPDX-License-Identifier: Apache-2.0
 */

import * as fs from 'node:fs/promises';
import * as path from 'node:path';
import { glob } from 'glob';
import yaml from 'js-yaml';
import { debugLogger } from '../utils/debugLogger.js';
import { coreEvents } from '../utils/events.js';

/**
 * Represents the definition of an Agent Skill.
 */
export interface SkillDefinition {
  /** The unique name of the skill. */
  name: string;
  /** A concise description of what the skill does. */
  description: string;
  /** The absolute path to the skill's source file on disk. */
  location: string;
  /** The core logic/instructions of the skill. */
  body: string;
  /** Whether the skill is currently disabled. */
  disabled?: boolean;
  /** Whether the skill is a built-in skill. */
  isBuiltin?: boolean;
}

export const FRONTMATTER_REGEX = /^---\r?\n([\s\S]*?)\r?\n---\r?\n([\s\S]*)/;

/**
 * Discovers and loads all skills in the provided directory.
 */
export async function loadSkillsFromDir(
  dir: string,
): Promise<SkillDefinition[]> {
  const discoveredSkills: SkillDefinition[] = [];

  try {
    const absoluteSearchPath = path.resolve(dir);
    const stats = await fs.stat(absoluteSearchPath).catch(() => null);
    if (!stats || !stats.isDirectory()) {
      return [];
    }

    const skillFiles = await glob(['SKILL.md', '*/SKILL.md'], {
      cwd: absoluteSearchPath,
      absolute: true,
      nodir: true,
    });

    for (const skillFile of skillFiles) {
      const metadata = await loadSkillFromFile(skillFile);
      if (metadata) {
        discoveredSkills.push(metadata);
      }
    }

    if (discoveredSkills.length === 0) {
      const files = await fs.readdir(absoluteSearchPath);
      if (files.length > 0) {
        debugLogger.debug(
          `Failed to load skills from ${absoluteSearchPath}. The directory is not empty but no valid skills were discovered. Please ensure SKILL.md files are present in subdirectories and have valid frontmatter.`,
        );
      }
    }
  } catch (error) {
    coreEvents.emitFeedback(
      'warning',
      `Error discovering skills in ${dir}:`,
      error,
    );
  }

  return discoveredSkills;
}

/**
 * Loads a single skill from a SKILL.md file.
 */
export async function loadSkillFromFile(
  filePath: string,
): Promise<SkillDefinition | null> {
  try {
    const content = await fs.readFile(filePath, 'utf-8');
    const match = content.match(FRONTMATTER_REGEX);
    if (!match) {
      return null;
    }

    const frontmatter = yaml.load(match[1]);
    if (!frontmatter || typeof frontmatter !== 'object') {
      return null;
    }

    const { name, description } = frontmatter as Record<string, unknown>;
    if (typeof name !== 'string' || typeof description !== 'string') {
      return null;
    }

    return {
      name,
      description,
      location: filePath,
      body: match[2].trim(),
    };
  } catch (error) {
    debugLogger.log(`Error parsing skill file ${filePath}:`, error);
    return null;
  }
}