import type { extname as ExtnameT, resolve as ResolveT } from "node:path";
import type { readdir as ReaddirT } from "node:fs/promises";
import { Document } from "../../document.js";
import { getEnv } from "../../util/env.js";
import { BaseDocumentLoader } from "../base.js";

// TypeScript enums are not tree-shakeable, so doing this instead
// See https://bargsten.org/jsts/enums/
export const UnknownHandling = {
  Ignore: "ignore",
  Warn: "warn",
  Error: "error",
} as const;
// eslint-disable-next-line @typescript-eslint/no-redeclare
/**
 * An enumeration of possible handling strategies for unknown file types.
 */
export type UnknownHandling =
  (typeof UnknownHandling)[keyof typeof UnknownHandling];

/**
 * A mapping of file extensions to loader functions. Each loader function
 * takes a file path as a parameter and returns a `BaseDocumentLoader`
 * instance.
 */
export interface LoadersMapping {
  [extension: string]: (filePath: string) => BaseDocumentLoader;
}

/**
 * A document loader that loads documents from a directory. It extends the
 * `BaseDocumentLoader` class and implements the `load()` method.
 */
export class DirectoryLoader extends BaseDocumentLoader {
  constructor(
    public directoryPath: string,
    public loaders: LoadersMapping,
    public recursive: boolean = true,
    public unknown: UnknownHandling = UnknownHandling.Warn
  ) {
    super();

    if (Object.keys(loaders).length === 0) {
      throw new Error("Must provide at least one loader");
    }
    for (const extension in loaders) {
      if (Object.hasOwn(loaders, extension)) {
        if (extension[0] !== ".") {
          throw new Error(`Extension must start with a dot: ${extension}`);
        }
      }
    }
  }

  /**
   * Loads the documents from the directory. If a file is a directory and
   * `recursive` is `true`, it recursively loads documents from the
   * subdirectory. If a file is a file, it checks if there is a
   * corresponding loader function for the file extension in the `loaders`
   * mapping. If there is, it loads the documents. If there is no
   * corresponding loader function and `unknown` is set to `Warn`, it logs a
   * warning message. If `unknown` is set to `Error`, it throws an error.
   * @returns A promise that resolves to an array of loaded documents.
   */
  public async load(): Promise<Document[]> {
    const { readdir, extname, resolve } = await DirectoryLoader.imports();
    const files = await readdir(this.directoryPath, { withFileTypes: true });

    const documents: Document[] = [];

    for (const file of files) {
      const fullPath = resolve(this.directoryPath, file.name);
      if (file.isDirectory()) {
        if (this.recursive) {
          const loader = new DirectoryLoader(
            fullPath,
            this.loaders,
            this.recursive,
            this.unknown
          );
          documents.push(...(await loader.load()));
        }
      } else {
        // I'm aware some things won't be files,
        // but they will be caught by the "unknown" handling below.
        const loaderFactory = this.loaders[extname(file.name)];
        if (loaderFactory) {
          const loader = loaderFactory(fullPath);
          documents.push(...(await loader.load()));
        } else {
          switch (this.unknown) {
            case UnknownHandling.Ignore:
              break;
            case UnknownHandling.Warn:
              console.warn(`Unknown file type: ${file.name}`);
              break;
            case UnknownHandling.Error:
              throw new Error(`Unknown file type: ${file.name}`);
            default:
              throw new Error(`Unknown unknown handling: ${this.unknown}`);
          }
        }
      }
    }

    return documents;
  }

  /**
   * Imports the necessary functions from the `node:path` and
   * `node:fs/promises` modules. It is used to dynamically import the
   * functions when needed. If the import fails, it throws an error
   * indicating that the modules failed to load.
   * @returns A promise that resolves to an object containing the imported functions.
   */
  static async imports(): Promise<{
    readdir: typeof ReaddirT;
    extname: typeof ExtnameT;
    resolve: typeof ResolveT;
  }> {
    try {
      const { extname, resolve } = await import("node:path");
      const { readdir } = await import("node:fs/promises");
      return { readdir, extname, resolve };
    } catch (e) {
      console.error(e);
      throw new Error(
        `Failed to load fs/promises. DirectoryLoader available only on environment 'node'. It appears you are running environment '${getEnv()}'. See https://<link to docs> for alternatives.`
      );
    }
  }
}