import type { readFile as ReadFileT } from "node:fs/promises";
import { Document } from "../../document.js";
import { getEnv } from "../../util/env.js";
import { BaseDocumentLoader } from "../base.js";

/**
 * Abstract class that extends the `BaseDocumentLoader` class. It
 * represents a document loader that loads documents from a buffer. The
 * `load()` method is implemented to read the buffer contents and metadata
 * based on the type of `filePathOrBlob`, and then calls the `parse()`
 * method to parse the buffer and return the documents.
 */
export abstract class BufferLoader extends BaseDocumentLoader {
  constructor(public filePathOrBlob: string | Blob) {
    super();
  }

  /**
   * Abstract method that needs to be implemented by subclasses. It is used
   * to parse the buffer and return the documents.
   * @param raw The buffer to be parsed.
   * @param metadata Metadata of the document.
   * @returns Promise that resolves with an array of `Document` objects.
   */
  protected abstract parse(
    raw: Buffer,
    metadata: Document["metadata"]
  ): Promise<Document[]>;

  /**
   * Method that reads the buffer contents and metadata based on the type of
   * `filePathOrBlob`, and then calls the `parse()` method to parse the
   * buffer and return the documents.
   * @returns Promise that resolves with an array of `Document` objects.
   */
  public async load(): Promise<Document[]> {
    let buffer: Buffer;
    let metadata: Record<string, string>;
    if (typeof this.filePathOrBlob === "string") {
      const { readFile } = await BufferLoader.imports();
      buffer = await readFile(this.filePathOrBlob);
      metadata = { source: this.filePathOrBlob };
    } else {
      buffer = await this.filePathOrBlob
        .arrayBuffer()
        .then((ab) => Buffer.from(ab));
      metadata = { source: "blob", blobType: this.filePathOrBlob.type };
    }
    return this.parse(buffer, metadata);
  }

  /**
   * Static method that imports the `readFile` function from the
   * `fs/promises` module in Node.js. It is used to dynamically import the
   * function when needed. If the import fails, it throws an error
   * indicating that the `fs/promises` module is not available in the
   * current environment.
   * @returns Promise that resolves with an object containing the `readFile` function.
   */
  static async imports(): Promise<{
    readFile: typeof ReadFileT;
  }> {
    try {
      const { readFile } = await import("node:fs/promises");
      return { readFile };
    } catch (e) {
      console.error(e);
      throw new Error(
        `Failed to load fs/promises. TextLoader available only on environment 'node'. It appears you are running environment '${getEnv()}'. See https://<link to docs> for alternatives.`
      );
    }
  }
}