Problem82.js

// Problem 82
//
// This problem was asked Microsoft.
//
// Using a read7() method that returns 7 characters from a file, implement readN(n) which reads n characters.
//
// For example, given a file with the content “Hello world”, three read7() returns “Hello w”, “orld” and then “”.
//
// O(1) Time complexity
// O(1) Space complexity

class FileReader {
  /**
   * Initializs a file reader
   */
  constructor(file) {
    this.file = file;
    this.offset = 0;
    this.buffer = '';
  }

  /**
   * Reads 7 characters from the file
   * @return {string}
   */
  read7() {
    const start = this.offset;
    const end = Math.min(this.offset + 7, this.file.length);
    // update the offset
    this.offset = end;
    return this.file.substring(start, end);
  }

  /**
   * Reads N characters using read7
   * @param {number} n
   * @return {string}
   */
  readN(n) {
    // adds extra characters to the buffer until the buffer's length is greater than or equal to n
    while (this.buffer.length < n) {
      const extra = this.read7();
      if (extra.length === 0) break;
      this.buffer += extra;
    }

    // updates buffer
    const readResult = this.buffer.substring(0, n);
    this.buffer = this.buffer.substring(n);
    return readResult;
  }
}

export default FileReader;