line-by-line.js

/**
 * Line-by-Line Reader Implementation
 * 
 * ## Task
 * Implement a line-by-line reader for a large file using the `getObject` function to fetch data in chunks.
 * The solution must process the file incrementally without loading it entirely into memory.
 * 
 * ## Functionality Requirements
 * - **Method:** Implement a `next()` method that:
 *   - Returns the next complete line as a string.
 *   - Returns `null` when no more lines remain.
 * - **Chunk Handling:** Use `getObject(s3Path, offsetChar, numChars)` to read chunks of data.
 *   - You decide how to manage `offsetChar` and `numChars`.
 * 
 * ## Provided Information
 * - **getObject(s3Path, offsetChar, numChars):**
 *   - Fetches up to `numChars` characters from the file starting at `offsetChar`.
 *   - Returns an empty string or `null` when no more data is available.
 * 
 * ## Constraints
 * - Process the file incrementally without loading it entirely into memory.
 * - Assume lines are separated by a single newline character (`\n`).
 * 
 * ## Deliverables
 * - A working implementation of `next()` (or an equivalent mechanism) that meets the requirements.
 * - The solution should be robust, handling edge cases effectively.
 * - Be prepared to discuss design decisions and testing approaches.
 * 
 * **Good luck!** 🚀
 */

function getObject(s3Path, offsetChar, numChars) {
  const simulatedData = `
{"type": "click", "on": "#subscribeBtn", "time": 167890123}
{"type": "click", "on": "#cancelOrderBtn", "time": 167890456}
{"type": "hover", "on": "#promoBanner", "time": 167891000}
{"type": "scroll", "on": "#productGallery", "time": 167891234}
{"type": "keypress", "on": "#feedbackInput", "time": 167891567}
{"type": "focus", "on": "#emailField", "time": 167891890}
  `.trim();
  
  const result = simulatedData.slice(offsetChar, offsetChar + numChars);
  return result.length > 0 ? result : null;
}

class S3LineReader {
  constructor(s3Path, chunkSize = 20) {
    this.s3Path = s3Path;
    this.chunkSize = chunkSize;
    this.offset = 0;
    this.buffer = "";
    this.eof = false;
  }

  next() {
    if (this.eof && this.buffer.length === 0) {
      return null;
    }

    while (!this.buffer.includes("\n") && !this.eof) {
      const chunk = getObject(this.s3Path, this.offset, this.chunkSize);
      if (!chunk) {
        this.eof = true;
        break;
      }
      this.buffer += chunk;
      this.offset += chunk.length;
    }

    if (this.buffer.includes("\n")) {
      const [line, ...rest] = this.buffer.split("\n");
      this.buffer = rest.join("\n");
      return line.trim();
    }

    if (this.eof) {
      const line = this.buffer.trim();
      this.buffer = "";
      return line || null;
    }

    return null;
  }
}

// Example Usage:
const reader = new S3LineReader('s3://mybucket/event_log');

let line;
while ((line = reader.next()) !== null) {
  console.log(line);
}