Function tailFile

src/filesystem/lib.ts:285–334

(filePath: string, numLines: number)

// Memory-efficient implementation to get the last N lines of a file
export async function tailFile(filePath: string, numLines: number): Promise<string> {
  const CHUNK_SIZE = 1024; // Read 1KB at a time
  const stats = await fs.stat(filePath);
  const fileSize = stats.size;
  
  if (fileSize === 0) return '';
  
  // Open file for reading
  const fileHandle = await fs.open(filePath, 'r');
  try {
    const lines: string[] = [];
    let position = fileSize;
    let chunk = Buffer.alloc(CHUNK_SIZE);
    let linesFound = 0;
    let remainingText = '';
    
    // Read chunks from the end of the file until we have enough lines
    while (position > 0 && linesFound < numLines) {
      const size = Math.min(CHUNK_SIZE, position);
      position -= size;
      
      const { bytesRead } = await fileHandle.read(chunk, 0, size, position);
      if (!bytesRead) break;
      
      // Get the chunk as a string and prepend any remaining text from previous iteration
      const readData = chunk.slice(0, bytesRead).toString('utf-8');
      const chunkText = readData + remainingText;
      
      // Split by newlines and count
      const chunkLines = normalizeLineEndings(chunkText).split('\n');
      
      // If this isn't the end of the file, the first line is likely incomplete
      // Save it to prepend to the next chunk
      if (position > 0) {
        remainingText = chunkLines[0];
        chunkLines.shift(); // Remove the first (incomplete) line
      }
      
      // Add lines to our result (up to the number we need)
      for (let i = chunkLines.length - 1; i >= 0 && linesFound < numLines; i--) {
        lines.unshift(chunkLines[i]);
        linesFound++;
      }
    }
    
    return lines.join('\n');
  } finally {
    await fileHandle.close();
  }
}

// New function to get the first N lines of a file
export async function headFile(filePath: string, numLines: number): Promise<string> {