How to read fragments of a word-based text file as a key separator?

Question

I have a .txt file with this format:

Part #368 - XXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Part #369 - XXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Part #370 - XXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

I read the file as follows:

var lines = fs.readFileSync('file.txt', 'utf-8')
.split('\n')
.filter(Boolean);

Thus, it returns an array of file strings. How can I get pieces of a file starting with the line "Part"?

var parts = _.filter(lines,function( s ) { return s.indexOf( 'Part' ) !== -1; });

Something like this, but instead of getting lines starting with "Part", I want all the lines from the line "Part" to be added to the next line of "Part".

Answer 1

This creates an array of array of strings.

var parts = _.reduce(lines, function( result, line ) {
    if (line.indexOf('Part') !== -1) result.push([]);
    _.last(result).push(line);
    return result;
}, []);

Answer 2

json stream

@Brad, , stream.Transform, JSON:

const { Transform } = require('stream');

class Delimited extends Transform {
  constructor({ delimiter = /\r?\n/g, encoding = 'utf8' } = {}) {
    super();

    // initialize internal values
    this._delimiter = delimiter instanceof RegExp ? delimiter : new RegExp(delimiter, 'g');
    this._encoding = encoding;
    this._buffer = '';
    this._first = true;
  }

  _transform(chunk, encoding, callback) {
    // convert input encoding into output encoding
    // and append to internal buffer
    if (encoding === 'buffer') {
      this._buffer += chunk.toString(this._encoding);
    } else if (encoding === this._encoding) {
      this._buffer += chunk;
    } else {
      this._buffer += Buffer.from(chunk, encoding).toString(this._encoding);
    }

    let partialJSON = '';

    // check if delimiter is found
    if (this._delimiter.test(this._buffer)) {
      // split internal buffer by delimiter
      let sections = this._buffer.split(this._delimiter);
      // put possibly incomplete section from array back into internal buffer
      this._buffer = sections.pop();
      // add each section to partial json array
      sections.forEach(section => {
        partialJSON += `${this._first ? '[' : ','}${JSON.stringify(section)}`;
        this._first = false;
      });
    }

    // push partial json array to readable stream
    callback(null, partialJSON);
  }

  _flush(callback) {
    // add remaining buffer as last section to json array
    callback(null, `${this._first ? '[' : ','}${JSON.stringify(this._buffer)}]`);
  }
}

:

const fs = require('fs');

let stream = fs.createReadStream('file.txt', 'utf8');
let transform = new Delimited({ delimiter: /\n\n(?=Part #\d)/g });
let json = '';

transform.on('data', (chunk) => json += chunk);
transform.on('end', () => console.log(JSON.parse(json)));

stream.pipe(transform);

!

, JSON , , , objectMode: true:

const { Transform } = require('stream');

class Delimited extends Transform {
  constructor(delimiter = /\r?\n/g) {
    super({ objectMode: true });

    // initialize internal values
    this._delimiter = delimiter instanceof RegExp ? delimiter : new RegExp(delimiter, 'g');
    this._encoding = 'utf8';
    this._buffer = '';
    this._first = true;
  }

  _transform(chunk, encoding, callback) {
    // convert input encoding into output encoding
    // and append to internal buffer
    if (encoding === 'buffer') {
      this._buffer += chunk.toString(this._encoding);
    } else if (encoding === this._encoding) {
      this._buffer += chunk;
    } else {
      this._buffer += Buffer.from(chunk, encoding).toString(this._encoding);
    }

    if (this._delimiter.test(this._buffer)) {
      // split internal buffer by delimiter
      let sections = this._buffer.split(this._delimiter);
      // put possibly incomplete section from array back into internal buffer
      this._buffer = sections.pop();
      // push each section to readable stream in object mode
      sections.forEach(this.push, this);
    }

    callback();
  }

  _flush(callback) {
    // push remaining buffer to readable stream
    callback(null, this._buffer);
  }
}

:

const fs = require('fs');

let stream = fs.createReadStream('file.txt', 'utf8');
let transform = new Delimited(/\n\n(?=Part #\d)/g);
let array = [];

transform.on('data', (chunk) => array.push(chunk));
transform.on('end', () => console.log(array));

stream.pipe(transform);

!