diff-stream

Compare two ordered object streams and produce a new stream consisting of the differences.

example

  var diffStream = require('diff-stream');

  var fromArray = require('read-stream').array;
  var stream1 = fromArray([{id: 1, name: 'albert'}, {id: 2, name: 'bob'}, {id: 3, name: 'cathy'}]);
  var stream2 = fromArray([{id: 1, name: 'albert'}, {id: 2, name: 'joe'}, {id: 4, name: 'thomas'}, {id: 5, name: 'xavier'}]);

  diffStream(stream1, stream2).pipe(process.stdout);

produces a stream of tuples:

  [{id: 2, name: 'bob'}, {id: 2, name: 'joe'}]
  [{id: 3, name: 'cathy'}, null]
  [null, {id: 4, name: 'thomas'}]
  [null, {id: 5, name: 'xavier'}]

This can be used, for example, to compare rows from streaming database queries, or long streams of JSON objects such as those produced by JSONStream, without having to actually buffer large amounts of data into memory.

Similar to the behavior of the ubiquitous diff command:

• Identical objects are not output
• Corresponding but differing objects are output as [obj1, obj2]
• Objects that only exist in stream1 are output as [obj1, null]
• Objects that only exist in stream2 are output as [null, obj2]

diffStream(stream1, stream2, [compare])

Compare the readable streams stream1 and stream2, returning a new readable stream, which consists of tuple pairs.

compare

The optional compare parameter may be a string key, used to determine the order of the objets. This defaults to id, which is a common key used to order objects in an object stream, such as results from a database query.

compare may also be a function, like the following:

  diffStream(stream1, stream1, function(obj1, obj2) {
    if (obj1.id < obj2.id)
      this.left();
    else if (obj1.id > obj2.id)
      this.right();
    else if (obj1.id == obj2.id && obj1.name == obj2.name)
      this.equal();
    else
      this.notEqual();
  }).pipe(process.stdout);

The function is called with pairs of objects from each stream. The function should call left, right, equal, or notEqual, to produce the corresponding output, and advance either stream1, or stream2, or both.

• left will output [obj1, null], and advance stream1
• right will output [null, obj2], and advance stream2
• equal will output nothing, and advance both streams
• notEqual will output [obj1, obj2], and advance both streams

ordering

Note, it is important that the two object streams be ordered, and that this ordering matches the compare method as described above. If the streams consist of unordered objects, then pairs will not be aligned appropriately.

license

MIT