hash-join

compute https://en.wikipedia.org/wiki/Hash_join on streaming document collections

example

hashing with functions

var join = require('hash-join');
var r = join(
    function (doc) { return doc.type === 'test' && doc.id },
    function (doc) { return doc.type === 'output' && doc.test }
);
 
r.reduce(function (acc, test, output) {
    var t = acc[test.id];
    if (!t) t = acc[test.id] = { commit : test.commit };
    
    if (!t.output) t.output = [];
    t.output.push(output.value);
    
    return acc;
}, {});
 
r.on('result', function (res) {
    console.dir(res);
});
 
r.insert({ type : 'output', test : 'abc123', value : 'beep ' });
r.insert({ type : 'output', test : 'abc123', value : 'boop\n' });
r.insert({
    type : 'test',
    id : 'abc123',
    commit : 'd8180b0778dd7a145b46f92b9a2d77db916debc5',
});

$ node example/hash.js
{ abc123: 
   { commit: 'd8180b0778dd7a145b46f92b9a2d77db916debc5',
     output: [ 'beep ' ] } }
{ abc123: 
   { commit: 'd8180b0778dd7a145b46f92b9a2d77db916debc5',
     output: [ 'beep ', 'boop\n' ] } }

using key path shortcuts

var join = require('hash-join');
var r = join('test', [ 'output', 'test' ]);
 
r.reduce(function (acc, test, output) {
    var t = acc[test.id];
    if (!t) t = acc[test.id] = { commit : test.commit };
    
    if (!t.output) t.output = [];
    t.output.push(output.value);
    
    return acc;
}, {});
 
r.on('result', function (res) {
    console.dir(res);
});
 
r.insert({ type : 'output', test : 'abc123', value : 'beep ' });
r.insert({ type : 'output', test : 'abc123', value : 'boop\n' });
r.insert({
    type : 'test',
    id : 'abc123',
    commit : 'd8180b0778dd7a145b46f92b9a2d77db916debc5',
});

$ node example/reduce.js
{ abc123: 
   { commit: 'd8180b0778dd7a145b46f92b9a2d77db916debc5',
     output: [ 'beep ' ] } }
{ abc123: 
   { commit: 'd8180b0778dd7a145b46f92b9a2d77db916debc5',
     output: [ 'beep ', 'boop\n' ] } }

using crdt

var join = require('hash-join');
var r = join('test', [ 'output', 'test' ]);
 
r.reduce(function (acc, test, output) {
    var t = acc[test.id];
    if (!t) t = acc[test.id] = { commit : test.commit };
    
    if (!t.output) t.output = [];
    t.output.push(output.value);
    
    return acc;
}, {});
 
r.on('result', function (res) {
    console.dir(res);
});
 
var doc = new(require('crdt').Doc);
doc.on('add', function (x) { r.insert(x.state) });
 
doc.add({ type : 'output', test : 'abc123', value : 'beep ' });
doc.add({ type : 'output', test : 'abc123', value : 'boop\n' });
doc.add({
    type : 'test',
    id : 'abc123',
    commit : 'd8180b0778dd7a145b46f92b9a2d77db916debc5',
});

$ node example/reduce_crdt.js
{ abc123: 
   { commit: 'd8180b0778dd7a145b46f92b9a2d77db916debc5',
     output: [ 'beep ' ] } }
{ abc123: 
   { commit: 'd8180b0778dd7a145b46f92b9a2d77db916debc5',
     output: [ 'beep ', 'boop\n' ] } }

methods

var join = require('hash-join')

var r = join(a, b)

Create a new 2-way join r from the hashing functions a and b.

Hashing functions should return falsy values if they don't match and otherwise should return the string id they should hash into.

If a or b is an array, they are treated as a path through the document to obtain the hashed element starting at the document's 'type' field and deeply traversing for the rest of the arguments. For example ['beep','boop','x',2] would resolve to the string "ghi" for this document:

{
  "type": "beep",
  "boop": { "x" : [ "abc", "def", "ghi" ] },
  "z" : 55
}

If a or b is a string, they are converted to [a,'id'] or [b,'id'] accordingly so that you can more tersely express a foreign key relation on the "id" field.

r.insert(doc)

Insert a document into the join r.

r.reduce(fn, init)

Compute a reduce over all the pairs that resolve to the same hash id with fn(acc, a, b) for an accumulator acc and the objects a and b that match the hashing functions supplied to or inferred from join() in the same order.

fn(acc, a, b) should return the new accumulator value acc. Each time a new value is derived, the 'result' event fires with the new accumulator result.

The accumulator starts off with the value given by init.

events

r.on('result', function (res) {})

Whenever a new result value is obtained from the reduce(), this event fires with the new accumulator value.

r.on('pair', function (a, b) {})

Whenever a pair is matched, this event fires with each item in the pair in the order specified to join().

install

With npm do:

npm install hash-join

license

MIT