LevelDOWN's chained batch

[juliangruber]

I added an implementation of LevelDOWN's chained batch with support for encodings and events.

This has a basic test but still needs tests for encodings and events.

Also, I think the Batch class should be moved to lib/batch.js which then requires a refactor of dispatchError since Batch needs that too.

[rvagg]

yeah, move it to a separate file.

The this.ops array is unfortunate and bound to slow performance down, not that I can see a way around that but it sort makes me wonder what the point of doing it the "proper" way in leveldown is/was. Why not just use db.batch(this.ops) at the end?

[rvagg]

perhaps this.ops isn't that bad, it's just an array of existing objects after all, there's no copying going on or anything.

@juliangruber can you think up a quick way to benchmark this to see if we're winning anything here?

[juliangruber]

I'll create a quick bench

[juliangruber]

old batch: 386.69760247486465 ops/s
new batch: 529.6610169491526 ops/s

~36% faster

[rvagg]

#win lets review this and get it in for 0.9 eh?

@maxogden @No9 this is going to mess up AbstractLevelDOWN for you by the way, the batch() tests in there are incomplete because we've been waiting for the new batch style to be exposed up here, but they're in LevelDOWN already. Your code will need to change to handle a chained batch as well as the current array-style of batch.

[rvagg]

merged