patrun

patrun

A fast pattern matcher on JavaScript object properties.

Need to pick out an object based on a subset of it's properties? Say you've got:

{ x:1,     } -> A
{ x:1, y:1 } -> B
{ x:1, y:2 } -> C

Then patrun can give you the following results:

{ x:1 }      -> A
{ x:2 }      -> no match
{ x:1, y:1 } -> B
{ x:1, y:2 } -> C
{ x:2, y:2 } -> no match
{ y:1 }      -> no match

It's basically query-by-example for property sets.

This module is used by the Seneca framework to pattern match actions.

Support

If you're using this library, feel free to contact me on twitter if you have any questions! :) @rjrodger

This module works on both Node.js and browsers.

Current Version: 0.1.0

Tested on: Node.js 0.10.19, Chrome 29

Quick example

Here's how you register some patterns, and then search for matches:

var patrun = require('patrun')

var pm = patrun()
      .add({a:1},'A')
      .add({b:2},'B')

// prints A
console.log( pm.find({a:1}) ) 

// prints null
console.log( pm.find({a:2}) ) 

// prints A, b:1 is ignored, it was never registered
console.log( pm.find({a:1,b:1}) ) 

// prints B, c:3 is ignored, it was never registered
console.log( pm.find({b:2,c:3}) ) 

You're matching a subset, so your input can contain any number of other properties.

Install

For Node.js:

npm install jsonic

For Bower:

bower install patrun

The Why

This module let's you build a simple decision tree so you can avoid writing if statements. It tries to make the minimum number of comparisons necessary to pick out the most specific match.

This is very useful for handling situations where you have lots of "cases", some of which have "sub-cases", and even "sub-sub-sub-cases".

For example, here are some sales tax rules:

• default: no sales tax
• here's a list of countries with known rates: Ireland: 23%, UK: 20%, Germany: 19%, ...
• but wait, that's only standard rates, here's the other rates
• Oh, and we also have the USA, where we need to worry about each state...

Do this:

// queries return a function, in case there is some
// really custom logic (and there is, see US, NY below)
// in the normal case, just pass the rate back out with
// an identity function
// also record the rate for custom printing later
function I(val) { var rate = function(){return val}; rate.val=val; return rate }


var salestax = patrun()
salestax
  .add({}, I(0.0) )

  .add({ country:'IE' }, I(0.25) )
  .add({ country:'UK' }, I(0.20) )
  .add({ country:'DE' }, I(0.19) )

  .add({ country:'IE', type:'reduced' }, I(0.135) )
  .add({ country:'IE', type:'food' },    I(0.048) )

  .add({ country:'UK', type:'food' },    I(0.0) )

  .add({ country:'DE', type:'reduced' }, I(0.07) )

  .add({ country:'US' }, I(0.0) ) // no federeal rate (yet!)

  .add({ country:'US', state:'AL' }, I(0.04) ) 
  .add({ country:'US', state:'AL', city:'Montgomery' }, I(0.10) ) 

  .add({ country:'US', state:'NY' }, I(0.07) ) 
  .add({ country:'US', state:'NY', type:'reduced' }, function under110(net){
    return net < 110 ? 0.0 : salestax.find( {country:'US', state:'NY'} )
  }) 


console.log('Default rate: ' + 
            salestax.find({})(99) )

console.log('Standard rate in Ireland on E99: ' + 
            salestax.find({country:'IE'})(99) )

console.log('Food rate in Ireland on E99:     ' + 
            salestax.find({country:'IE',type:'food'})(99) )

console.log('Reduced rate in Germany on E99:  ' + 
            salestax.find({country:'IE',type:'reduced'})(99) )

console.log('Standard rate in Alabama on $99: ' + 
            salestax.find({country:'US',state:'AL'})(99) )

console.log('Standard rate in Montgomery, Alabama on $99: ' + 
            salestax.find({country:'US',state:'AL',city:'Montgomery'})(99) )

console.log('Reduced rate in New York for clothes on $99: ' + 
            salestax.find({country:'US',state:'NY',type:'reduced'})(99) )


// prints:
// Default rate: 0
// Standard rate in Ireland on E99: 0.25
// Food rate in Ireland on E99:     0.048
// Reduced rate in Germany on E99:  0.135
// Standard rate in Alabama on $99: 0.04
// Standard rate in Montgomery, Alabama on $99: 0.1
// Reduced rate in New York for clothes on $99: 0

You can take a look a the decision tree at any time:

// print out decision tree, using a custom format function
console.log(salestax.toString( function(f){return f.name+':'+f.val} ))


// prints:
:0
city:
 Montgomery ->
  country:
   US ->
    state:
     AL ->      :0.1
 * ->
  country:
   IE ->    :0.25
    type:
     reduced ->      :0.135
     food ->      :0.048
   UK ->    :0.2
    type:
     food ->      :0
   DE ->    :0.19
    type:
     reduced ->      :0.07
   US ->    :0
    state:
     AL ->      :0.04
     NY ->      :0.07
      type:
       reduced ->        under110:undefined

The Rules

• 1: More specific mathches beat less specific matches. That is, more property values beat fewer.
• 2: Property names are checked in alphabetical order.

And that's it.

Development

From the Irish patrún: pattern. Pronounced pah-troon.

sudo npm install phantomjs@1.9.1-0 uglify-js -g