walk.js

An extensible tree walk(..) framework...

Theory and operation

This module generalizes structure traverse (walking). This is done via a walk(..) function that recieves a user-defined getter(..) function and returns a walker.

Constructing the walker and walking

walk(getter)(state, ...nodes) -> state
walk(getter, state)(...nodes) -> state
walk(getter, state, ...nodes) -> state

• Recieves a getter function a state and a list of nodes,
• Iterates through nodes calling the getter(..) per node, threading the state through each call,
• Returns the state when there are no more nodes.

The getter

getter(state, node, next, down, stop) -> state

• Recieves state, node and three control functions: next, down and stop,
• Called in a context (this), persistent within one walk(..) call, inherited from walker.prototype and usable to store data between getter(..) calls,
• Can process node and state,
• Can queue nodes for walking via next(...nodes)
• Can walk nodes directly via down(state, ...nodes) -> state
• Can abbort walking and return a state via stop() or stop(state)
• Returns state,

Putting it all together

A trivial flat example...

walk(function(r, n){ return r+n }, 0, ...[1,2,3]) // -> 6

The above is essentially equivalent to...

[1,2,3].reduce(function(r, n){ return r+n }, 0) // -> 6

And for flat lists .reduce(..) and friends are simpler and more logical. walk(..) is designed to simplify more complex cases:

• The input is not flat:

  // sum the items in a *deep* array (depth-first)...
  var sum = walk(function(r, n){
  	return n instanceof Array ?
  		down(r, ...n)
  		: r + n }, 0) 
  
  sum( [1, [2, 3], 4, [[5], 6]] ) // -> 21
  

  For reference here is a recursive .reduce(..) example:

  function sumr(l){
  	return l.reduce(function(r, e){
  		return r + (e instanceof Array ?
  			sumr(e)
  			: e) }, 0) }
  
  sumr( [1, [2, 3], 4, [[5], 6]] ) // -> 21
  

• Need to abort the recursion prematurelly:

  // check if structure contains 0...
  var containsZero = walk(function(r, e, next, down, stop){
  	// NOTE: we'll only count leaf nodes...
  	this.nodes_visited = (this.nodes_visited || 0)
  	return e === 0 ? 
  			// target found...
  			//...abort search, report number of nodes visited...
  			stop(this.nodes_visited+1)
  		: e instanceof Array ?
  			// breadth-first walk...
  			!!next(...e)
  		: (this.nodes_visited++, r) }, false)
  
  containsZero( [1, [2, 0], 4, [[5], 6]] ) // -> 3
  containsZero( [1, [2, 5], 4, [[5], 6]] ) // -> false
  

  See a more usefull search in examples...

Installation and loading

$ npm install --save generic-walk

var walk = require('generic-walk').walk

Note: This module supports both AMD and node's require(..)*

API

walk(getter(..)) -> walker(state, ...nodes)
Construct a reusable walker.

walk(getter(..), state) -> walker(...nodes)
Construct a reusable walker with fixed initial state.

walk(getter(..), state, ...nodes) -> result
Walk the nodes.

The getter

getter(state, node, next(..), down(..), stop(..)) -> state
User provided function, called to process a node.

next(...nodes)
Queue nodes for walking. The queued nodes will get walked after this level of nodes is done (i.e. the getter(..) is called for each node on level).

down(state, ...nodes) -> state
Walk nodes and return state. The nodes will get walked immidiately.

stop()
stop(state)
Stop walking and return state. The passed state is directly returned from the walker.

Note that stop(..) behaves in a similar manner to return, i.e. execution is aborted immidiately.

Examples

Sum all the values of a nested array (breadth-first)...

var sum = walk(function(res, node, next){
	return node instanceof Array ?
		// compensate for that next(..) returns undefined...
		next(...node) 
			|| res
		: res + node }, 0)

sum([1, [2], 3, [[4, 5]]]) // -> 15 ...walks the nodes: 1, 3, 2, 4, 5

Sum all the values of a nested array (depth-first)...

var sumd = walk(function(res, node, next, down, stop){
	return node instanceof Array ?
		down(res, ...node)
		: res + node }, 0)

sumd([1, [2], 3, [[4, 5]]]) // -> 15 ...walks the nodes: 1, 2, 3, 4, 5

To explicitly see the paths the sum/sumd take we need to modify them a little:

var sum = walk(function(res, node, next){
	this.log(node)
	return node instanceof Array ?
		// compensate for that next(..) returns undefined...
		next(...node) 
			|| res
		: res + node }, 0)
var sumd = walk(function(res, node, next, down, stop){
	this.log(node)
	return node instanceof Array ?
		down(res, ...node)
		: res + node }, 0)

// define the path logger...
sum.prototype.log = 
sumd.prototype.log =
function(node){
	this.path = node instanceof Array ?
		this.path
		: (this.path || []).concat([node])
} 
// XXX need a more natural way to catch the end of the walk...
sum.prototype.onWalkEnd = 
sumd.prototype.onWalkEnd = 
function(res){ 
	console.log('-->', this.path)
	return res
}

sum([1, [2], 3, [[4, 5]]]) // -> 15

sumd([1, [2], 3, [[4, 5]]]) // -> 15

FInd first zero in tree and return it's path...

// NOTE: the only reason this is wrapped into a function is that we need
//  	to restrict the number of items (L) this is passed to 1...
var firstZero = function(L){
	return walk(function(res, node, next, down, stop){
		// setup...
		if(this.path == null){
			this.path = []
			node = [null, node]
		}
		var path = this.path
		var k = node[0]
		var v = node[1]
		return v === 0 ?
				// NOTE: we .slice(1) here to remove the initial null
				//		we added in firstZero(..)...
				stop(path.slice(1).concat([k]))
			: v instanceof Object?
				(path.push(k), 
				down(
					res, 
					...Object.entries(v)))
			: res}, false, L) }

firstZero([10, 5, [{x: 1, y: 0}, 4]]) // -> ['2', '0', 'y']