paul

tree traversal and transform functions

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Readme

Paul
Paul is a library of functions for working with tree data structures such as abstract syntax trees, binary trees, and other nested data structures.
npm install paul # hey, that rhymes

Usage

var Paul = require('paul');
var Walker = new Paul(['children']);

var tree = {
	value: 10,
	children: [{
		value: 6
	}, {
		value: 4
	}]
};

var sum = Walker.depthReduce(tree, function(num, node) {
	return num + node.value;
}, 0);

require('assert').equal(20, sum);

Documentation

- [`walker(walkFn) func`](https://github.com/andrejewski/paul#walkerwalkfn-func)
- [`walk(tree, walkFn)`](https://github.com/andrejewski/paul#walktree-walkfn)
- `prototype`
	- [`map(tree, transform) tree`](https://github.com/andrejewski/paul#maptree-transform-tree)
	- [`filter(tree, predicate) tree`](https://github.com/andrejewski/paul#filtertree-predicate-tree)
	- [`where(tree, properties) tree`](https://github.com/andrejewski/paul#wheretree-properties-tree)
	- [`iterator(tree) Iterator`](https://github.com/andrejewski/paul#iteratortree-iterator)
	- [`forEach(tree, iteratee)`](https://github.com/andrejewski/paul#foreachtree-iteratee)
	- [`find(tree, predicate) node`](https://github.com/andrejewski/paul#findtree-predicate-node)
	- [`findWhere(tree, properties) node`](https://github.com/andrejewski/paul#findwheretree-properties-node)
	- [`reduce(tree, iteratee, [memo]) memo`](https://github.com/andrejewski/paul#reducetree-iteratee-memo-memo)
	- [`parent(tree, node) parentNode`](https://github.com/andrejewski/paul#parenttree-node-parentnode)
	- [`siblings(tree, node) {left, right}`](https://github.com/andrejewski/paul#siblingstree-node-left-right)
Note: Methods iterator() through siblings() are not actual methods and instead there are two methods that differ solely on traversal method. For instance, find() must be called as either depthFind() or breadthFind(). An explanation of the difference between these traversals may be helpful.
Assume all code examples are preceded by the following:
var Paul = require('paul');
var assert = require('assert');

===

Paul(walkFn)

An instance of Paul will have all of the prototype functions listed above. See walker(walkFn) for a description of a valid walking function.
Note: the new keyword is optional but recommended.
===

walker(walkFn) func

The function walker takes a walking function and returns a function to be used on a node to return its children.
What makes Paul useful is this function. Supply walker a walking function with the signature function(node, walk), which decides how a node's children are walked or ignored.
Say you have an AST where you could have one or more child nodes on the properties left, right, and/or children for any given node. The appropriate walk function would be:
function walkFn(node, walk) {
	if(node.left) walk('left');
	if(node.right) walk('right');
	if(node.children) walk('children');
}
var walker1 = Paul.walker(walkFn);

Paul walks through the given tree node-by-node with your walk function until no more child nodes are found and the walking ends.
If an AST has a lot of properties that could have child nodes values, walker also accepts an array of property keys.
var walker2 = Paul.walker(['left', 'right', 'children']);
// walker 2 does the same as walker 1.

===

walk(tree, walkFn)

Takes a tree and a function that accepts a node and walk(node) callback.
This function is not a method of an instance of Paul for the simple reason that you walk the nodes yourself.
All this function does is pass the given function walkFn the node to be processed and the function walk and return the value computed by the walkFn.
Confused? An example will help us both honestly.
var tree = {
	op: '+',
	left: {value: 8},
	right: {
		op: '/',
		left: {value: 20},
		right: {value: 4}
	}
};

var math = Paul.walk(tree, function(node, walk) {
	if(node.op) {
		return '(' + walk(node.left) + node.op + walk(node.right) + ')';
	}
	return node.value;
});

assert.equal(math, '(8+(20/4))');

If only the walkFn is provided, the function will return a generic function to call on any tree.
===

map(tree, transform) tree

Produces a new tree of nodes by mapping each node in tree through a transformation function transform. The function is passed one argument node.
var pine = {id: "A", kids: [{id: "B"}, {id: "C"}]};
var paul = new Paul(['kids']);

var palm = paul.map(pine, function(node) {
	node.id += node.id;
	return node;
});

var tree = {id: "AA", kids: [{id: "BB"}, {id: "CC"}]};
assert.deepEqual(palm, tree);

===

filter(tree, predicate) tree

Looks through each node in the tree, returning a tree of all the nodes that pass a truth test predicate. Any nodes that fail the truth test and their sub-nodes will be removed. Sub-nodes will not be tested if the parent node fails.
var pine = {id: "A", kids: [{id: "B"}, {id: "C"}]};
var paul = new Paul(['kids']);

var palm = paul.filter(pine, function(node) {
	return node.id === "A";
});

var tree = {id: "A", kids: []};
assert.deepEqual(palm, tree);

===

where(tree, properties) tree

Looks through each node in the tree, returning a tree of all the nodes that contain all of the key-value pairs listed in properties. Any nodes that fail the truth test and their sub-nodes will be removed. Sub-nodes will not be tested if the parent node fails.
var pine = {id: "A", kids: [{id: "B"}, {id: "C"}]};
var paul = new Paul(['kids']);

var palm = paul.where(pine, {id: "A"});

var tree = {id: "A", kids: []};
assert.deepEqual(palm, tree);

===

iterator(tree) Iterator

Returns an iterator that returns each node in the tree in the specified traversal order.
  • depthIterator(tree) Iterator: depth-first
  • breadthIterator(tree) Iterator: breadth-first

An explanation of the difference between these traversals may be helpful.
Each next() call on the returned iterator returns an object containing a boolean done which is true when there are no more nodes to iterator over and value which is the node. This conforms to the ES6 Iterator protocol.
var tree = {id: "A", kids: [{id: "B"}, {id: "C"}]};
var paul = new Paul(['kids']);

var text = "";
var iter = paul.depthIterator(tree);

while(true) {
	var res = iter.next();
	if(res.done) break;
	text += node.id;
}

assert.equal(text, "ABC");

===

forEach(tree, iteratee)

Iterates over a tree, yielding each node in turn to an iteratee function. Each invocation of iteratee is called with one argument node. Iteration order is decided by which method is used.
  • depthForEach(tree, iteratee): depth-first
  • breadthForEach(tree, iteratee): breadth-first

var tree = {id: "A", kids: [{id: "B"}, {id: "C"}]};
var paul = new Paul(['kids']);

var text = "";
paul.depthForEach(tree, function(node) {
	text += node.id;
});

assert.equal(text, "ABC");

===

find(tree, predicate) node

Looks through each node in the tree, returning the first one that passes a truth test predicate, or undefined if no node passes the test. The function returns as soon as it finds an acceptable node, and doesn't traverse the entire tree.
  • depthFind(tree, predicate): depth-first
  • breadthFind(tree, predicate): breadth-first

var tree = {id: "A", kids: [{id: "B"}, {id: "C"}]};
var paul = new Paul(['kids']);

var node = paul.depthFind(tree, function(node) {
	return node.id === "B";
});

assert.deepEqual(node, {id: "B"});

===

findWhere(tree, properties) node

Looks through each node in the tree, returning the first node that contains all of the key-value pairs listed in properties, or undefined if no node passes the test. The function returns as soon as it finds an acceptable node, and doesn't traverse the entire tree.
  • depthFindWhere(tree, properties): depth-first
  • breadthFindWhere(tree, properties): breadth-first

var tree = {id: "A", kids: [{id: "B"}, {id: "C"}]};
var paul = new Paul(['kids']);

var node = paul.depthFindWhere(tree, {id: "B"}});

assert.deepEqual(node, {id: "B"});

===

reduce(tree, iteratee, memo) memo

Also known as inject and foldl, reduce boils down a tree of nodes into a single value. Memo is the initial state of the reduction, and each successive step of it should be returned by iteratee. The iteratee is passed two arguments: the memo, then the node.
If no memo is passed to the initial invocation of reduce, the iteratee is not invoked on the root node of the tree. The root node is instead passed as the memo in the invocation of the iteratee on the next node in the tree.
  • depthReduce(tree, iteratee, [memo]): depth-first
  • breadthReduce(tree, iteratee, [memo]): breadth-first

var tree = {id: "A", kids: [{id: "B"}, {id: "C"}]};
var paul = new Paul(['kids']);

var text = paul.depthReduce(tree, function(str, node) {
	return str + node.id;
}, "");

assert.equal(text, "ABC");

===

parent(tree, node) parentNode

Returns the parent node of the given node. If the node is not found in the tree or the node has no parent (meaning the node is the tree), undefined is returned.
  • depthParent(tree, node) parentNode: depth-first
  • breadthParent(tree, node) parentNode: breadth-first

var tree = {id: "A", kids: [{id: "B"}, {id: "C"}]};
var paul = new Paul(['kids']);

var nodeB = tree.kids[0];
var parentOfB = paul.depthParent(tree, nodeB);

assert.equal(tree, parentOfB);

===

siblings(tree, node) {left, right}

Returns the sibling nodes of the given node. If the node is not found in the tree or the node has no parent (meaning the node is the tree), undefined is returned. Also if the node is not in an Array member (i.e {child: node}), the node cannot have any siblings and thus undefined is returned.
Siblings of a node are returned in an object containing two properties left and right. Siblings that come before the given node are placed in the left property. Siblings that come after the given node are placed in the right property.
  • depthSiblings(tree, node) {left, right}: depth-first
  • breadthSiblings(tree, node) {left, right}: breadth-first

var tree = {id: "A", kids: [{id: "B"}, {id: "C"}, {id: "D"}]};
var paul = new Paul(['kids']);

var nodeC = tree.kids[1];
var siblings = paul.depthSiblings(tree, nodeC);

assert.deepEqual(siblings, {
	left: [{id: "B"}],
	right: [{id: "D"}]
});

Why "Paul"?

I have a history of naming awesome libraries after awesome people I know with four letter names. I need this library for further work with my HTML parser project Himalaya. Also, have you ever seen The Fast and the Furious?
Yes, this project is dedicated to Paul Walker.

Contributing

We can always have more tests: if you find a bug, create an issue or be fabulous and fix the problem and write the tests up yourself in a coherent pull request. Same goes for documentation: typo fixes and clearer function descriptions are appreciated.
Run tests with the npm test command.
Follow me on Twitter for updates or just for the lolz and please check out my other repositories if I have earned it. I thank you for reading.