Walk u a graph, son
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README.markdown

Giraphe

This is an API stub for a generic graph-climbing library in JavaScript, extracted from (and developed for) Paws.js. It's intended to present the the same API for different use-cases, while simultaneously optimizing for each use-case individually. (It does not currently do this.)

The primary purpose of Giraphe's walk() function is, given an arbitrary graph structure expressed in JavaScript objects, to allow you to explore that graph in a static fashion — given a set of callbacks, to walk through the graph, both 1. ‘discovering’ new subgraphs that need to be recursed into, and 2. ‘collecting’ nodes according to some critereon.

Interface

The Walker() constructor

The default export, this must be called with an options argument, and it returns a walk() function as described by those options.

import Walker from 'giraphe'
const walk = new Walker({ options... })

The passed options-object describes the structure of your graph to Giraphe. It must describe the classes or objects participating in your graph, as follows:

  1. It must include exactly one of the keyer: or key: properties:

    • key — the most basic mode of operation, this expresses that nodes beloning to your graph have unique String-ish identifiers, stored on the node-Object at the property with the given name.

      const walk = new Walker({ key: 'id', ... })
          , root = { id: 'foobar' }
      
      walk(root, ...)
    • keyer — for more flexibility, you may instead provide a function to generate a unique key for a given node. The function you provide as the keyer will be invoked with the object to be uniquely identified, and must return a String unique to that Object.

      const identify = node => /* generate key! */
          , walk = new Walker({ keyer: identify, ... })
          , root = new MyNodeType
      
      walk(root, ...)
  2. It must include exactly one of the class: or predicate: properties:

    • class — again, this causes the simplest mode of operation; when passed a JavaScript Function (i.e. a “class”), this will cause the walking-function to use a simple instanceof check to determine whether a touched JavaScript object is a node in your graph or not.

      const walk = new Walker({ key: ..., class: MyNodeType })
          , root = new MyNodeType
      
      walk(root, ...)
    • predicate — when functioning across JavaScript contexts, or otherwise operating on a graph of noes of non-homogenous JavaScript type, you may instead provide a predicate function which will indicate to Giraphe whether a given JavaScript Object is a node in your graph or not.

      const isNode = node => /* verify nodey-ness! */
          , walk = new Walker({ key: ..., predicate: isNode })
          , root = new MyNodeType
      
      walk(root, ...)
  3. If your graph includes annotated edges (that is, seperate objects ‘wrapping’ the nodes, when belonging to data-structures existing on other nodes), the options may also include an edge: property, itself with, again, either a class: or predicate: sub-property, and either a extract_key: or extractor: property:

    • edge.class — as with the root class option above, this should contain the javascript constructor for a ‘class’ used as edges. as with the node-class type, this will cause giraphe to test the return-type against the edge.class with instanceof.

      const walk = new Walker({
         key: ...,
         class: ...,
         edge: { class: MyEdgeType, ... }
      })
      const root = new MyNodeType
      
      walk(root, ...)
    • edge.predicate — again identical to the corresponding option to the root predicate option, above, this should contain a Function to be used as an alternative to the simple instanceof test that edge.class would produce.

      const walk = new Walker({
         key: ...,
         class: ...,
         edge: { predicate: isEdge, ... }
      })
          , root = new MyNodeType
      
      walk(root, ...)
    • edge.extract_path — if expressing edge-typing to Giraphe, then you may specify to Giraphe how to extract the node-object wrapped by a given edge-object, by indicating at which key on the edge-object the actual child-node is stored.

      const walk = new Walker({
         key: ...,
         class: ...,
         edge: { class: ..., extract_path: 'target' }
      })
          , root = new MyNodeType
      
      walk(root, ...)
    • edge.extractor — if extracting the final node from an edge-object is more complicated than simply accessing the value at a particular key, then you may instead pass an extractor- function. It should accept an edge-object, and return a node-object.

      const walk = new Walker({
         key: ...,
         class: ...,
         edge: { class: ..., extract_key: 'target' }
      })
          , root = new MyNodeType
      
      walk(root, ...)

    Of note, if your graph uses annotated edge-objects, those objects are expected to be unique. That is, in edgeless operation, Giraphe only visits a given child once for a particular parent (that is, if multiple supplybacks on a given parent produce a given child, that child will only be visited once; but if a child is supplied via multiple walk-paths, it has multiple opportunities to be accepted); but if navigating via edge-objects, Giraphe, will re-visit any given child once for every edge-object supplied during the parent's walk-step (that is, there may be several edges per parent-child pair.)

The produced walk() function

Once configured, Giraphe will return to you a function-object, which is to be called with a root node and a collection of various sorts of callbacks. The returned walk() function may be invoked either function-style, or method-style:

const walk = new Walker({ ... })
walk(root, funcs...)

MyNodeClass.prototype.walk = walk
root.walk(funcs...)

(If invoked without any root whatsoever, i.e. only with Function objects, the walk() function effectively partially-applies those functions — the walk() function immediately returns a version of itself that prepends those functions to any others passed.)

const walk = new Walker({ ... })
MyNodeClass.prototype.walk = walk(funcs...)
root.walk(more_funcs...)

Otherwise, the first argument to walk() (or alternatively the object upon which it is invoked, if it is invoked method-style) must be an object of the class passed to the Walker constructor (or one satisfying the predicate, if such was passed instead — henceforth, I'll just call such an object “a node.”). That node will be the first walked, that is, the root of the subgraph that walk() will process.

All other arguments must be functions; these behave as callbacks manipulating the behaviour of the recursive walk() process. Such callbacks must behave in one of two ways: as a so-called ‘supplier’, or as a ‘filter.’

All callbacks are invoked with the same arguments (which must not be modified during the callback's evaluation):

  • Either the current_node being visited, or if annotated, the current_edge whence it's reached,
  • the parent node that was being visited when a supplier-callback yielded the current node,
  • and the complete list of callbacks with which the current walk() is operating.

Each callback is invoked with the current node being visited as this. (If the graph being walked has annotated edge-objects, this may differ from the value of the first argument!)

When all discovered nodes in the graph have been exhausted, walk() produces a final object-mapping of nodes; the properties of which are the unique-keys of all collected nodes, each with the corresponding node-object as the value.

‘Supplier’ callbacks

Suppliers are how walk() recurses into your graph-structure. At each node, additional nodes may be ‘discovered’ by your supplier-callback; these'll be added to the set of nodes pending visitation.

Suppliers may indicate further nodes for visitation by,

  • returning a node directly:

    root.walk(function(){ return this.child })
  • returning an Array of nodes:

    root.walk(function(){ return [this.left, this.right] })
  • returning an object-mapping of nodes (such as that returned from another walk() process):

    root.walk(function(){ return this.walk_children() })

If edge-structure was provided to walk()'s constructor, the node may optionally be replaced with an edge, instead, in the first and second styles. (Edges may not be returned in a mapping — key- value maps may only contain the node directly associated with each key.)

(Note that, although returning undefined technically makes a callback a ‘filter’, it counts as a pass; so it's equivalent to a supplier that adds nothing. However, filter-vs-supplier behaviour may change in future releases, especially w.r.t. caching!)

‘Filter’ callbacks

If suppliers are how you find new nodes to visit, filters are how you select which of those nodes contribute to the overall result of the walk(). Where suppliers operate on descendants of the current node being visited, filters operate on that current node itself.

Any callback returning a boolean value is treated as a filter.

The default behaviour of a filter (i.e. if it returns undefined), is to pass the current node — i.e. as a noop, simply let it, and any nodes supplied by other callbacks, through into the final output of walk().

root.walk(function(){ return this.is_blue || this.is_green })

However, if a callback explicitly returns boolean false, then it's considered to reject the current node — despite having been ‘supplied’ by a previous callback, this node will not be included in the results of the walk(); and any newly-supplied nodes from the current walk-step will be discarded as well.

root.walk(function(){ return false if this.age >= 12 })

This rejection is short-circuiting — the current walk-step is aborted, no further callbacks (of either type) are evaluated, any nodes collected via the rejected current node during this step for future walking will be discarded, so on and so forth.

That said, however, rejection is not permanent — rejection by a callback only implies it was not valid for collection as a child of the parent through which it was discovered. The same node can be discovered through multiple parent nodes; and for each such re-discovery, the callbacks will be re-evaluated / the node will be given another chance to be included in the output. Correspondingly, acceptance is permanent — if every single filterback on a given step accepts a node, then that node will be included in the results of walk() (and will not be walked again.)

If the walk() is configured for annotated graphs, then rejection is evaluated per-edge: if multiple edges to the same child were discovered via a given parent, then that child must be separately rejected via each edge on subsequent walk-steps.

Examples

const MyNode = function(){
   this._children = new Array
   this.id = MyNode.max = (MyNode.max || 0) + 1 }

const walk = new Walker({ key: 'id', class: MyNode })

MyNode.prototype.walk = walk
MyNode.prototype.children = function(){ return this._children }
MyNode.prototype.descendants = walk(node => node._children() )

Why?

This exists because I got tired of writing almost the same graph-walking function, over and over, for Paws.js; while each function was slightly different from the previous one, and yet occured in a ‘hot’ enough location to preclude using a truly generic walk() function for each situation.

The intention, unfulfilled, was to pre-‘compile’ optimized graph-walking functions for each situation, based on the options passed to the Walker constructor; this would allow me to centralize testing efforts and interface design (the API) to this single library. My first attempt at that task was laughable (it involved mustache templates. Seriously.); and thus, for now, this library is simply going to be an external API, with a generic (slow) walk() implementation satisfying all of my needs; however, I still hope to eventually extract the necessary optimizations into this library.