n-d-map

About

n-d-map provides two very implementations of a Map with multiple dimensions: NDMap and NDEMap. Their interfaces follow closely the standard-Map.

The difference is in methods where you have to provide a key. Instead of one key for a Map, you provide multiple keys (one per dimension) for a NDMap (and NDEMap of course):

// map with one dimension:
const m = new Map();
m.set(key, value);
const value = m.get(key);

// map with two dimensions:
const m = new NDMap(2);
m.set(key1, key2, value);
const value = m.get(key1, key2);

Installation

npm install n-d-map

The code obviously uses Map and generators, so the js-environment needs to support ES6. Thus the minimal supported node.js-version is 6.8.1.

Usage

Both implementations are to be used just like a normal Map, with the follwing differences:

• Constructor: The constructor does not support (yet) an iteratable-argument. The one and only argument is the number of dimensions this instance should have.
• Every function taking a key argument in Map (delete,get,has,set) takes multiple key arguments in a ND(E)Map. The function will check if all keys are present, and throw if this is not the case.

// There are transpiled and minified versions, along with the ES6-sources.

// The transpiled versions are:
const NDMap = require('n-d-map/target/NDMap');
// or
const NDMap = require('n-d-map/target/NDMap.min');
// or via the index
const NDMap = require('n-d-map').NDMap;

// The source files are in lib:
const NDMap = require('n-d-map/lib/NDMap');

// or equally for NDEMap

// create a map with two dimensions
const m = new NDMap(2);

// set a value (e.g. a CMS-page via language and ID)
m.set('en', 'index', page);

// get a value
const page = m.get('en', 'index');

// iterate over all entries
for (let entry of m) { ... }

// or just keys
for (let entry of m.keys()) { ... }

Implementations

Basically ND-maps are trees with a fixed depth. So you could as well use a tree. Just that with a fixed depth, the implementation can be greatly simplified, and that's what n-d-map does.

There are two implementations available: NDMap, a true multidimensional map, and NDEMap, that uses just one flat map, and tampers with the keys to do so.

In most cases you should be fine with either of them. There are, however, reasons to choose one over the other.

You should use NDMap when

• you want to use anything as a key, not just strings
• you can't decide on a separator
• you think you need to mess with the branches (and not just the leaves), like you need to know the number of entries per dimension or so. In this case NDMap._getEntry(keys) will be your friend.

You should use NDEMap when

• order matters.
  Iterators of a Map return elements in the order as they were inserted. This order may be different in a NDMap, due to the simplicity of the implementation. NDMap iterates strictly depth-first, and does keep track of the insertion order across dimensions.
  However, within a dimension the order is preserved.
• you have a huge map, and you frequently need to know the overall size. NDMap determines the size when you ask for it (via Map.size) by adding the sizes of all leaf-maps. Means, it iterates through all branches. This can take some time.

NDMap is implemented as a Map (of Maps (of Maps (...))). This means, everything that is valid about keys in a Map is as well valid for an NDMap (types, equality). But this goes for the price of messing up the order.

NDEMap is implemented by simply mapping arrays of keys to strings:

// separator
const sep = '/';
const keys = ['a','b','c'];

// keys to key
const key = keys.join(sep);
// key is "a/b/c"

This preserves the order of insertion, but your keys have to be strings without the seprator char.

So the bottom line is: In the most common usecase (string keys) you would most likely use NDEMap, since it preserves the insertion order, is quick with Map.size and is implemented in a much simpler way.

For the full documentation checkout the docs.