Question 348004 in Javascript Question posted in
A very good W3school tutorial can be found here.

Javascript Map() most performant way to create usable keys from [int, int]

Javascript Map()s compare object keys by identity, which means Map<[number, number], any> is a bad idea:

const map = new Map();
const coordinates = [1, 3];

map.set(coordinates, 0);
map.get([1, 3]);
# undefined

To avoid this, we commonly see keys set as `${a}_${b}`. That’s reasonably fast, but it involves number-to-string conversion. If we know the range of A and B, we could do (a << 8) + b which is 2-3x faster in my testing: http://jsben.ch/JRG9z

For integers with unknown domain, is there a similar approach to the bitshift the uniqueness of the resulting key, but doesn’t involve a string cast?

Tags:

2

Answers


  1. 0

    I usually use a nested object for this. Could be replaced with map also (more coding):

    class CoordMap{
  map = {};
  set(coord, val){
    (this.map[coord[0]] ??= {})[coord[1]] = val;
  }
  get(coord){
    return this.map[coord[0]]?.[coord[1]];
  }
}

const map = new CoordMap;
const coordinates = [1, 3];

map.set(coordinates, 0);
console.log(map.get([1, 3]));
    Login or Signup to reply.
  2. 0

    For integers with unknown domain, is there a similar approach to the bitshift the uniqueness of the resulting key, but doesn’t involve a string cast?

    No. Integers with an unknown domain carry at least 53 bits of information, so there’s no way to fit two integers into a single JS number (max 64 bits of information) without collisions.

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search