Javascript – Calculating the shortest route between two points

algorithmgraph-algorithmjavascriptmathnode.js

I have been working in the past weeks on a multiplayer HTML5 game, using nodejs and websockets.

I've been stuck in this problem for a little while.
Imagine that I have this tilesheet map implemented with an array (as shown below).

1 or brown tiles – there is an obstacle in the way and the player can not pass through it.

0 or green tiles – are free paths where the player is allowed to move.

Access any tile on the map by calling:

 array[x][y]

I would like to create the fastest algorithm possible to find out the shortest route (if there is one) between two points of the map. How would you approach this problem? I know this is common problem.

Example:

Player at the position (1,7) fires a bullet with some AI that will toward the enemy player at the position (6,0). Bullet has to calculate the shortest route between the 2 players and if there aren´t any it would just explode against a wall.

Question:

How to efficiently find the shortest route between two points?

Best Answer

This is a common graph theory problem algorithm

In graph theory, the shortest path problem is the problem of finding a path between two vertices (or nodes) in a graph such that the sum of the weights of its constituent edges is minimized.

The problem of finding the shortest path between two intersections on a road map (the graph's vertices correspond to intersections and the edges correspond to road segments, each weighted by the length of its road segment) may be modeled by a special case of the shortest path problem in graphs.

For now exists lot of implementations of this algorithm. More simpler in implementation is a Dijkstra's algorithm with worst case performance as O(|E|+|V|log|V|) where

|V| is the number of nodes
|E| is the number of edges

Illustration of algorithm work

Definition of Dijkstra's Shortest Path Algorithm

initial node - the node at which we are starting.
distance of node Y - be the distance from the initial node to Y.

Algorithm will assign some initial distance values and will try to improve them step by step:

Assign to every node a tentative distance value: set it to 0 for our initial node and to ∞ for all other nodes.
Set the initial node as current. Mark all other nodes unvisited. Create a set of all the unvisited nodes called the unvisited set.
For the current node, consider all of its unvisited neighbors and calculate their tentative distances. Compare the newly calculated tentative distance to the current assigned value and assign the smaller one.
When we are done considering all of the neighbors of the current node, mark the current node as visited and remove it from the unvisited set. A visited node will never be checked again.
If the destination node has been marked visited (when planning a route between two specific nodes) or if the smallest tentative distance among the nodes in the unvisited set is ∞ (when planning a complete traversal; occurs when there is no connection between the initial node and remaining unvisited nodes), then stop. The algorithm has finished.
Otherwise, select the unvisited node that is marked with the smallest tentative distance, set it as the new "current node", and go back to step 3.

More implementations of Dijkstra algorithm you can find on github repository mburst/dijkstras-algorithm.

For example here is JavaScript implementation

Scoping rules

The main difference is scoping rules. Variables declared by var keyword are scoped to the immediate function body (hence the function scope) while let variables are scoped to the immediate enclosing block denoted by { } (hence the block scope).

function run() {
  var foo = "Foo";
  let bar = "Bar";

  console.log(foo, bar); // Foo Bar

  {
    var moo = "Mooo"
    let baz = "Bazz";
    console.log(moo, baz); // Mooo Bazz
  }

  console.log(moo); // Mooo
  console.log(baz); // ReferenceError
}

run();

The reason why let keyword was introduced to the language was function scope is confusing and was one of the main sources of bugs in JavaScript.

Take a look at this example from another stackoverflow question:

var funcs = [];
// let's create 3 functions
for (var i = 0; i < 3; i++) {
  // and store them in funcs
  funcs[i] = function() {
    // each should log its value.
    console.log("My value: " + i);
  };
}
for (var j = 0; j < 3; j++) {
  // and now let's run each one to see
  funcs[j]();
}

My value: 3 was output to console each time funcs[j](); was invoked since anonymous functions were bound to the same variable.

People had to create immediately invoked functions to capture correct values from the loops but that was also hairy.

Hoisting

While variables declared with var keyword are hoisted (initialized with undefined before the code is run) which means they are accessible in their enclosing scope even before they are declared:

function run() {
  console.log(foo); // undefined
  var foo = "Foo";
  console.log(foo); // Foo
}

run();

let variables are not initialized until their definition is evaluated. Accessing them before the initialization results in a ReferenceError. The variable is said to be in "temporal dead zone" from the start of the block until the initialization is processed.

function checkHoisting() {
  console.log(foo); // ReferenceError
  let foo = "Foo";
  console.log(foo); // Foo
}

checkHoisting();

Creating global object property

At the top level, let, unlike var, does not create a property on the global object:

var foo = "Foo";  // globally scoped
let bar = "Bar"; // not allowed to be globally scoped

console.log(window.foo); // Foo
console.log(window.bar); // undefined

Redeclaration

In strict mode, var will let you re-declare the same variable in the same scope while let raises a SyntaxError.

'use strict';
var foo = "foo1";
var foo = "foo2"; // No problem, 'foo1' is replaced with 'foo2'.

let bar = "bar1"; 
let bar = "bar2"; // SyntaxError: Identifier 'bar' has already been declared

Best Answer

Illustration of algorithm work

Definition of Dijkstra's Shortest Path Algorithm

Related Solutions

Calculate distance between two latitude-longitude points? (Haversine formula)

Javascript – What’s the difference between using “let” and “var”

Scoping rules

Hoisting

Creating global object property

Redeclaration

Related Topic