5kyu_DirectionsReduction.js

// 5 kyu - Directions Reduction

// Description:

// Once upon a time, on a way through the old wild west,…
// … a man was given directions to go from one point to another. The directions were "NORTH", 
// "SOUTH", "WEST", "EAST". Clearly "NORTH" and "SOUTH" are opposite, "WEST" and "EAST" too. 
// Going to one direction and coming back the opposite direction is a needless effort. Since this 
// is the wild west, with dreadfull weather and not much water, it's important to save yourself 
// some energy, otherwise you might die of thirst!
// How I crossed the desert the smart way.

// The directions given to the man are, for example, the following:

// ["NORTH", "SOUTH", "SOUTH", "EAST", "WEST", "NORTH", "WEST"].

// [North, South, South, East, West, North, West]
// You can immediatly see that going "NORTH" and then "SOUTH" is not reasonable, better stay to 
// the same place! So the task is to give to the man a simplified version of the plan. A better 
// plan in this case is simply:

// ["WEST"]

// Other examples:

// In ["NORTH", "SOUTH", "EAST", "WEST"], the direction "NORTH" + "SOUTH" is going north and 
// coming back right away. What a waste of time! Better to do nothing.

// The path becomes ["EAST", "WEST"], now "EAST" and "WEST" annihilate each other, therefore, 
// the final result is [] (nil in Clojure).

// In ["NORTH", "EAST", "WEST", "SOUTH", "WEST", "WEST"], "NORTH" and "SOUTH" are not directly 
// opposite but they become directly opposite after the reduction of "EAST" and "WEST" so the 
// whole path is reducible to ["WEST", "WEST"].

// Task

// Write a function dirReduc which will take an array of strings and returns an array of strings 
// with the needless directions removed (W<->E or S<->N side by side).

// The Haskell version takes a list of directions with data Direction = North | East | West | 
// South. The Clojure version returns nil when the path is reduced to nothing. The Rust version 
// takes a slice of enum Direction {NORTH, SOUTH, EAST, WEST}.

// Examples

// See more examples in "Example Tests"

// Note

// All paths can't be made simpler. The path ["NORTH", "WEST", "SOUTH", "EAST"] is not reducible. 
// "NORTH" and "WEST", "WEST" and "SOUTH", "SOUTH" and "EAST" are not directly opposite of each 
// other and can't become such. Hence the result path is itself : ["NORTH", "WEST", "SOUTH", 
// "EAST"].

let dirReduc = function(arr) {
  let str = arr.join(' ');
  let re = /NORTH\s+SOUTH|SOUTH\s+NORTH|WEST\s+EAST|EAST\s+WEST/;
  while (re.test(str)){
    str = str.replace(re,'');
  }
  return str.split(" ").filter(a=>!!a);
}