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PHP how to recreate the random order of an array after chunk

For an online game I create an initial PHP array with 36 values: A-Z 0-9. A random script remix the array, i.e

[0] R
[1] 4
[2] N
[3] d
...
[35] 8

Once done the script assigns 6 signs to each of the 6 players via a chunk($arr,6).

Player 1 [0] = [R,4,n,d,O,M]
Player 2 [1] = [p,2,s,Z,k,u]
...
Player 6 [5] = [J,r,V,5,z,8]

The question is: once the table of players has been reordered alphabetically, is there an algorithmic way to recreate the initial distribution?

To simplify the question:

Initial array

[0] = abc
[1] = def
[2] = ghi
[3] = jkl
[4] = mno
[5] = pqr

Random order permutation [0,1,2,3,4,5] -> [2,5,0,3,1,4]:

[2] = ghi
[5] = pqr
[0] = abc
[3] = jkl
[1] = def
[4] = mno

Result:

new order     [0]   [1]   [2]   [3]   [4]   [5] 
value         ghi   pqr   abc   jkl   def   mno
former order  [2]   [5]   [0]   [3]   [1]   [4] -> random permutation

To retrieve former order I must find the way to produce the array [2,4,0,3,5,1]:

 new order   [2]   [4]   [0]   [3]   [5]   [1]          
 value       abc   def   ghi   jkl   mno   pqr  -> initial array order

How to obtain this array [2,4,0,3,5,1] from random permutation [2,5,0,3,1,4] ?

Tags:

2

Answers


  1. 0

    Yes, there is a way to do that. Let’s assume that you have an array called $array and you shuffle it inside a function to which (very important!) you pass your array by value rather than by address. Example:

    ?php

$array = ['a','b','c','d','e','f'];

function shuffleMyArray($array) {
    shuffle($array);
    return $array;
}

$shuffled = shuffleMyArray($array);

echo json_encode([
    'original' => $array,
    'shuffled' => $shuffled
]);

    Output example:

    {"original":["a","b","c","d","e","f"],"shuffled":["a","d","c","f","b","e"]}
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  2. 0
    1. Store your former order using array_flip
    2. Shuffle the array (you may use lajos-arpad suggestion to preserve the original array)
    3. Obtain the new order using array_map by mapping shuffled value to its original position
        $array = ['abc','def','ghi','jkl','mno','pqr'];
    $former_order = array_flip($array);
    // ['abc' => 0, 'def' => 1, 'ghi' => 2, 'jkl' => 3, 'mno' => 4, 'pqr' => 5]
    shuffle($array);
    // ['abc','mno','def','ghi','pqr','jkl']
    $new_order = array_map(fn($item) => $former_order[$item], $array);
    // [0,4,1,2,5,3]
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