数组洗牌和撤销

Question

如下代码，

<?php
// sort array in a fixed order with given seed
function do_shuffle( $array, $seed ) {
mt_srand( $seed );
usort( $array, fn() => mt_rand( - 9, 9 ) );
return $array;
}
// the reverse operation of do_shuffle
function undo_shuffle( $array, $seed ) {
return $array; // to implement
}
$origin_array = str_split( 'olive tree' );
$seed = 20220502;
$shuffled_array = do_shuffle( $origin_array, $seed );
var_dump( $shuffled_array );
// undo shuffle
$origin_array = undo_shuffle( $shuffled_array, $seed );
var_dump( $origin_array );

如何实现数组洗牌及其反向操作?

如何返回$shuffled_array $origin_array?

Answer 1

关键思想是重新创建一个进程，但不是打乱数组值，而是打乱数组索引。这给了我们一个洗牌索引和原始索引之间的映射:

// the reverse operation of do_shuffle
function undo_shuffle( $array, $seed ) {
mt_srand( $seed );
$indices_map = range(0, count($array) - 1);
// We do just the same shuffle operation but
// shuffling array indices instead of values
usort( $indices_map, function(){ 
return mt_rand( - 9, 9 );
} );
// Restore original array using the map we've created
$out = [];
foreach( $indices_map as $shuffled_index => $original_index )
$out[ $original_index ] = $array[ $shuffled_index ];
// Sort our output by integer key
ksort($out);
return $out;
}