Recursive function in PHP to find path between arbitrary nodes

Question

I have an array representing connections between nodes.

Given a specific end node, I'd like to return a subset of the nodes that lead to this end node.

E.g. If my nodes definitions are "a-b", "a-c","c-e", "e-f", "e-k","k-z","q-z", "x-q" "k-m", "c-t" which represent the following:

and I'd like to see the path(s) leading to 'z', which are:

a-c, c-e, e-k, k-z
q-z, x-q

I have this PHP code:

<?php

$end   = "z";  
$items = array("a-b", "a-c", "c-e", "e-f", "e-k", "k-z", "q-z", "x-q", "k-m", "c-t"); 

$graph = getPre($items, $end);  

print_r($graph);  // This is what I want to populate

exit();

function getPre($array, $char) {  

  $predecessors = array_filter($array, function($line) use($char) {
      return strpos(substr($line, 2, 1), $char) === FALSE ? FALSE : TRUE;
            }
          );
  if (count($predecessors) == 0) {
    return;
  } else {
      print_r($predecessors)."<br>";
    echo "<br>";
    foreach ($predecessors as $pre) {getPre($array, substr($pre, 0,1));}
  }
}

?>

which outputs:

Array ( [5] => k-z [6] => q-z )
Array ( [4] => e-k )
Array ( [2] => c-e )
Array ( [1] => a-c )
Array ( [7] => x-q )

which at least is the correct nodes list, so I'm almost there.

Question: How do I have have the results returned in my $graph array?

Somewhere I need to return $predecessors; and have it merged with the previous results. I will also want to add functionality to be able to specify a start node, but I think that will just be an extra test in my recursive end test.

php is not very recursion friendly, Even if you get the syntax right, you are still likely to hit memory limits very fast — Nathanael, Oct 14 '19 at 16:12
@Nathanael Which is somewhat ironic, gioven what PHP stands for... Hopefully I'll only be descending into single-figure depths. — Steve Ives, Oct 14 '19 at 16:25

Nigel Ren · Accepted Answer · 2019-10-14T20:32:50.620

The main thing you needed (as you said) is to build the arrays to pass the data back. This code will build a sub element if there are more elements to add or just set the return value to the item if nothing is found. Not sure if the format of the output is exactly what you are after, but you may be able to work with it.

function getPre($array, $char) {
    $graph = [];
    foreach ($array as $pre) {
        if ( $pre[2] == $char ) {
            $graph[$pre] = getPre($array, $pre[0]) ?: $pre;
        }
    }

    return $graph;
}

Note than rather than having to use substr() to extract a character, you can use the string as an array and [2] is the 3rd character.

This outputs...

Array
(
    [k-z] => Array
        (
            [e-k] => Array
                (
                    [c-e] => Array
                        (
                            [a-c] => a-c
                        )

                )

        )

    [q-z] => Array
        (
            [x-q] => x-q
        )

)

To output the codes as strings, I've processed the result of the above function using iterators. The idea being to start at the leaf nodes and then to track back (using getSubIterator()) to the base of the tree...

$graph = getPre($items, $end);
$objects = new RecursiveIteratorIterator( new RecursiveArrayIterator( $graph ), RecursiveIteratorIterator::LEAVES_ONLY );
foreach ( $objects as $name => $object )    {
    $path = $object;
    for ( $depth = $objects->getDepth() - 1; $depth >= 0; $depth-- ) {
        $path .= ",".$objects->getSubIterator( $depth )->key();
    }
    echo $path.PHP_EOL;
}

Which outputs...

a-c,c-e,e-k,k-z
x-q,q-z

Thanks Nigel. I can see that all necessary nodes are in there - is there anyway to get all the nodes in a single 1-dimensional array? Or a way to iterate over all the elements simply? — Steve Ives, Oct 14 '19 at 19:43

Recursive function in PHP to find path between arbitrary nodes

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