Working with a forest of binary trees stored in a large XML file (PHP)

Question

I have an array like \'var1\'=>1.05, \'var2\'=>0.76,... and a forest of binary trees stored in a 100+ MB XML file.

Answer 1

You're on the right track with XMLReader. Rather conveniently it includes the method expand() which will return a copy of the current node as a DOMNode. This will let you handle each individual Tree in memory with the DOM API.

As for handling nodes - evaluate and descend recursively.

---

Example:

$data = [
    'var1' => 1.05,
    'var2' => 0.76
];

$dom    = new DOMDocument();
$xpath  = new DOMXPath($dom);
$reader = new XMLReader();
$reader->open('forest.xml');

// Read until reaching the first Tree.
while ($reader->read() && $reader->localName !== 'Tree');

while ($reader->localName === 'Tree') {
    $tree = $dom->importNode($reader->expand(), true);

    echo evaluateTree($data, $tree, $xpath), "\n";

    // Move on to the next.
    $reader->next('Tree');
}

$reader->close();

function evaluateTree(array $data, DOMElement $tree, DOMXPath $xpath)
{
    foreach ($xpath->query('./Node', $tree) as $node) {
        $field    = $xpath->evaluate('string(./SimplePredicate/@field)', $node);
        $operator = $xpath->evaluate('string(./SimplePredicate/@operator)', $node);
        $value    = $xpath->evaluate('string(./SimplePredicate/@value)', $node);

        if (evaluatePredicate($data[$field], $operator, $value)) {
            // Descend recursively.
            return evaluateTree($data, $node, $xpath);
        }
    }

    // Reached the end of the line.
    return $tree->getAttribute('id');
}

function evaluatePredicate($left, $operator, $right)
{
    switch ($operator) {
        case "lessOrEqual":
            return $left <= $right;
        case "greaterThan":
            return $left > $right;
        default:
            return false;
    }
}

---

Output:

4