find_largest_product.php

<?php

// For the following sequence, the correct answer is set '79778' produces product '24696'

$numbers = <<<EOD
7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450
EOD;

$nonzero = preg_split('/[^0]{0,5}0[^0]{0,5}/', trim($numbers), -1, PREG_SPLIT_NO_EMPTY);
$product = 0;
$set = null;
foreach($nonzero as $sequence) {
  while(strlen($sequence)>4) {
    $cur_set = substr($sequence, 0, 5);
    $cur_product = array_reduce(preg_split('//', $cur_set, -1, PREG_SPLIT_NO_EMPTY), 'rprod', 1);
    if ($product < $cur_product) {
      $set = $cur_set;
      $product = $cur_product;
    }
    print $cur_set.': '.$cur_product."\n";
    $sequence = substr($sequence, 1);
  }
}

if (null === $set) {
  $product = 0;
  $set = 'all sets produce 0';
}

print 'Greatest product: '.$product."\n";
print 'Greatest set: '.$set."\n";

function rprod($v, $w) {
  $v *= $w;
  return $v;
}