Print the time to wait before each pool has < 1% probability of producing a longer chain, based on current pool hashrates.

gistfile1.php

<?php

// Average block time in minutes.
$avg_block_time = file_get_contents("https://blockchain.info/q/interval")/60;

// Pools page..
$pools_page = file_get_contents("https://blockchain.info/pools?timespan=4days");

// Obtain the JSON string blockchain embeds in their page for the graph
$lines = explode("\n",$pools_page);
foreach($lines as $i => $l) {
	if($i == 16){
		$len = strlen($l);
		$start = 568;
		$from_end = 6;
		$delta = $len-($start+$from_end);
		$str = "{\"data\"".str_replace("&#039;",'"',substr($l, $start, $delta))."}";
		$pool_hashrate = json_decode($str);
	}
}

echo "Average block time: {$avg_block_time} minutes<br /><br />";

foreach($pool_hashrate->data as $pool) {
	$blocks = prob($pool[1]);
	$time = ($avg_block_time)*$blocks;
	echo "{$pool[0]} has {$pool[1]}% of the hashrate: <br />\nWait {$blocks} blocks, or {$time} minutes for 99% certainty a double spend cannot occur<br /><br />";

}

// Taken from bitcoin whitepaper
function AttackerSuccessProbability($ratio, $blocks_deep) {
	$q = $ratio;
	$z = $blocks_deep;
	$p = 1.0 - $q;
	
	$sum = 1.0;
	$lambda = $z * ($q/$p);
	for($k = 0; $k <= $z; $k++) {
		$poisson = exp((0-$lambda));
		for($i = 1; $i <= $k; $i++)
			$poisson *= $lambda / $i;
		$sum -= $poisson * (1 - pow($q/$p, ($z - $k)));
	}
	return number_format($sum,7);
}

function prob($percentage_hash) {
	// Convert % hashrate to ratio.
	$ratio = $percentage_hash/100;
	$i = 0;
	$a = AttackerSuccessProbability($ratio, $i);
	// Loop until probability finally is greater than 0.01
	while($a > 0.01) {
		$i++;
		$a = AttackerSuccessProbability($ratio, $i);
	}
	// Return number of blocks taken to reach this probability, at this hashrate
	return $i;
}