Question

Suppose you have an array “value => timestamp”. The values are increasing with the time but they can be reset at any moment.

For example :

$array = array(
1 => 6000,
2 => 7000,
3 => 8000,
7 => 9000,
8 => 10000,
9 => 11000,
55 => 1000,
56 => 2000,
57 => 3000,
59 => 4000,
60 => 5000,
);

I would like to retrieve all the missing values from this array.

This example would return :

array(4,5,6,58)

I don’t want all the values between 9 and 55 because 9 is newer than the other higher values.

In real condition the script will deal with thousands of values so it need to be efficient.

Thanks for your help!

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UPDATE :
The initial array can be ordered by timestamps if it is easier for the algorithm.

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UPDATE 2 :
In my example the values are UNIX timestamps so they would look more like this : 1285242603 but for readability reason I simplified it.

Answer 1

Here’s another solution:

$prev = null;
$missing = array();
foreach ($array as $curr => $value) {
    if (!is_null($prev)) {
        if ($curr > $prev+1 && $value > $array[$prev]) {
            $missing = array_merge($missing, range($prev+1, $curr-1));
        }
    }
    $prev = $curr;
}