在整数数组上进行数学运算时如何翻转？

So I am trying to do math on an array of integers while enforcing a maximum integer in each piece of the array. Similar to this:

function add($amount) {
  $result = array_reverse([0, 0, 0, 100, 0]);
  $max = 100;

  for ($i = 0; $i < count($result); ++$i) {
    $int = $result[$i];
    $new = $int + $amount;
    $amount = 0;

    while ($new > $max) {
      $new = $new - $max;
      ++$amount;
    }

    $result[$i] = $new;
  }

  return array_reverse($result);
}

add(1); // [0, 0, 0, 100, 1]
add(100); // [0, 0, 0, 100, 100]
add(101); // [0, 0, 1, 0, 100]

So what I have above works but it is slow when adding larger integers. I've tried to do this with bitwise shifts and gotten close but I just can't get it to work for some reason. I think I need a third-party perspective. Does anyone have some tips?

Use min($max, $number) to get $number limited to $max.

for ($i = 0; $i < count($result); ++$i) {
    $result[$i] = min($max, $result[$i] + $amount);
}

The part that is taking up the majority of the time is the while loop. You are reducing the value down repeatedly until you have a sub-100 value. However, using PHP to loop down like that takes an incredible amount of time (a 12-digit integer clocked in at over 20 seconds on my local machine). Instead, use multiplication and division (along with an if). It is magnitudes faster. The same 12-digit integer took less than a second to complete with this code:

function add($amount) {
    $result = array_reverse([0, 0, 0, 100, 0]);
    $max = 100;

    for ($i = 0, $size = count($result); $i < $size; ++$i) {
        $int = $result[$i];
        $new = $int + $amount;
        $amount = 0;

        if( $new > $max ) {
            $remainder = $new % $max;
            // Amount is new divided by max (subtract 1 if remainder is 0 [see next if])
            $amount = ((int) ($new / $max));
            // If remainder exists, new is the the number of times max goes into new 
            // minus the value of max. Otherwise it is the remainder
            if( $remainder == 0 ) {
                $amount -= 1;
                $new = $new - ((($new / $max) * $max) - $max);
            } else {
                $new = $remainder;
            }
        }

        $result[$i] = $new;
    }

    return array_reverse($result);
}

Also note that I moved your count($result) call into the variable initialization section of the for loop. When it is inside the expression section it gets executed each time the for loop repeats which can also add to the overall time of executing the function.

Also note that with a large math change like this you may want to assert a range of values you expect to calculate to ensure there are no outliers. I did a small range and they all came out the same but I encourage you to run your own.