Question

I have to calculate a price based on a rate structure along these lines:

$303.00 fixed price up to 500 units
$0.023 additional per unit from 501-10,000 units
$0.022 additional per unit from 10,001-25,000 units
$0.021 additional per unit from 25,001-50,000 units

I’m a little lost on setting up a database structure and algorithm (the larger sticking point) for calculating this. Has anyone done this? Is there a nice, elegant way of calculating this sort of thing?

edit: As an example, a 25,100 unit run would cost $303.00 for the first 500 units, $218.50 for the next 9,500 units, $330.00 for the next 15,000 units, and $2.10 for the next 100 units, for a total of $853.60.

It wouldn’t be a simple 25,100 * $0.021 calculation – I’m well aware of how to select and calculate that.

Similar to the way income tax is assessed – on a marginal basis.

Answer 1

What I wound up doing:

size  units     fixed     per
1       500   303.000   0.000
1     10000     0.000   0.023
1     25000     0.000   0.022
1     50000     0.000   0.021



function calculate_price($size, $quantity) {
  global $db;

  $price = 0;
  $count = 0;

  // fetch rates from the database
  // note: $size is already sanitised by the calling function
  $query = "SELECT units, flat, per FROM rates WHERE size={$size} ORDER BY units ASC";
  $result = $db->query($query);

  // step through the rates
  while($rate = $result->fetch_object()) {
    // figure out how many of our units fall within this tier
    $tier_count = max(0, min($quantity - $count, $rate->units - $count));

    // calculate the price for this tier, including any flat rate
    $tier_price = $rate->flat + ($rate->per * $tier_count);

    // add tier price and count to the totals
    $price += $tier_price;
    $count += $tier_count;

    // store the last, largest number of units rate for any leftovers outside our tiers
    $last_rate = $rate;
  }

  // if some of our units fall outside our defined tiers, use the last tier's values for them
  if($count < $quantity) {
    $tier_count = $quantity - $count;
    $tier_price = $last_rate->flat + ($last_rate->per * $tier_count);
    $price += $tier_price;
    $count += $tier_count;
  }

  return $price;
}