PHP incremntal Price Calculator based upon quantity

Question

I am trying to write a script that calculates a price based upon the quantities.

Database :
pid | prodict | unit | price
1 | coke   |100 | 2
2 | coke   |100 | 1.8
3 | coke   |300 | 1.5
4 | coke   |       | 1.1
1 | pepsi   |100 | 2.2
2 | pepsi   |50 | 2
3 | pepsi   |       | .8

Pid is product id, Product is Product name/Product code. Unit is Incremental unit, or in addition to the existing unit by the number, price is price per unit for incremental unit.

Real Problems:

• if unit is '', or Null, that is that identifies the maximum cumulative unit and then the price in that null column applies to that.
• Not all the products have same level of price. some might end up with 1 value (flat rate, in that case, unit would be null or ''. some might go as up as possible.
• There is no cumulative column and i can not add that. It is not there, and is not possible to add now becuase products might be in millions (asssumption, to make the code flexible for unlimited products)
• All you would get is a lumpsum of units and product code. eg. 100 units of coke, or 200 units of pepsi.
• Number in unit column is Inclusive, that is Less than equal to.

I was a big fan of For loop and break(bad programming), but now i think i need either if condition or while loop, for both i do not have much confidence.

Thank you in advance

NoteIf you feel difficult to understand the problem, then just assume the income tax calculator, same or similar thing - upto x amount, base tax, then for NEXT y amount, y tax rate, for next z amount, z tax rate more than z, z+ tax

Answer 1

Well, you certainly want to loop through the products first, and then work out a total based on the quantity passed in. Something like this?

// Using PostgreSQL as an example here
$entries = pg_fetch_all(pg_query('SELECT * FROM database ORDER BY prodict, pid ASC'));

// Have the quantities ready;
$quantities = array(
    'coke' => 1024,
    'pepsi' => 512,
);

// Prepare an array to hold the total values.
$totals = array();

// Loop through the entries in the array in a non-conventional way
while($entry = current($entries)) {
    // Get the name of the product from the array
    $product = $entry['prodict'];
    // And the quantity
    $quantity = $quantities[$product];
    // Prepare a price for this product starting at zero
    $price = 0;
    // Now use a do-while to figure out the price
    do {
        // At this point '$entry' contains information about the pricing for the first 'x' quanitity
        $quantityAtThisPrice = $entry['unit'];
        // Check the price
        $priceForThisUnit = $entry['price'];
        // Check we have any quantity remaining of this product
        if($quantity > 0) {
            // Check if the quantity at this price is null or if that quantity at this price is more than what we have left
            if($quantityAtThisPrice === null || $quantityAtThisPrice > $quantity) {
                // This means the rest of the quantity is at this price
                $price += ($quantity * $priceForThisUnit);
                // No more quantity left to price
                $quantity = 0;
            }
            // Otherwise we'll add for this unit, and move on to the next
            else {
                // Add to the price
                $price += ($quantityAtThisPrice * $priceForThisUnit);
                // Subtract the quantity we just priced
                $quantity -= $quantityAtThisPrice;
            }
        }
        // Now fetch the next entry
        $entry = next($entries);
    } while ($entry['prodict'] === $product);

    // Add the calculated price to the totals array
    $totals[$product] = $price;
}

var_dump($totals);

Got a bit carried away but I think that should work.