Modeling exercise: Quantities

quantities.php

<?php

// provided as method arguments:
$quantityInCurrentDelivery = 10.0;

// product-specific:
$quantityPrecision = 5;

// these quantities are all floats:
$quantityInitiallyOrdered = 12.5;
$quantitySoFarDelivered = 3.5;
$quantityOpen = 9.0;

// when we accept a delivery:
    $quantitySoFarDelivered = round($quantitySoFarDelivered + $quantityInCurrentDelivery, $quantityPrecision);

    $quantityOpen = $quantityInitiallyOrdered - $quantityInCurrentDelivery;
    if ($quantityOpen < 0) {
        // over-delivering is fine, but we'll set quantity open to 0
        $quantityOpen = 0.0;
    }

// when we have to undo a delivery:
    $quantitySoFarDelivered = round($quantitySoFarDelivered - $quantityInCurrentDelivery, $quantityPrecision);
    if ($quantitySoFarDelivered < 0) {
        $quantitySoFarDelivered = 0.0;
    }

    $quantityOpen = round($quantityOpen + $quantityInCurrentDelivery, $quantityPrecision);
    if ($quantityOpen > $quantityInitiallyOrdered) {
        // maybe we have been over-delivered, but we shouldn't expect more than we ordered
        $quantityOpen = $quantityInitiallyOrdered;
    }

Design issues that should be fixed by introducing value objects:

• Dealing with "quantity precision" (i.e. number of decimals taken into account).
• Preventing "quantity so far delivered" to end up being less than 0.
• Preventing "quantity open" to end up being more than "quantity initially ordered".

Question: what is the relation between these different quantities? Which ones do we need to "remember" (i.e. store in a database, etc.), which ones can we derive?

See also: https://github.com/matthiasnoback/modeling-exercises