# Issue

I have a data set like this.

```
name: tea, pricePerUnit: 5.3, quantity: 2, category: drinks
name: coffee, pricePerUnit: 3.5, quantity: 2, category: drinks
name: cheese, pricePerUnit: 8, quantity: 2, category: food
```

Based on category and the priceUnit I have to give some discount like

**If the client buys more than 3 items of the product within the same category, we are giving him 10% discount for all product in this category.**

So, I have implemented this method but It has a bug.

```
public BigDecimal calculateTotalWithDiscountAndCategory(ShoppingCart shoppingCart){
return shoppingCart.getProducts()
.stream()
.collect(Collectors.groupingBy(Product::getCategory))
.entrySet()
.stream()
.map( v -> v.getValue()
.stream()
.map(product ->{
if(product.getQuantity() > 3){
BigDecimal currentTotal = product.getPricePerUnit().multiply(BigDecimal.valueOf(product.getQuantity()));
return currentTotal.multiply(BigDecimal.valueOf(0.9));
}else {
return product.getPricePerUnit().multiply(BigDecimal.valueOf(product.getQuantity()));
}
}
)
.reduce(BigDecimal::add)
.orElse(BigDecimal.ZERO)
).reduce(BigDecimal::add)
.orElse(BigDecimal.ZERO);
}
```

If I executed this, Every time no discount calculating because it is taking care of one object which quantity < 2.

**But correct one would be, As there are 4 drinks discounts should apply to both drinks.**

How can I modify this code?

# Solution

You need to aggregate the quantity before applying the discount. Here's one way to do that:

```
public BigDecimal calculateTotalWithDiscountAndCategory(ShoppingCart shoppingCart){
Map<Category, List<Product>> products = shoppingCart.getProducts()
.stream()
.collect(Collectors.groupingBy(Product::getCategory));
return products.values()
.stream()
.map(this::categoryTotal)
.reduce(BigDecimal.ZERO, BigDecimal::add);
}
private BigDecimal categoryTotal(List<Product> products) {
BigDecimal sum = products.stream()
.map(Product::getPricePerUnit)
.reduce(BigDecimal.ZERO, BigDecimal::add);
int quantity = products.stream()
.mapToInt(Product::getQuantity)
.sum();
if (quantity > 3) {
sum = sum.multiply(new BigDecimal("0.9"));
}
return sum;
}
```

A more efficient implementation might aggregate the quantity and price in the initial pass so you don't have to iterate each category twice, but for that you'll need a mutable container or a clunky merge function. I'll leave that as an exercise to the reader.

Answered By - shmosel Answer Checked By - Senaida (PHPFixing Volunteer)

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