Fix amount off discount calculation when not perfectly divisible (#168)

Author: duncanmcclean

src/Discounts/Types/AmountOff.php

@@ -13,9 +13,8 @@ public function calculate(Cart $cart, LineItem $lineItem): int
     {
         $discountValue = (int) $this->discount->get('amount_off');
 
-        $eligibleSubtotal = $cart->lineItems()
-            ->filter(fn (LineItem $line) => $this->isValidForLineItem($cart, $line))
-            ->sum(fn (LineItem $line) => $line->total());
+        $eligibleLineItems = $cart->lineItems()->filter(fn (LineItem $line) => $this->isValidForLineItem($cart, $line));
+        $eligibleSubtotal = $eligibleLineItems->sum(fn (LineItem $line) => $line->total());
 
         if ($eligibleSubtotal <= 0) {
             return 0;
@@ -30,6 +29,18 @@ public function calculate(Cart $cart, LineItem $lineItem): int
         $lineTotal = $lineItem->total();
         $proportion = $lineTotal / $eligibleSubtotal;
 
+        // If this is the last eligible line item, give it the remainder to avoid
+        // losing cents due to rounding errors when the discount isn't perfectly divisible.
+        $lastEligibleLineItem = $eligibleLineItems->last();
+
+        if ($lastEligibleLineItem && $lastEligibleLineItem->id() === $lineItem->id()) {
+            $allocatedToOthers = $eligibleLineItems
+                ->filter(fn (LineItem $line) => $line->id() !== $lineItem->id())
+                ->sum(fn (LineItem $line) => (int) floor($discountValue * ($line->total() / $eligibleSubtotal)));
+
+            return $discountValue - $allocatedToOthers;
+        }
+
         return (int) floor($discountValue * $proportion);
     }
 

tests/Discounts/Types/AmountOffTest.php

@@ -64,4 +64,30 @@ public function it_distributes_fixed_amount_proportionally_by_line_item_value()
         $this->assertEquals(75, $amountLine2);
         $this->assertEquals(100, $amountLine1 + $amountLine2);
     }
+
+    /**
+     * @see https://github.com/duncanmcclean/statamic-cargo/issues/164
+     */
+    #[Test]
+    public function it_distributes_fixed_amount_without_rounding_errors_when_not_perfectly_divisible()
+    {
+        // When 1000 (CHF 10) is divided by 3 equal line items, each gets 333.33...
+        // Using floor() would give 333 + 333 + 333 = 999, losing 1 cent
+        $discount = Discount::make()->type('amount_off')->set('amount_off', 1000);
+
+        $cart = Cart::make();
+        $cart->lineItems()->create(['id' => 'line1', 'total' => 3333]);
+        $cart->lineItems()->create(['id' => 'line2', 'total' => 3333]);
+        $cart->lineItems()->create(['id' => 'line3', 'total' => 3334]);
+
+        $discountType = (new AmountOff)->setDiscount($discount);
+
+        $amountLine1 = $discountType->calculate($cart, $cart->lineItems()->find('line1'));
+        $amountLine2 = $discountType->calculate($cart, $cart->lineItems()->find('line2'));
+        $amountLine3 = $discountType->calculate($cart, $cart->lineItems()->find('line3'));
+
+        // The total discount applied should equal the full discount amount (1000)
+        // not 999 due to rounding errors
+        $this->assertEquals(1000, $amountLine1 + $amountLine2 + $amountLine3);
+    }
 }