diff --git a/src/GildedRose.php b/src/GildedRose.php
index 533411b..96f039c 100644
--- a/src/GildedRose.php
+++ b/src/GildedRose.php
@@ -4,6 +4,19 @@
 
 class GildedRose
 {
+    private const AGED_BRIE = 'Aged Brie';
+    private const BACKSTAGE_PASSES = 'Backstage passes to a TAFKAL80ETC concert';
+    private const SULFURAS = 'Sulfuras, Hand of Ragnaros';
+    
+    private const MAX_QUALITY = 50;
+    private const MIN_QUALITY = 0;
+    
+    private const QUALITY_INCREASE = 1;
+    private const QUALITY_DECREASE = 1;
+    
+    private const BACKSTAGE_FIRST_THRESHOLD = 11;
+    private const BACKSTAGE_SECOND_THRESHOLD = 6;
+    
     /**
      * @var Item[]
      */
@@ -20,52 +33,101 @@ public function __construct(array $items)
     public function updateQuality(): void
     {
         foreach ($this->items as $item) {
-            if ($item->name != 'Aged Brie' && $item->name != 'Backstage passes to a TAFKAL80ETC concert') {
-                if ($item->quality > 0) {
-                    if ($item->name != 'Sulfuras, Hand of Ragnaros') {
-                        $item->quality = $item->quality - 1;
-                    }
-                }
-            } else {
-                if ($item->quality < 50) {
-                    $item->quality = $item->quality + 1;
-                    if ($item->name == 'Backstage passes to a TAFKAL80ETC concert') {
-                        if ($item->sellIn < 11) {
-                            if ($item->quality < 50) {
-                                $item->quality = $item->quality + 1;
-                            }
-                        }
-                        if ($item->sellIn < 6) {
-                            if ($item->quality < 50) {
-                                $item->quality = $item->quality + 1;
-                            }
-                        }
-                    }
-                }
-            }
-
-            if ($item->name != 'Sulfuras, Hand of Ragnaros') {
-                $item->sellIn = $item->sellIn - 1;
-            }
-
-            if ($item->sellIn < 0) {
-                if ($item->name != 'Aged Brie') {
-                    if ($item->name != 'Backstage passes to a TAFKAL80ETC concert') {
-                        if ($item->quality > 0) {
-                            if ($item->name != 'Sulfuras, Hand of Ragnaros') {
-                                $item->quality = $item->quality - 1;
-                            }
-                        }
-                    } else {
-                        $item->quality = $item->quality - $item->quality;
-                    }
-                } else {
-                    if ($item->quality < 50) {
-                        $item->quality = $item->quality + 1;
-                    }
-                }
-            }
+            $this->updateItemQuality($item);
         }
     }
+
+    private function updateItemQuality(Item $item): void
+    {
+        if ($this->isSulfuras($item)) {
+            return;
+        }
+
+        $this->updateQualityBeforeSellIn($item);
+        $this->decreaseSellIn($item);
+        $this->updateQualityAfterSellIn($item);
+    }
+
+    private function updateQualityBeforeSellIn(Item $item): void
+    {
+        if ($this->isAgedBrie($item)) {
+            $this->increaseQuality($item);
+        } elseif ($this->isBackstagePass($item)) {
+            $this->updateBackstagePassQuality($item);
+        } else {
+            $this->decreaseQuality($item);
+        }
+    }
+
+    private function updateQualityAfterSellIn(Item $item): void
+    {
+        if ($item->sellIn >= 0) {
+            return;
+        }
+
+        if ($this->isAgedBrie($item)) {
+            $this->increaseQuality($item);
+            return;
+        }
+
+        if ($this->isBackstagePass($item)) {
+            $this->resetQuality($item);
+            return;
+        }
+
+        $this->decreaseQuality($item);
+    }
+
+    private function updateBackstagePassQuality(Item $item): void
+    {
+        $this->increaseQuality($item);
+
+        if ($item->sellIn < self::BACKSTAGE_FIRST_THRESHOLD) {
+            $this->increaseQuality($item);
+        }
+
+        if ($item->sellIn < self::BACKSTAGE_SECOND_THRESHOLD) {
+            $this->increaseQuality($item);
+        }
+    }
+
+    private function increaseQuality(Item $item): void
+    {
+        if ($item->quality < self::MAX_QUALITY) {
+            $item->quality += self::QUALITY_INCREASE;
+        }
+    }
+
+    private function decreaseQuality(Item $item): void
+    {
+        if ($item->quality > self::MIN_QUALITY) {
+            $item->quality -= self::QUALITY_DECREASE;
+        }
+    }
+
+    private function decreaseSellIn(Item $item): void
+    {
+        $item->sellIn--;
+    }
+
+    private function resetQuality(Item $item): void
+    {
+        $item->quality = self::MIN_QUALITY;
+    }
+
+    private function isSulfuras(Item $item): bool
+    {
+        return $item->name === self::SULFURAS;
+    }
+
+    private function isAgedBrie(Item $item): bool
+    {
+        return $item->name === self::AGED_BRIE;
+    }
+
+    private function isBackstagePass(Item $item): bool
+    {
+        return $item->name === self::BACKSTAGE_PASSES;
+    }
 }
 
diff --git a/tests/GildedRoseTest.php b/tests/GildedRoseTest.php
new file mode 100644
index 0000000..647ac2a
--- /dev/null
+++ b/tests/GildedRoseTest.php
@@ -0,0 +1,278 @@
+<?php
+
+namespace GildedRose\Tests;
+
+use GildedRose\GildedRose;
+use GildedRose\Item;
+use PHPUnit\Framework\TestCase;
+
+class GildedRoseTest extends TestCase
+{
+    // ========================================
+    // NORMAL ITEM TESTS
+    // ========================================
+
+    public function testNormalItemQualityDecreasesBy1BeforeSellDate(): void
+    {
+        $items = [new Item('Normal Item', 10, 20)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(19, $items[0]->quality);
+        $this->assertSame(9, $items[0]->sellIn);
+    }
+
+    public function testNormalItemQualityDecreasesBy2AfterSellDate(): void
+    {
+        $items = [new Item('Normal Item', 0, 20)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(18, $items[0]->quality);
+        $this->assertSame(-1, $items[0]->sellIn);
+    }
+
+    public function testNormalItemQualityNeverNegative(): void
+    {
+        $items = [new Item('Normal Item', 5, 0)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(0, $items[0]->quality);
+        $this->assertSame(4, $items[0]->sellIn);
+    }
+
+    public function testNormalItemQualityDoesNotGoNegativeAfterSellDate(): void
+    {
+        $items = [new Item('Normal Item', -1, 1)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(0, $items[0]->quality);
+        $this->assertSame(-2, $items[0]->sellIn);
+    }
+
+    // ========================================
+    // AGED BRIE TESTS
+    // ========================================
+
+    public function testAgedBrieIncreasesInQuality(): void
+    {
+        $items = [new Item('Aged Brie', 10, 20)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(21, $items[0]->quality);
+        $this->assertSame(9, $items[0]->sellIn);
+    }
+
+    public function testAgedBrieIncreasesBy2AfterSellDate(): void
+    {
+        $items = [new Item('Aged Brie', 0, 20)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(22, $items[0]->quality);
+        $this->assertSame(-1, $items[0]->sellIn);
+    }
+
+    public function testAgedBrieQualityNeverExceeds50(): void
+    {
+        $items = [new Item('Aged Brie', 10, 50)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(50, $items[0]->quality);
+        $this->assertSame(9, $items[0]->sellIn);
+    }
+
+    public function testAgedBrieQualityNeverExceeds50AfterSellDate(): void
+    {
+        $items = [new Item('Aged Brie', -1, 49)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(50, $items[0]->quality);
+        $this->assertSame(-2, $items[0]->sellIn);
+    }
+
+    // ========================================
+    // BACKSTAGE PASSES TESTS
+    // ========================================
+
+    public function testBackstagePassesIncreaseBy1WhenMoreThan10Days(): void
+    {
+        $items = [new Item('Backstage passes to a TAFKAL80ETC concert', 15, 20)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(21, $items[0]->quality);
+        $this->assertSame(14, $items[0]->sellIn);
+    }
+
+    public function testBackstagePassesIncreaseBy2When10DaysOrLess(): void
+    {
+        $items = [new Item('Backstage passes to a TAFKAL80ETC concert', 10, 20)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(22, $items[0]->quality);
+        $this->assertSame(9, $items[0]->sellIn);
+    }
+
+    public function testBackstagePassesIncreaseBy3When5DaysOrLess(): void
+    {
+        $items = [new Item('Backstage passes to a TAFKAL80ETC concert', 5, 20)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(23, $items[0]->quality);
+        $this->assertSame(4, $items[0]->sellIn);
+    }
+
+    public function testBackstagePassesDropTo0AfterConcert(): void
+    {
+        $items = [new Item('Backstage passes to a TAFKAL80ETC concert', 0, 20)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(0, $items[0]->quality);
+        $this->assertSame(-1, $items[0]->sellIn);
+    }
+
+    public function testBackstagePassesQualityNeverExceeds50(): void
+    {
+        $items = [new Item('Backstage passes to a TAFKAL80ETC concert', 5, 49)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(50, $items[0]->quality);
+        $this->assertSame(4, $items[0]->sellIn);
+    }
+
+    // ========================================
+    // SULFURAS TESTS
+    // ========================================
+
+    public function testSulfurasNeverChangesQuality(): void
+    {
+        $items = [new Item('Sulfuras, Hand of Ragnaros', 10, 80)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(80, $items[0]->quality);
+        $this->assertSame(10, $items[0]->sellIn);
+    }
+
+    public function testSulfurasNeverChangesSellIn(): void
+    {
+        $items = [new Item('Sulfuras, Hand of Ragnaros', 0, 80)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(80, $items[0]->quality);
+        $this->assertSame(0, $items[0]->sellIn);
+    }
+
+    public function testSulfurasNeverChangesEvenWithNegativeSellIn(): void
+    {
+        $items = [new Item('Sulfuras, Hand of Ragnaros', -1, 80)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(80, $items[0]->quality);
+        $this->assertSame(-1, $items[0]->sellIn);
+    }
+
+    // ========================================
+    // EDGE CASE TESTS
+    // ========================================
+
+    public function testMultipleItemsUpdateCorrectly(): void
+    {
+        $items = [
+            new Item('Normal Item', 5, 10),
+            new Item('Aged Brie', 5, 10),
+            new Item('Sulfuras, Hand of Ragnaros', 5, 80),
+            new Item('Backstage passes to a TAFKAL80ETC concert', 5, 10),
+        ];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(9, $items[0]->quality);
+        $this->assertSame(11, $items[1]->quality);
+        $this->assertSame(80, $items[2]->quality);
+        $this->assertSame(13, $items[3]->quality);
+    }
+
+    public function testMultipleDaysSimulation(): void
+    {
+        $items = [new Item('Normal Item', 2, 10)];
+        $gildedRose = new GildedRose($items);
+        
+        // Day 1: sellIn=1, quality=9
+        $gildedRose->updateQuality();
+        $this->assertSame(9, $items[0]->quality);
+        $this->assertSame(1, $items[0]->sellIn);
+        
+        // Day 2: sellIn=0, quality=8
+        $gildedRose->updateQuality();
+        $this->assertSame(8, $items[0]->quality);
+        $this->assertSame(0, $items[0]->sellIn);
+        
+        // Day 3: sellIn=-1, quality=6 (degrades by 2 after sell date)
+        $gildedRose->updateQuality();
+        $this->assertSame(6, $items[0]->quality);
+        $this->assertSame(-1, $items[0]->sellIn);
+    }
+
+    public function testBackstagePassesAt6Days(): void
+    {
+        $items = [new Item('Backstage passes to a TAFKAL80ETC concert', 6, 20)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(22, $items[0]->quality);
+        $this->assertSame(5, $items[0]->sellIn);
+    }
+
+    public function testBackstagePassesAt11Days(): void
+    {
+        $items = [new Item('Backstage passes to a TAFKAL80ETC concert', 11, 20)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(21, $items[0]->quality);
+        $this->assertSame(10, $items[0]->sellIn);
+    }
+
+    public function testAgedBrieAt49Quality(): void
+    {
+        $items = [new Item('Aged Brie', 5, 49)];
+        $gildedRose = new GildedRose($items);
+        
+        $gildedRose->updateQuality();
+        
+        $this->assertSame(50, $items[0]->quality);
+        $this->assertSame(4, $items[0]->sellIn);
+    }
+}