Add unit tests for sellIn value

src/test/kotlin/com/gildedrose/GildedRoseTest.kt

@@ -1,19 +1,38 @@
 package com.gildedrose
 
-import org.junit.Assert.*
+import org.assertj.core.api.Java6Assertions.assertThat
 import org.junit.Test
+import java.util.*
 
 class GildedRoseTest {
 
-    @Test fun foo() {
-        val items = arrayOf<Item>(Item("foo", 0, 0))
-        val app = GildedRose(items)
-        app.updateQuality()
-        assertEquals("fixme", app.items[0].name)
+    @Test
+    fun givenRandomItem_afterRandomNumberOfDays_shouldDecreaseSellInByNumberOfDays() {
+        val days = generateRandomNumber()
+        val initialSellIn = generateRandomNumber()
+        val item = Item(generateRandomName(), initialSellIn, 0)
+        val app = GildedRose(arrayOf(item))
 
+        app.advanceTimeBy(days)
+
+        assertThat(item.sellIn).isEqualTo(initialSellIn-days)
     }
 
+    @Test
+    fun givenSulfuras_afterRandomNumberOfDays_shouldHaveSameSellIn() {
+        val days = generateRandomNumber()
+        val initialSellIn = generateRandomNumber()
+        val item = Item("Sulfuras, Hand of Ragnaros", initialSellIn, 0)
+        val app = GildedRose(arrayOf(item))
 
+        app.advanceTimeBy(days)
+
+        assertThat(item.sellIn).isEqualTo(initialSellIn)
+    }
+
+    private fun GildedRose.advanceTimeBy(days: Int) = repeat(days, { this.updateQuality() })
+
+    private fun generateRandomName() = UUID.randomUUID().toString()
+
+    private fun generateRandomNumber() = (0 until 9999).shuffled().last()
 }
-
-