feat(banner): 배너 카운터와 상태 계약을 추가한다

app/src/main/java/kr/co/vividnext/sodalive/v2/widget/banner/BannerCounterFormatter.kt

@@ -0,0 +1,7 @@
+package kr.co.vividnext.sodalive.v2.widget.banner
+
+object BannerCounterFormatter {
+    fun format(currentIndex: Int, count: Int): String {
+        return "%02d / %02d".format(currentIndex + 1, count)
+    }
+}

app/src/main/java/kr/co/vividnext/sodalive/v2/widget/banner/BannerState.kt

@@ -0,0 +1,33 @@
+package kr.co.vividnext.sodalive.v2.widget.banner
+
+data class BannerState(
+    val displayMode: BannerDisplayMode,
+    val currentIndex: Int,
+    private val count: Int
+) {
+    fun nextIndex(): Int = (currentIndex + 1) % count
+
+    fun previousIndex(): Int = (currentIndex - 1 + count) % count
+
+    companion object {
+        fun from(count: Int, currentIndex: Int): BannerState {
+            val displayMode = when (count) {
+                0 -> BannerDisplayMode.Hidden
+                1 -> BannerDisplayMode.Single
+                else -> BannerDisplayMode.Carousel
+            }
+            val correctedIndex = if (currentIndex in 0 until count) currentIndex else 0
+            return BannerState(
+                displayMode = displayMode,
+                currentIndex = correctedIndex,
+                count = count
+            )
+        }
+    }
+}
+
+enum class BannerDisplayMode {
+    Hidden,
+    Single,
+    Carousel
+}

app/src/test/java/kr/co/vividnext/sodalive/v2/widget/banner/BannerCounterFormatterTest.kt

@@ -0,0 +1,28 @@
+package kr.co.vividnext.sodalive.v2.widget.banner
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class BannerCounterFormatterTest {
+
+    @Test
+    fun `20개 배너의 첫 번째 아이템은 1부터 시작하는 카운터로 표시한다`() {
+        val text = BannerCounterFormatter.format(currentIndex = 0, count = 20)
+
+        assertEquals("01 / 20", text)
+    }
+
+    @Test
+    fun `20개 배너의 열 번째 아이템은 1부터 시작하는 카운터로 표시한다`() {
+        val text = BannerCounterFormatter.format(currentIndex = 9, count = 20)
+
+        assertEquals("10 / 20", text)
+    }
+
+    @Test
+    fun `1개 배너도 formatter는 양쪽을 두 자리로 표시한다`() {
+        val text = BannerCounterFormatter.format(currentIndex = 0, count = 1)
+
+        assertEquals("01 / 01", text)
+    }
+}

app/src/test/java/kr/co/vividnext/sodalive/v2/widget/banner/BannerStateTest.kt

@@ -0,0 +1,49 @@
+package kr.co.vividnext.sodalive.v2.widget.banner
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class BannerStateTest {
+
+    @Test
+    fun `배너 개수가 0이면 숨김 모드다`() {
+        val state = BannerState.from(count = 0, currentIndex = 0)
+
+        assertEquals(BannerDisplayMode.Hidden, state.displayMode)
+    }
+
+    @Test
+    fun `배너 개수가 1이면 단일 모드다`() {
+        val state = BannerState.from(count = 1, currentIndex = 0)
+
+        assertEquals(BannerDisplayMode.Single, state.displayMode)
+    }
+
+    @Test
+    fun `배너 개수가 2이면 캐러셀 모드다`() {
+        val state = BannerState.from(count = 2, currentIndex = 0)
+
+        assertEquals(BannerDisplayMode.Carousel, state.displayMode)
+    }
+
+    @Test
+    fun `마지막 배너의 다음 index는 첫 번째로 순환한다`() {
+        val state = BannerState.from(count = 3, currentIndex = 2)
+
+        assertEquals(0, state.nextIndex())
+    }
+
+    @Test
+    fun `첫 번째 배너의 이전 index는 마지막으로 순환한다`() {
+        val state = BannerState.from(count = 3, currentIndex = 0)
+
+        assertEquals(2, state.previousIndex())
+    }
+
+    @Test
+    fun `현재 index가 범위를 벗어나면 첫 번째로 보정한다`() {
+        val state = BannerState.from(count = 3, currentIndex = 3)
+
+        assertEquals(0, state.currentIndex)
+    }
+}