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
+}