Add tests for MissingChapters function

domain/src/main/java/tachiyomi/domain/chapter/service/MissingChapters.kt

@@ -1,23 +1,21 @@
 package tachiyomi.domain.chapter.service
 
-import kotlin.math.floor
-
-fun countMissingChapters(chaptersInput: List<Float>): Int? {
-    if (chaptersInput.isEmpty()) {
+fun List<Float>.missingChaptersCount(): Int {
+    if (this.isEmpty()) {
         return 0
     }
 
-    val chapters = chaptersInput
-        // Remove any invalid chapters
-        .filter { it != -1f }
+    val chapters = this
+        // Ignore unknown chapter numbers
+        .filterNot { it == -1f }
         // Convert to integers, as we cannot check if 16.5 is missing
-        .map { floor(it.toDouble()).toInt() }
+        .map(Float::toInt)
         // Only keep unique chapters so that -1 or 16 are not counted multiple times
         .distinct()
         .sorted()
 
     if (chapters.isEmpty()) {
-        return null
+        return 0
     }
 
     var missingChaptersCount = 0

domain/src/test/java/tachiyomi/domain/chapter/service/MissingChaptersTest.kt

@@ -0,0 +1,30 @@
+package tachiyomi.domain.chapter.service
+
+import io.kotest.matchers.shouldBe
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.parallel.Execution
+import org.junit.jupiter.api.parallel.ExecutionMode
+
+@Execution(ExecutionMode.CONCURRENT)
+class MissingChaptersTest {
+
+    @Test
+    fun `returns 0 when empty list`() {
+        emptyList<Float>().missingChaptersCount() shouldBe 0
+    }
+
+    @Test
+    fun `returns 0 when all unknown chapter numbers`() {
+        listOf(-1f, -1f, -1f).missingChaptersCount() shouldBe 0
+    }
+
+    @Test
+    fun `handles repeated base chapter numbers`() {
+        listOf(1f, 1.0f, 1.1f, 1.5f, 1.6f, 1.99f).missingChaptersCount() shouldBe 0
+    }
+
+    @Test
+    fun `returns number of missing chapters`() {
+        listOf(-1f, 1f, 2f, 2.2f, 4f, 6f, 10f, 11f).missingChaptersCount() shouldBe 5
+    }
+}