Use smaller window to calculate fetch interval if there's less total chapters

This is sort of a workaround for sources that tend to only give you the first few and
most recent few chapters, which would have been 28 day intervals before due to
the big gap in the middle.

domain/src/main/java/tachiyomi/domain/manga/interactor/FetchInterval.kt

@@ -46,6 +46,8 @@ class FetchInterval(
     }
 
     internal fun calculateInterval(chapters: List<Chapter>, zone: ZoneId): Int {
+        val chapterWindow = if (chapters.size <= 8) 3 else 10
+
         val uploadDates = chapters.asSequence()
             .filter { it.dateUpload > 0L }
             .sortedByDescending { it.dateUpload }
@@ -55,7 +57,7 @@ class FetchInterval(
                     .atStartOfDay()
             }
             .distinct()
-            .take(10)
+            .take(chapterWindow)
             .toList()
 
         val fetchDates = chapters.asSequence()
@@ -66,7 +68,7 @@ class FetchInterval(
                     .atStartOfDay()
             }
             .distinct()
-            .take(10)
+            .take(chapterWindow)
             .toList()
 
         val interval = when {

domain/src/test/java/tachiyomi/domain/manga/interactor/FetchIntervalTest.kt

@@ -54,6 +54,21 @@ class FetchIntervalTest {
         fetchInterval.calculateInterval(chapters, testZoneId) shouldBe 1
     }
 
+    @Test
+    fun `returns interval based on smaller subset of recent chapters if very few chapters`() {
+        val oldChapters = (1..3).map {
+            chapterWithTime(chapter, (it * 7).days)
+        }
+        // Significant gap between chapters
+        val newChapters = (1..3).map {
+            chapterWithTime(chapter, oldChapters.lastUploadDate() + 365.days + (it * 7).days)
+        }
+
+        val chapters = oldChapters + newChapters
+
+        fetchInterval.calculateInterval(chapters, testZoneId) shouldBe 7
+    }
+
     @Test
     fun `returns interval of 7 days when multiple chapters in 1 day`() {
         val chapters = (1..10).map {