Fix app trying to split long strip when not needed (#3121)

Co-authored-by: AntsyLich <59261191+AntsyLich@users.noreply.github.com>

core/common/src/main/kotlin/tachiyomi/core/common/util/system/ImageUtil.kt

@@ -205,7 +205,11 @@ object ImageUtil {
      */
     private fun isTallImage(imageSource: BufferedSource): Boolean {
         val options = extractImageOptions(imageSource)
-        return (options.outHeight / options.outWidth) > 3
+        return TallImageSplitCalculator.shouldSplit(
+            imageWidth = options.outWidth,
+            imageHeight = options.outHeight,
+            optimalImageHeight = optimalImageHeight,
+        )
     }
 
     /**
@@ -226,7 +230,6 @@ object ImageUtil {
         val options = extractImageOptions(imageSource).apply {
             inJustDecodeBounds = false
         }
-
         val splitDataList = options.splitData
 
         return try {
@@ -273,8 +276,7 @@ object ImageUtil {
             val imageHeight = outHeight
             val imageWidth = outWidth
 
-            // -1 so it doesn't try to split when imageHeight = optimalImageHeight
-            val partCount = (imageHeight - 1) / optimalImageHeight + 1
+            val partCount = TallImageSplitCalculator.calculatePartCount(imageHeight, optimalImageHeight)
             val optimalSplitHeight = imageHeight / partCount
 
             logcat {

core/common/src/main/kotlin/tachiyomi/core/common/util/system/TallImageSplitCalculator.kt

@@ -0,0 +1,17 @@
+package tachiyomi.core.common.util.system
+
+internal object TallImageSplitCalculator {
+
+    fun calculatePartCount(imageHeight: Int, optimalImageHeight: Int): Int {
+        require(imageHeight > 0) { "imageHeight must be positive" }
+        require(optimalImageHeight > 0) { "optimalImageHeight must be positive" }
+        // -1 so it doesn't try to split when imageHeight = optimalImageHeight
+        return (imageHeight - 1) / optimalImageHeight + 1
+    }
+
+    fun shouldSplit(imageWidth: Int, imageHeight: Int, optimalImageHeight: Int): Boolean {
+        require(imageWidth > 0) { "imageWidth must be positive" }
+        return imageHeight > imageWidth * 3 &&
+            calculatePartCount(imageHeight, optimalImageHeight) > 1
+    }
+}

core/common/src/test/java/tachiyomi/core/common/util/system/TallImageSplitCalculatorTest.kt

@@ -0,0 +1,37 @@
+package tachiyomi.core.common.util.system
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertFalse
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+
+class TallImageSplitCalculatorTest {
+
+    @Test
+    fun `does not split when aspect ratio is tall but computed split count is one`() {
+        assertFalse(
+            TallImageSplitCalculator.shouldSplit(
+                imageWidth = 1024,
+                imageHeight = 4385,
+                optimalImageHeight = 4386,
+            ),
+        )
+    }
+
+    @Test
+    fun `splits when aspect ratio is tall and computed split count is greater than one`() {
+        assertTrue(
+            TallImageSplitCalculator.shouldSplit(
+                imageWidth = 1024,
+                imageHeight = 4385,
+                optimalImageHeight = 4384,
+            ),
+        )
+    }
+
+    @Test
+    fun `calculate part count rounds boundary correctly`() {
+        assertEquals(1, TallImageSplitCalculator.calculatePartCount(imageHeight = 4384, optimalImageHeight = 4384))
+        assertEquals(2, TallImageSplitCalculator.calculatePartCount(imageHeight = 4385, optimalImageHeight = 4384))
+    }
+}