extract the artist grid scroll estimate and test it

The index to pixel arithmetic was inline and fiddly (it was missing the row gap), so pull it
out as estimateScrollPx and pin the 2-column cases. ROW_GAP is shared with the grid spacing.

app/src/main/kotlin/com/subtracks/ui/library/ArtistDetailScreen.kt

@@ -82,6 +82,21 @@ import org.koin.core.parameter.parametersOf
 
 private val ART_HEIGHT = 420.dp
 private val TITLE_INSET = 16.dp
+private val ROW_GAP = 16.dp
+
+internal fun estimateScrollPx(
+    index: Int,
+    offset: Float,
+    imageHeightPx: Float,
+    albumHeightPx: Float,
+    albumRowGapPx: Float,
+): Float =
+    if (index <= 0) {
+        offset
+    } else {
+        imageHeightPx + albumRowGapPx + (albumHeightPx + albumRowGapPx) * ((index - 1) / 2) + offset
+    }
+
 private val FADE_LEAD = 24.dp
 private const val THEME_TRANSITION_MS = 100
 private const val SCRIM_FADE_MS = 180
@@ -164,16 +179,16 @@ fun ArtistDetailScreen(
     }
     var imageHeightPx by remember { mutableFloatStateOf(0f) }
     var albumHeightPx by remember { mutableFloatStateOf(0f) }
-    val albumRowGapPx = with(density) { 16.dp.toPx() }
+    val albumRowGapPx = with(density) { ROW_GAP.toPx() }
     val scrollPx by remember {
         derivedStateOf {
-            val index = listState.firstVisibleItemIndex
-            val offset = listState.firstVisibleItemScrollOffset.toFloat()
-            if (index <= 0) {
-                offset
-            } else {
-                imageHeightPx + albumRowGapPx + (albumHeightPx + albumRowGapPx) * ((index - 1) / 2) + offset
-            }
+            estimateScrollPx(
+                index = listState.firstVisibleItemIndex,
+                offset = listState.firstVisibleItemScrollOffset.toFloat(),
+                imageHeightPx = imageHeightPx,
+                albumHeightPx = albumHeightPx,
+                albumRowGapPx = albumRowGapPx,
+            )
         }
     }
     val nameTextStyle = MaterialTheme.typography.headlineLarge
@@ -207,7 +222,7 @@ fun ArtistDetailScreen(
                     columns = GridCells.Fixed(2),
                     contentPadding = PaddingValues(bottom = 16.dp + navBarBottom),
                     horizontalArrangement = Arrangement.spacedBy(12.dp),
-                    verticalArrangement = Arrangement.spacedBy(16.dp),
+                    verticalArrangement = Arrangement.spacedBy(ROW_GAP),
                     modifier = Modifier.fillMaxSize(),
                 ) {
                     item(span = { GridItemSpan(maxLineSpan) }) {

app/src/test/kotlin/com/subtracks/ui/library/EstimateScrollPxTest.kt

@@ -0,0 +1,30 @@
+package com.subtracks.ui.library
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class EstimateScrollPxTest {
+    private val image = 432f
+    private val album = 200f
+    private val gap = 16f
+
+    @Test
+    fun theHeroUsesTheRawOffset() {
+        assertEquals(50f, estimateScrollPx(0, 50f, image, album, gap), 0.01f)
+    }
+
+    @Test
+    fun theFirstAlbumRowAddsTheHeroAndOneGap() {
+        assertEquals(image + gap + 10f, estimateScrollPx(1, 10f, image, album, gap), 0.01f)
+    }
+
+    @Test
+    fun theSecondCellOfARowDoesNotAdvance() {
+        assertEquals(estimateScrollPx(1, 10f, image, album, gap), estimateScrollPx(2, 10f, image, album, gap), 0.01f)
+    }
+
+    @Test
+    fun theNextRowAddsACellAndAGap() {
+        assertEquals(image + gap + (album + gap) + 10f, estimateScrollPx(3, 10f, image, album, gap), 0.01f)
+    }
+}