better pagination

composeApp/src/commonMain/kotlin/net/sergeych/toread/ReaderContent.kt

@@ -31,6 +31,7 @@ import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateMapOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.rememberCoroutineScope
@@ -77,6 +78,7 @@ import net.sergeych.toread.text.SoftHyphen
 import kotlinx.coroutines.launch
 import kotlin.math.max
 import kotlin.math.min
+import kotlin.math.roundToInt
 
 @Composable
 internal fun ContinuousBookReader(
@@ -90,6 +92,7 @@ internal fun ContinuousBookReader(
 ) {
     val hyphenation = remember { HyphenationRegistry() }
     val scope = rememberCoroutineScope()
+    val textLineMetricsByItem = remember(contentPlan) { mutableStateMapOf<Int, TextLineMetrics>() }
     val contentPadding = if (isAndroidPlatform()) {
         PaddingValues(start = 0.dp, top = 6.dp, end = 0.dp, bottom = 6.dp)
     } else {
@@ -103,12 +106,12 @@ internal fun ContinuousBookReader(
             .pageTurnOnTouchTap(
                 onPageDown = {
                     scope.launch {
-                        listState.animateScrollBy(listState.pageScrollDistance())
+                        listState.pageScrollByPage(1, textLineMetricsByItem)
                     }
                 },
                 onPageUp = {
                     scope.launch {
-                        listState.animateScrollBy(-listState.pageScrollDistance())
+                        listState.pageScrollByPage(-1, textLineMetricsByItem)
                     }
                 },
             ),
@@ -175,6 +178,7 @@ internal fun ContinuousBookReader(
                     textAlign = TextAlign.Justify,
                     // so we add 6.dp to make it look symmetric
                     modifier = Modifier.padding(start = (element.depth * 8).dp + 6.dp, end = 0.dp),
+                    onTextLayout = { textLineMetricsByItem[itemIndex] = it.toTextLineMetrics() },
                 )
                 is ReaderElement.Subtitle -> ReaderText(
                     text = element.text,
@@ -184,6 +188,7 @@ internal fun ContinuousBookReader(
                     highlightedRange = highlightedRange,
                     textAlign = TextAlign.Center,
                     modifier = Modifier.fillMaxWidth().padding(top = 18.dp, bottom = 8.dp),
+                    onTextLayout = { textLineMetricsByItem[itemIndex] = it.toTextLineMetrics() },
                 )
             }
         }
@@ -195,14 +200,34 @@ private fun Modifier.pageTurnOnTouchTap(
     onPageUp: () -> Unit,
 ): Modifier = pointerInput(onPageDown, onPageUp) {
     awaitEachGesture {
-        val down = awaitFirstDown(requireUnconsumed = false, pass = PointerEventPass.Final)
+        val down = awaitFirstDown(requireUnconsumed = false, pass = PointerEventPass.Initial)
         if (down.type != PointerType.Touch) {
             waitForUpOrCancellation(pass = PointerEventPass.Final)
             return@awaitEachGesture
         }
 
-        val up = waitForUpOrCancellation(pass = PointerEventPass.Final) ?: return@awaitEachGesture
+        val touchSlop = viewConfiguration.touchSlop
-        if (up.isConsumed) return@awaitEachGesture
+        val touchSlopSquared = touchSlop * touchSlop
+        var upPosition: Offset? = null
+        var cancelled = false
+
+        while (upPosition == null && !cancelled) {
+            val event = awaitPointerEvent(pass = PointerEventPass.Initial)
+            val change = event.changes.firstOrNull { it.id == down.id }
+            if (change == null) {
+                cancelled = true
+            } else if (change.pressed) {
+                val dx = change.position.x - down.position.x
+                val dy = change.position.y - down.position.y
+                if (dx * dx + dy * dy > touchSlopSquared) {
+                    cancelled = true
+                }
+            } else {
+                upPosition = change.position
+            }
+        }
+
+        if (cancelled) return@awaitEachGesture
 
         if (down.position.x < size.width / 2f) {
             onPageDown()
@@ -212,12 +237,68 @@ private fun Modifier.pageTurnOnTouchTap(
     }
 }
 
+private suspend fun LazyListState.pageScrollByPage(
+    direction: Int,
+    textLineMetricsByItem: Map<Int, TextLineMetrics>,
+) {
+    val target = pageScrollTarget(direction, textLineMetricsByItem)
+    if (target != null) {
+        animateScrollToItem(target.itemIndex, target.scrollOffset)
+    } else {
+        animateScrollBy(pageScrollDistance() * direction)
+    }
+}
+
 private fun LazyListState.pageScrollDistance(): Float {
     val layoutInfo = layoutInfo
     val viewportHeight = layoutInfo.viewportEndOffset - layoutInfo.viewportStartOffset
     return viewportHeight.toFloat().coerceAtLeast(0f)
 }
 
+private fun LazyListState.pageScrollTarget(
+    direction: Int,
+    textLineMetricsByItem: Map<Int, TextLineMetrics>,
+): PageScrollTarget? {
+    val layoutInfo = layoutInfo
+    val viewportHeight = layoutInfo.viewportEndOffset - layoutInfo.viewportStartOffset
+    val targetViewportOffset = if (direction > 0) {
+        layoutInfo.viewportEndOffset
+    } else {
+        layoutInfo.viewportStartOffset - viewportHeight
+    }
+    val targetItem = layoutInfo.visibleItemsInfo.firstOrNull { item ->
+        targetViewportOffset >= item.offset && targetViewportOffset < item.offset + item.size
+    } ?: return null
+    val metrics = textLineMetricsByItem[targetItem.index] ?: return null
+    val targetItemOffset = targetViewportOffset - targetItem.offset
+    val lineTop = metrics.lineTopContaining(targetItemOffset) ?: return null
+
+    return PageScrollTarget(targetItem.index, lineTop)
+}
+
+private data class PageScrollTarget(
+    val itemIndex: Int,
+    val scrollOffset: Int,
+)
+
+private data class TextLineMetrics(
+    val lineTops: List<Int>,
+    val lineBottoms: List<Int>,
+) {
+    fun lineTopContaining(offsetPx: Int): Int? {
+        val lineIndex = lineTops.indices.firstOrNull { index ->
+            offsetPx >= lineTops[index] && offsetPx < lineBottoms[index]
+        } ?: return null
+        return lineTops[lineIndex]
+    }
+}
+
+private fun TextLayoutResult.toTextLineMetrics(): TextLineMetrics =
+    TextLineMetrics(
+        lineTops = List(lineCount) { line -> getLineTop(line).roundToInt() },
+        lineBottoms = List(lineCount) { line -> getLineBottom(line).roundToInt() },
+    )
+
 @Composable
 private fun DetailsPane(
     book: Fb2Book,
@@ -388,6 +469,7 @@ private fun ReaderText(
     textAlign: TextAlign,
     modifier: Modifier = Modifier,
     highlightedRange: ReaderSentenceRange? = null,
+    onTextLayout: (TextLayoutResult) -> Unit = {},
 ) {
     val highlightColor = MaterialTheme.colorScheme.secondaryContainer
     val annotatedText = text.toAnnotatedString(language, hyphenation, highlightedRange, highlightColor)
@@ -433,7 +515,10 @@ private fun ReaderText(
         style = style,
         textAlign = textAlign,
         modifier = textModifier,
-        onTextLayout = { textLayout = it },
+        onTextLayout = {
+            textLayout = it
+            onTextLayout(it)
+        },
     )
 }