New feature: Reorder pages (#20) (#38)

* Page reordering: backend

* Page reordering: user interface

* Improve handling of data inconsistencies

app/build.gradle.kts

@@ -4,6 +4,7 @@ plugins {
     alias(libs.plugins.kotlin.compose)
     alias(libs.plugins.aboutLibraries)
     alias(libs.plugins.protobuf)
+    alias(libs.plugins.kotlin.serialization)
 }
 
 val abiCodes = mapOf(
@@ -126,7 +127,9 @@ dependencies {
     }
     implementation(libs.icons.extended)
     implementation(libs.zoomable)
+    implementation(libs.reorderable)
     implementation(libs.aboutlibraries.compose.m3)
+    implementation(libs.kotlinx.serialization.json)
 
     testImplementation(libs.junit)
     testImplementation(libs.assertj)

app/src/main/java/org/fairscan/app/ImageRepository.kt

@@ -14,6 +14,11 @@
  */
 package org.fairscan.app
 
+import kotlinx.collections.immutable.ImmutableList
+import kotlinx.collections.immutable.toImmutableList
+import kotlinx.serialization.json.Json
+import org.fairscan.app.data.DocumentMetadata
+import org.fairscan.app.data.Page
 import java.io.File
 
 const val SCAN_DIR_NAME = "scanned_pages"
@@ -24,19 +29,52 @@ class ImageRepository(appFilesDir: File, val transformations: ImageTransformatio
         if (!exists()) mkdirs()
     }
 
-    val fileNames = scanDir.listFiles()
+    private val metadataFile = File(scanDir, "document.json")
-        ?.map { f -> f.name }?.toMutableList()
-        ?:mutableListOf()
 
-    fun imageIds(): List<String> {
+    private var fileNames: MutableList<String> =
-        return fileNames.toList()
+        loadFileNames()
+
+    private fun loadFileNames(): MutableList<String> {
+        val filesOnDisk: Set<String> = scanDir.listFiles()
+            ?.filter { it.extension == "jpg" }
+            ?.map { it.name }
+            ?.toSet()
+            ?: emptySet()
+
+        val metadataFiles: List<String>? = loadMetadata()
+            ?.pages
+            ?.map { it.file }
+
+        return when {
+            metadataFiles != null -> metadataFiles
+                .filter { it in filesOnDisk }
+                .toMutableList()
+            else -> filesOnDisk
+                .sorted()
+                .toMutableList()
         }
+    }
+
+    private fun loadMetadata(): DocumentMetadata? =
+        if (metadataFile.exists()) {
+            runCatching {
+                Json.decodeFromString<DocumentMetadata>(metadataFile.readText())
+            }.getOrNull()
+        } else null
+
+    private fun saveMetadata() {
+        val metadata = DocumentMetadata(version = 1, pages = fileNames.map { id -> Page(id) })
+        metadataFile.writeText(Json.encodeToString(metadata))
+    }
+
+    fun imageIds(): ImmutableList<String> = fileNames.toImmutableList()
 
     fun add(bytes: ByteArray) {
         val fileName = "${System.currentTimeMillis()}.jpg"
         val file = File(scanDir, fileName)
         file.writeBytes(bytes)
         fileNames.add(fileName)
+        saveMetadata()
     }
 
     val idRegex = Regex("([0-9]+)(-(90|180|270))?\\.jpg")
@@ -57,6 +95,7 @@ class ImageRepository(appFilesDir: File, val transformations: ImageTransformatio
                 val index = fileNames.indexOf(id)
                 if (index >= 0) {
                     fileNames[index] = rotatedId
+                    saveMetadata()
                 }
                 delete(id)
             }
@@ -64,17 +103,25 @@ class ImageRepository(appFilesDir: File, val transformations: ImageTransformatio
     }
 
     fun getContent(id: String): ByteArray? {
-        if (fileNames.contains(id)) {
         val file = File(scanDir, id)
+        if (file.exists()) {
             return file.readBytes()
         }
         return null
     }
 
+    fun movePage(id: String, newIndex: Int) {
+        if (!fileNames.remove(id)) return
+        val safeIndex = newIndex.coerceIn(0, fileNames.size)
+        fileNames.add(safeIndex, id)
+        saveMetadata()
+    }
+
     fun delete(id: String) {
         val file = File(scanDir, id)
         file.delete()
         fileNames.remove(id)
+        saveMetadata()
     }
 
     fun clear() {
@@ -82,5 +129,6 @@ class ImageRepository(appFilesDir: File, val transformations: ImageTransformatio
         scanDir.listFiles()?.forEach {
             file -> file.delete()
         }
+        saveMetadata()
     }
 }

app/src/main/java/org/fairscan/app/MainActivity.kt

@@ -122,7 +122,8 @@ class MainActivity : ComponentActivity() {
                             initialPage = screen.initialPage,
                             navigation = navigation,
                             onDeleteImage =  { id -> viewModel.deletePage(id) },
-                            onRotateImage = { id, clockwise -> viewModel.rotateImage(id, clockwise) }
+                            onRotateImage = { id, clockwise -> viewModel.rotateImage(id, clockwise) },
+                            onPageReorder = { id, newIndex -> viewModel.movePage(id, newIndex) },
                         )
                     }
                     is Screen.Main.Export -> {

app/src/main/java/org/fairscan/app/MainViewModel.kt

@@ -26,6 +26,7 @@ import androidx.lifecycle.ViewModel
 import androidx.lifecycle.ViewModelProvider
 import androidx.lifecycle.viewModelScope
 import androidx.lifecycle.viewmodel.CreationExtras
+import kotlinx.collections.immutable.persistentListOf
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.Job
 import kotlinx.coroutines.flow.MutableStateFlow
@@ -87,7 +88,7 @@ class MainViewModel(
         }.stateIn(
             scope = viewModelScope,
             started = SharingStarted.Eagerly,
-            initialValue = DocumentUiModel(emptyList(), ::getBitmap)
+            initialValue = DocumentUiModel(persistentListOf(), ::getBitmap)
         )
 
     private val _captureState = MutableStateFlow<CaptureState>(CaptureState.Idle)
@@ -228,6 +229,11 @@ class MainViewModel(
         }
     }
 
+    fun movePage(id: String, newIndex: Int) {
+        imageRepository.movePage(id, newIndex)
+        _pageIds.value = imageRepository.imageIds()
+    }
+
     fun afterCaptureError() {
         _captureState.value = CaptureState.Idle
     }
@@ -238,7 +244,7 @@ class MainViewModel(
     }
 
     fun startNewDocument() {
-        _pageIds.value = listOf()
+        _pageIds.value = persistentListOf()
         viewModelScope.launch {
             imageRepository.clear()
         }

app/src/main/java/org/fairscan/app/data/DocumentMetadata.kt

@@ -0,0 +1,28 @@
+/*
+ * Copyright 2025 Pierre-Yves Nicolas
+ *
+ * This program is free software: you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation, either version 3 of the License, or (at your option)
+ * any later version.
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ * You should have received a copy of the GNU General Public License along with
+ * this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.fairscan.app.data
+
+import kotlinx.serialization.Serializable
+
+@Serializable
+data class DocumentMetadata(
+    val version: Int = 1,
+    val pages: List<Page>
+)
+
+@Serializable
+data class Page(
+    val file: String,
+)

app/src/main/java/org/fairscan/app/view/CameraScreen.kt

@@ -76,6 +76,7 @@ import androidx.compose.ui.tooling.preview.Preview
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.unit.sp
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import kotlinx.collections.immutable.toImmutableList
 import kotlinx.coroutines.delay
 import org.fairscan.app.CameraPermissionState
 import org.fairscan.app.LiveAnalysisState
@@ -157,6 +158,7 @@ fun CameraScreen(
             CommonPageListState(
                 document = document,
                 onPageClick = { index -> viewModel.navigateTo(Screen.Main.Document(index)) },
+                onPageReorder = { id, index -> viewModel.movePage(id, index) },
                 listState = listState,
                 onLastItemPosition = { offset -> thumbnailCoords.value = offset },
             ),
@@ -468,9 +470,12 @@ private fun ScreenPreview(captureState: CaptureState, rotationDegrees: Float = 0
             pageListState =
                 CommonPageListState(
                     document = fakeDocument(
-                        listOf(1, 2, 2, 2).map { "gallica.bnf.fr-bpt6k5530456s-$it.jpg" },
+                        listOf(1, 2)
+                            .map { "gallica.bnf.fr-bpt6k5530456s-$it.jpg" }
+                            .toImmutableList(),
                         LocalContext.current),
                     onPageClick = {},
+                    onPageReorder = { _,_ -> },
                     listState = LazyListState(),
                 ),
             cameraUiState = CameraUiState(pageCount = 4, LiveAnalysisState(), captureState,

app/src/main/java/org/fairscan/app/view/DocumentScreen.kt

@@ -52,6 +52,7 @@ import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.tooling.preview.Preview
 import androidx.compose.ui.unit.dp
+import kotlinx.collections.immutable.toImmutableList
 import net.engawapg.lib.zoomable.ZoomState
 import net.engawapg.lib.zoomable.zoomable
 import org.fairscan.app.Navigation
@@ -66,6 +67,7 @@ fun DocumentScreen(
     navigation: Navigation,
     onDeleteImage: (String) -> Unit,
     onRotateImage: (String, Boolean) -> Unit,
+    onPageReorder: (String, Int) -> Unit,
 ) {
     // TODO Check how often images are loaded
     val showDeletePageDialog = rememberSaveable { mutableStateOf(false) }
@@ -81,7 +83,7 @@ fun DocumentScreen(
 
     val listState = rememberLazyListState()
     LaunchedEffect(currentPageIndex.intValue) {
-        listState.animateScrollToItem(currentPageIndex.intValue)
+        listState.scrollToItem(currentPageIndex.intValue)
     }
 
     MyScaffold(
@@ -89,6 +91,7 @@ fun DocumentScreen(
         pageListState = CommonPageListState(
             document,
             onPageClick = { index -> currentPageIndex.intValue = index },
+            onPageReorder = onPageReorder,
             currentPageIndex = currentPageIndex.intValue,
             listState = listState,
         ),
@@ -226,12 +229,13 @@ fun DocumentScreenPreview() {
     FairScanTheme {
         DocumentScreen(
             fakeDocument(
-                listOf(1, 2, 2, 2).map { "gallica.bnf.fr-bpt6k5530456s-$it.jpg" },
+                listOf(1, 2).map { "gallica.bnf.fr-bpt6k5530456s-$it.jpg" }.toImmutableList(),
                 LocalContext.current),
             initialPage = 1,
             navigation = dummyNavigation(),
             onDeleteImage = { _ -> },
             onRotateImage = { _,_ -> },
+            onPageReorder = { _,_ -> },
         )
     }
 }

app/src/main/java/org/fairscan/app/view/DocumentUiModel.kt

@@ -15,9 +15,10 @@
 package org.fairscan.app.view
 
 import android.graphics.Bitmap
+import kotlinx.collections.immutable.ImmutableList
 
 data class DocumentUiModel(
-    private val pageIds: List<String>,
+    val pageIds: ImmutableList<String>,
     private val imageLoader: (String) -> Bitmap?
 ) {
     fun pageCount(): Int {

app/src/main/java/org/fairscan/app/view/HomeScreen.kt

@@ -54,6 +54,7 @@ import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.tooling.preview.Preview
 import androidx.compose.ui.unit.dp
+import kotlinx.collections.immutable.persistentListOf
 import org.fairscan.app.CameraPermissionState
 import org.fairscan.app.Navigation
 import org.fairscan.app.R
@@ -284,7 +285,7 @@ fun HomeScreenPreviewWithCurrentDocument() {
         HomeScreen(
             cameraPermission = rememberCameraPermissionState(),
             currentDocument = fakeDocument(
-                listOf("gallica.bnf.fr-bpt6k5530456s-1.jpg"),
+                persistentListOf("gallica.bnf.fr-bpt6k5530456s-1.jpg"),
                 LocalContext.current),
             navigation = dummyNavigation(),
             onClearScan = {},

app/src/main/java/org/fairscan/app/view/PageList.kt

@@ -14,6 +14,7 @@
  */
 package org.fairscan.app.view
 
+import android.annotation.SuppressLint
 import android.content.res.Configuration
 import android.graphics.Bitmap
 import androidx.compose.foundation.Image
@@ -31,6 +32,7 @@ import androidx.compose.foundation.lazy.LazyColumn
 import androidx.compose.foundation.lazy.LazyListScope
 import androidx.compose.foundation.lazy.LazyListState
 import androidx.compose.foundation.lazy.LazyRow
+import androidx.compose.foundation.lazy.itemsIndexed
 import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Text
@@ -47,12 +49,15 @@ import androidx.compose.ui.text.style.TextAlign
 import androidx.compose.ui.unit.Density
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.unit.sp
+import sh.calvin.reorderable.ReorderableItem
+import sh.calvin.reorderable.rememberReorderableLazyListState
 
 const val PAGE_LIST_ELEMENT_SIZE_DP = 120
 
 data class CommonPageListState(
     val document: DocumentUiModel,
     val onPageClick: (Int) -> Unit,
+    val onPageReorder: (String, Int) -> Unit,
     val listState: LazyListState,
     val currentPageIndex: Int? = null,
     val onLastItemPosition: ((Offset) -> Unit)? = null,
@@ -64,12 +69,18 @@ fun CommonPageList(
     modifier: Modifier = Modifier,
 ) {
     val isLandscape = LocalConfiguration.current.orientation == Configuration.ORIENTATION_LANDSCAPE
+
+    val reorderableLazyListState = rememberReorderableLazyListState(state.listState) { from, to ->
+        state.onPageReorder(from.key as String, to.index)
+    }
     val content: LazyListScope.() -> Unit = {
-        items(state.document.pageCount()) { index ->
+        itemsIndexed(state.document.pageIds, key = { _, item -> item}) { index, item ->
+            ReorderableItem(reorderableLazyListState, key = item) { _ ->
                 // TODO Use small images rather than big ones
                 val image = state.document.load(index)
                 if (image != null) {
-                PageThumbnail(image, index, state)
+                    PageThumbnail(image, index, state, Modifier.draggableHandle())
+                }
             }
         }
     }
@@ -103,33 +114,34 @@ private fun PageThumbnail(
     image: Bitmap,
     index: Int,
     state: CommonPageListState,
+    @SuppressLint("ModifierParameter") draggableModifier: Modifier,
 ) {
     val bitmap = image.asImageBitmap()
     val isSelected = index == state.currentPageIndex
     val borderColor =
         if (isSelected) MaterialTheme.colorScheme.secondary else Color.Transparent
     val maxImageSize = PAGE_LIST_ELEMENT_SIZE_DP.dp
-    var modifier =
+    var imageModifier =
         if (bitmap.height > bitmap.width)
             Modifier.height(maxImageSize)
         else
             Modifier.width(maxImageSize)
     if (index == state.document.lastIndex()) {
         val density = LocalDensity.current
-        modifier = modifier.addPositionCallback(state.onLastItemPosition, density, 1.0f)
+        imageModifier = imageModifier.addPositionCallback(state.onLastItemPosition, density, 1.0f)
     }
     Box (modifier = Modifier.height(PAGE_LIST_ELEMENT_SIZE_DP.dp)) {
         Image(
             bitmap = bitmap,
             contentDescription = null,
-            modifier = modifier
+            modifier = imageModifier
                 .align(Alignment.Center)
                 .padding(4.dp)
                 .border(2.dp, borderColor)
                 .clickable { state.onPageClick(index) }
         )
         Box(
-            modifier = Modifier
+            modifier = draggableModifier
                 .padding(8.dp)
                 .align(Alignment.BottomCenter)
                 .background(Color.Black.copy(alpha = 0.5f), shape = RoundedCornerShape(4.dp))

app/src/main/java/org/fairscan/app/view/PreviewUtils.kt

@@ -16,6 +16,8 @@ package org.fairscan.app.view
 
 import android.content.Context
 import android.graphics.BitmapFactory
+import kotlinx.collections.immutable.ImmutableList
+import kotlinx.collections.immutable.persistentListOf
 import org.fairscan.app.Navigation
 
 fun dummyNavigation(): Navigation {
@@ -23,10 +25,10 @@ fun dummyNavigation(): Navigation {
 }
 
 fun fakeDocument(): DocumentUiModel {
-    return DocumentUiModel(listOf()) { _ -> null }
+    return DocumentUiModel(persistentListOf()) { _ -> null }
 }
 
-fun fakeDocument(pageIds: List<String>, context: Context): DocumentUiModel {
+fun fakeDocument(pageIds: ImmutableList<String>, context: Context): DocumentUiModel {
     return DocumentUiModel(pageIds) { id ->
         context.assets.open(id).use { input ->
             BitmapFactory.decodeStream(input)

app/src/test/java/org/fairscan/app/ImageRepositoryTest.kt

@@ -61,14 +61,21 @@ class ImageRepositoryTest {
     }
 
     @Test
-    fun `should find existing files at initialization`() {
+    fun `should find existing files at initialization with no json`() {
-        val bytes = byteArrayOf(101, 102, 103)
+        val scanDir = File(getFilesDir(), SCAN_DIR_NAME)
-        val repo1 = repo()
+        scanDir.mkdirs()
-        assertThat(repo1.imageIds()).isEmpty()
+        File(scanDir, "1.jpg").writeBytes(byteArrayOf(101, 102, 103))
-        repo1.add(bytes)
+        assertThat(repo().imageIds()).containsExactly("1.jpg")
-        val repo2 = repo()
+    }
-        assertThat(repo2.imageIds()).hasSize(1)
+
-        assertThat(repo2.getContent(repo2.imageIds()[0])).isEqualTo(bytes)
+    @Test
+    fun `should filter pages in json at initialization`() {
+        val scanDir = File(getFilesDir(), SCAN_DIR_NAME)
+        scanDir.mkdirs()
+        val json = """{"pages":[{"file":"1.jpg"}, {"file":"2.jpg"}]}"""
+        File(scanDir, "document.json").writeText(json)
+        File(scanDir, "2.jpg").writeBytes(byteArrayOf(101, 102, 103))
+        assertThat(repo().imageIds()).containsExactly("2.jpg")
     }
 
     @Test
@@ -117,4 +124,18 @@ class ImageRepositoryTest {
         val id5 = repo.imageIds().last()
         assertThat(id5).isEqualTo("$baseId-270.jpg")
     }
+
+    @Test
+    fun movePage() {
+        val repo = repo()
+        repo.add(byteArrayOf(101))
+        repo.add(byteArrayOf(110))
+        val id0 = repo.imageIds().first()
+        val id1 = repo.imageIds().last()
+        repo.movePage(id1, 0)
+        assertThat(repo.imageIds()).containsExactly(id1, id0)
+
+        val repo2 = repo()
+        assertThat(repo2.imageIds()).containsExactly(id1, id0)
+    }
 }

gradle/libs.versions.toml

@@ -19,6 +19,8 @@ zoomable = "2.8.1"
 aboutLibraries = "12.2.4"
 protobuf = "0.9.5"
 protobufJavaLite = "4.32.0"
+kotlinSerialization = "1.9.0"
+reorderable = "3.0.0"
 
 [libraries]
 androidx-core-ktx = { group = "androidx.core", name = "core-ktx", version.ref = "coreKtx" }
@@ -51,7 +53,9 @@ litert-metadata = { group = "com.google.ai.edge.litert", name = "litert-metadata
 opencv = { group="org.opencv", name="opencv", version.ref = "opencv" }
 pdfbox = { group = "com.tom-roush", name = "pdfbox-android", version.ref = "pdfbox" }
 zoomable = { group = "net.engawapg.lib", name = "zoomable", version.ref = "zoomable" }
+reorderable = { module = "sh.calvin.reorderable:reorderable", version.ref = "reorderable" }
 aboutlibraries-compose-m3 = { module = "com.mikepenz:aboutlibraries-compose-m3", version.ref = "aboutLibraries" }
+kotlinx-serialization-json = { module = "org.jetbrains.kotlinx:kotlinx-serialization-json", version.ref = "kotlinSerialization" }
 
 assertj =  { group="org.assertj", name="assertj-core", version.ref = "assertj" }
 
@@ -59,7 +63,7 @@ assertj =  { group="org.assertj", name="assertj-core", version.ref = "assertj" }
 android-application = { id = "com.android.application", version.ref = "agp" }
 kotlin-android = { id = "org.jetbrains.kotlin.android", version.ref = "kotlin" }
 kotlin-compose = { id = "org.jetbrains.kotlin.plugin.compose", version.ref = "kotlin" }
+kotlin-serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }
 license =  { id = "com.github.hierynomus.license", version.ref = "license" }
 aboutLibraries = { id = "com.mikepenz.aboutlibraries.plugin", version.ref = "aboutLibraries" }
 protobuf = { id = "com.google.protobuf", version.ref = "protobuf" }
-