New feature: allow user to rotate a page

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

@@ -18,7 +18,7 @@ import java.io.File
 
 const val SCAN_DIR_NAME = "scanned_pages"
 
-class ImageRepository(appFilesDir: File) {
+class ImageRepository(appFilesDir: File, val transformations: ImageTransformations) {
 
     private val scanDir: File = File(appFilesDir, SCAN_DIR_NAME).apply {
         if (!exists()) mkdirs()
@@ -39,6 +39,30 @@ class ImageRepository(appFilesDir: File) {
         fileNames.add(fileName)
     }
 
+    val idRegex = Regex("([0-9]+)(-(90|180|270))?\\.jpg")
+
+    fun rotate(id: String, clockwise: Boolean) {
+        val originalFile = File(scanDir, id)
+        if (!originalFile.exists()) {
+            return
+        }
+        idRegex.matchEntire(id)?.let {
+            val baseId = it.groupValues[1]
+            val degrees = it.groupValues[3].ifEmpty { "0" }.toInt()
+            val targetDegrees = (degrees + (if (clockwise) 90 else 270)) % 360
+            val rotatedId = if (targetDegrees == 0) "$baseId.jpg" else "$baseId-$targetDegrees.jpg"
+            val rotatedFile = File(scanDir, rotatedId)
+            transformations.rotate(originalFile, rotatedFile, clockwise)
+            if (rotatedFile.exists()) {
+                val index = fileNames.indexOf(id)
+                if (index >= 0) {
+                    fileNames[index] = rotatedId
+                }
+                delete(id)
+            }
+        }
+    }
+
     fun getContent(id: String): ByteArray? {
         if (fileNames.contains(id)) {
             val file = File(scanDir, id)

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

@@ -0,0 +1,23 @@
+/*
+ * 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
+
+import java.io.File
+
+fun interface ImageTransformations {
+
+    fun rotate(inputFile: File, outputFile: File, clockwise: Boolean)
+
+}

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

@@ -110,7 +110,8 @@ class MainActivity : ComponentActivity() {
                             onStartNew = {
                                 viewModel.startNewDocument()
                                 viewModel.navigateTo(Screen.Main.Home) },
-                            onDeleteImage =  { id -> viewModel.deletePage(id) }
+                            onDeleteImage =  { id -> viewModel.deletePage(id) },
+                            onRotateImage = { id, clockwise -> viewModel.rotateImage(id, clockwise) }
                         )
                     }
                     is Screen.Overlay.About -> {

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

@@ -58,7 +58,7 @@ class MainViewModel(
             override fun <T : ViewModel> create(modelClass: Class<T>, extras: CreationExtras): T {
                 return MainViewModel(
                     ImageSegmentationService(context),
-                    ImageRepository(context.filesDir),
+                    ImageRepository(context.filesDir, OpenCvTransformations()),
                     PdfFileManager(
                         File(context.cacheDir, "pdfs"),
                         Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS),
@@ -221,6 +221,13 @@ class MainViewModel(
         _captureState.value = CaptureState.Idle
     }
 
+    fun rotateImage(id: String, clockwise: Boolean) {
+        viewModelScope.launch {
+            imageRepository.rotate(id, clockwise)
+            _pageIds.value = imageRepository.imageIds()
+        }
+    }
+
     fun afterCaptureError() {
         _captureState.value = CaptureState.Idle
     }

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

@@ -0,0 +1,40 @@
+/*
+ * 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
+
+import org.opencv.core.Core
+import org.opencv.core.Mat
+import org.opencv.imgcodecs.Imgcodecs
+import java.io.File
+
+class OpenCvTransformations : ImageTransformations {
+    override fun rotate(inputFile: File, outputFile: File, clockwise: Boolean) {
+        val src: Mat = Imgcodecs.imread(inputFile.absolutePath)
+
+        require (!src.empty()) { "Could not load image from ${inputFile.absolutePath}" }
+
+        val dst = Mat()
+        Core.rotate(src, dst,
+            if (clockwise) Core.ROTATE_90_CLOCKWISE else Core.ROTATE_90_COUNTERCLOCKWISE
+        )
+
+        if (!Imgcodecs.imwrite(outputFile.absolutePath, dst)) {
+            throw RuntimeException("Could not write image to ${outputFile.absolutePath}")
+        }
+
+        src.release()
+        dst.release()
+    }
+}

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

@@ -29,6 +29,8 @@ import androidx.compose.foundation.layout.width
 import androidx.compose.foundation.lazy.rememberLazyListState
 import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.automirrored.filled.RotateLeft
+import androidx.compose.material.icons.automirrored.filled.RotateRight
 import androidx.compose.material.icons.filled.Add
 import androidx.compose.material.icons.filled.Close
 import androidx.compose.material.icons.filled.PictureAsPdf
@@ -72,6 +74,7 @@ fun DocumentScreen(
     pdfActions: PdfGenerationActions,
     onStartNew: () -> Unit,
     onDeleteImage: (String) -> Unit,
+    onRotateImage: (String, Boolean) -> Unit,
 ) {
     // TODO Check how often images are loaded
     val showNewDocDialog = rememberSaveable { mutableStateOf(false) }
@@ -112,7 +115,12 @@ fun DocumentScreen(
             )
         },
     ) { modifier ->
-        DocumentPreview(document, currentPageIndex, { showDeletePageDialog.value = true }, modifier)
+        DocumentPreview(
+            document,
+            currentPageIndex,
+            { showDeletePageDialog.value = true },
+            onRotateImage,
+            modifier)
         if (showNewDocDialog.value) {
             NewDocumentDialog(onConfirm = onStartNew, showNewDocDialog, stringResource(R.string.close_document))
         }
@@ -137,6 +145,7 @@ private fun DocumentPreview(
     document: DocumentUiModel,
     currentPageIndex: MutableIntState,
     onDeleteImage: (String) -> Unit,
+    onRotateImage: (String, Boolean) -> Unit,
     modifier: Modifier,
 ) {
     val imageId = document.pageId(currentPageIndex.intValue)
@@ -170,6 +179,7 @@ private fun DocumentPreview(
                     )
                 }
             }
+            RotationButtons(imageId, onRotateImage, Modifier.align(Alignment.BottomCenter))
             SecondaryActionButton(
                 Icons.Outlined.Delete,
                 contentDescription = stringResource(R.string.delete_page),
@@ -193,6 +203,27 @@ private fun DocumentPreview(
     }
 }
 
+@Composable
+fun RotationButtons(
+    imageId: String,
+    onRotateImage: (String, Boolean) -> Unit,
+    modifier: Modifier = Modifier
+) {
+    Row(modifier = modifier.padding(4.dp)) {
+        SecondaryActionButton(
+            icon = Icons.AutoMirrored.Default.RotateLeft,
+            contentDescription = "Rotate left",
+            onClick = { onRotateImage(imageId, false) }
+        )
+        Spacer(Modifier.width(8.dp))
+        SecondaryActionButton(
+            icon = Icons.AutoMirrored.Default.RotateRight,
+            contentDescription = "Rotate right",
+            onClick = { onRotateImage(imageId, true) }
+        )
+    }
+}
+
 @Composable
 private fun BottomBar(
     showPdfDialog: MutableState<Boolean>,
@@ -265,7 +296,8 @@ fun DocumentScreenPreview() {
                 MutableStateFlow(PdfGenerationUiState()),
                 {}, {}, {}),
             onStartNew = {},
-            onDeleteImage = { _ -> {} }
+            onDeleteImage = { _ -> },
+            onRotateImage = { _,_ -> },
         )
     }
 }

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

@@ -35,7 +35,7 @@ class ImageRepositoryTest {
     }
 
     fun repo(): ImageRepository {
-        return ImageRepository(getFilesDir())
+        return ImageRepository(getFilesDir(), {f1,f2,_->f1.copyTo(f2)})
     }
 
     @Test
@@ -89,4 +89,32 @@ class ImageRepositoryTest {
         val repo2 = repo()
         assertThat(repo2.imageIds()).isEmpty()
     }
+
+    @Test
+    fun rotate() {
+        val repo = repo()
+        repo.add(byteArrayOf(101, 102, 103))
+        val id0 = repo.imageIds().last()
+        val baseId = id0.substring(0, id0.length - 4)
+
+        repo.rotate(id0, true)
+        val id1 = repo.imageIds().last()
+        assertThat(id1).isEqualTo("$baseId-90.jpg")
+
+        repo.rotate(id1, true)
+        val id2 = repo.imageIds().last()
+        assertThat(id2).isEqualTo("$baseId-180.jpg")
+
+        repo.rotate(id2, true)
+        val id3 = repo.imageIds().last()
+        assertThat(id3).isEqualTo("$baseId-270.jpg")
+
+        repo.rotate(id3, true)
+        val id4 = repo.imageIds().last()
+        assertThat(id4).isEqualTo("$baseId.jpg")
+
+        repo.rotate(id4, false)
+        val id5 = repo.imageIds().last()
+        assertThat(id5).isEqualTo("$baseId-270.jpg")
+    }
 }