improvement(set-due-date): simplify for FSRS

When FSRS is enabled, there is no reason
for due date to be inconsistent with interval

So, hide 'set interval to same value' and use `true`
Property name: `updateIntervalToMatchDueDate`

Confirmed by Jarrett and Expertium paraphrased:

> Expertium: when FSRS is enabled, is there any reason for keeping `ivl`
  not consistent with due date? Because I'm pretty sure there isn't
> Jarrett: Yeah, there isn’t.

https://discord.com/channels/368267295601983490/1282005522513530952/1324758732122624020

The current backend implementation is not ideal:
* `1!` => interval is set to days from today, rather than days from last review
* `1` => interval does not match due date

So I suspect this will be improved upstream in the future

Fixes 17707

AnkiDroid/src/main/java/com/ichi2/anki/Reviewer.kt

@@ -684,10 +684,11 @@ open class Reviewer :
         }
     }
 
-    private fun showDueDateDialog() {
-        val dialog = SetDueDateDialog.newInstance(listOf(currentCardId!!))
-        showDialogFragment(dialog)
-    }
+    private fun showDueDateDialog() =
+        launchCatchingTask {
+            val dialog = SetDueDateDialog.newInstance(listOf(currentCardId!!))
+            showDialogFragment(dialog)
+        }
 
     private fun showResetCardDialog() {
         Timber.i("showResetCardDialog() Reset progress button pressed")

AnkiDroid/src/main/java/com/ichi2/anki/scheduling/SetDueDateDialog.kt

@@ -28,6 +28,7 @@ import android.widget.TextView
 import androidx.annotation.CheckResult
 import androidx.core.content.ContextCompat
 import androidx.core.os.bundleOf
+import androidx.core.view.isVisible
 import androidx.core.widget.doOnTextChanged
 import androidx.fragment.app.DialogFragment
 import androidx.fragment.app.Fragment
@@ -48,6 +49,7 @@ import com.ichi2.anki.R
 import com.ichi2.anki.launchCatchingTask
 import com.ichi2.anki.requireAnkiActivity
 import com.ichi2.anki.scheduling.SetDueDateViewModel.Tab
+import com.ichi2.anki.servicelayer.getFSRSStatus
 import com.ichi2.anki.showThemedToast
 import com.ichi2.anki.snackbar.showSnackbar
 import com.ichi2.anki.ui.internationalization.toSentenceCase
@@ -59,6 +61,7 @@ import com.ichi2.utils.create
 import com.ichi2.utils.negativeButton
 import com.ichi2.utils.neutralButton
 import com.ichi2.utils.positiveButton
+import com.ichi2.utils.requireBoolean
 import com.ichi2.utils.title
 import kotlinx.coroutines.launch
 import timber.log.Timber
@@ -84,10 +87,15 @@ class SetDueDateDialog : DialogFragment() {
     // used to determine if a rotation has taken place
     private var initialRotation: Int = 0
 
+    val cardIds: LongArray
+        get() = requireNotNull(requireArguments().getLongArray(ARG_CARD_IDS)) { ARG_CARD_IDS }
+
+    val fsrsEnabled: Boolean
+        get() = requireArguments().requireBoolean(ARG_FSRS)
+
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
-        val cardIds = requireNotNull(requireArguments().getLongArray(ARG_CARD_IDS)) { ARG_CARD_IDS }
-        viewModel.init(cardIds)
+        viewModel.init(cardIds, fsrsEnabled)
         Timber.d("Set due date dialog: %d card(s)", cardIds.size)
         this.initialRotation = getScreenRotation()
     }
@@ -151,9 +159,12 @@ class SetDueDateDialog : DialogFragment() {
                 )
 
                 // setup 'set interval to same value' checkbox
-                findViewById<MaterialCheckBox>(R.id.change_interval)!!.apply {
-                    isChecked = viewModel.updateIntervalToMatchDueDate
-                    setOnCheckedChangeListener { _, isChecked ->
+                findViewById<MaterialCheckBox>(R.id.change_interval)!!.also { cb ->
+                    // `.also` is used as .isVisible is an extension, so Kotlin prefers
+                    // incorrectly setting Fragment.isVisible
+                    cb.isVisible = viewModel.canSetUpdateIntervalToMatchDueDate
+                    cb.isChecked = viewModel.updateIntervalToMatchDueDate
+                    cb.setOnCheckedChangeListener { _, isChecked ->
                         viewModel.updateIntervalToMatchDueDate = isChecked
                     }
                 }
@@ -184,12 +195,17 @@ class SetDueDateDialog : DialogFragment() {
 
     companion object {
         const val ARG_CARD_IDS = "ARGS_CARD_IDS"
+        const val ARG_FSRS = "ARGS_FSRS"
         const val MAX_WIDTH_DP = 450
 
         @CheckResult
-        fun newInstance(cardIds: List<CardId>) =
+        suspend fun newInstance(cardIds: List<CardId>) =
             SetDueDateDialog().apply {
-                arguments = bundleOf(ARG_CARD_IDS to cardIds.toLongArray())
+                arguments =
+                    bundleOf(
+                        ARG_CARD_IDS to cardIds.toLongArray(),
+                        ARG_FSRS to (getFSRSStatus() ?: false.also { Timber.w("FSRS Status error") }),
+                    )
                 Timber.i("Showing 'set due date' dialog for %d cards", cardIds.size)
             }
     }

AnkiDroid/src/main/java/com/ichi2/anki/scheduling/SetDueDateViewModel.kt

@@ -44,6 +44,25 @@ class SetDueDateViewModel : ViewModel() {
     /** The cards to change the due date of */
     lateinit var cardIds: List<CardId>
 
+    /** Whether the Free Spaced Repetition Scheduler is enabled */
+    // initialized in init()
+    private var fsrsEnabled: Boolean = false
+        set(value) {
+            field = value
+            Timber.d("fsrsEnabled : %b", value)
+            if (value) {
+                Timber.d("updateIntervalToMatchDueDate forced to true: FSRS is enabled")
+                this.updateIntervalToMatchDueDate = true
+            }
+        }
+
+    /** Whether the user can set [updateIntervalToMatchDueDate] */
+    val canSetUpdateIntervalToMatchDueDate
+        // this only makes sense in SM-2, where the due date does not directly impact the next
+        // interval calculation. In FSRS, the current date is taken into account
+        // so ivl should match due date for simplicity
+        get() = !fsrsEnabled
+
     /**
      * The number of cards which will be affected
      *
@@ -75,15 +94,30 @@ class SetDueDateViewModel : ViewModel() {
             refreshIsValid()
         }
 
-    /** If `true`, the interval of the card is updated to match the calculated due date */
+    /**
+     * If `true`, the interval of the card is updated to match the calculated due date
+     *
+     * This is only supported when using SM-2. In FSRS, there's no reason for ivl != due date,
+     * as the date when a card is seen affects the scheduling.
+     *
+     * @throws UnsupportedOperationException if unset when FSRS is enabled
+     */
     var updateIntervalToMatchDueDate: Boolean = false
+        get() = if (fsrsEnabled) true else field
         set(value) {
             Timber.d("updateIntervalToMatchDueDate: %b", value)
+            if (fsrsEnabled && !value) {
+                throw UnsupportedOperationException("due date must match interval if using FSRS")
+            }
             field = value
         }
 
-    fun init(cardIds: LongArray) {
+    fun init(
+        cardIds: LongArray,
+        fsrsEnabled: Boolean,
+    ) {
         this.cardIds = cardIds.toList()
+        this.fsrsEnabled = fsrsEnabled
     }
 
     fun setNextDateRangeStart(value: Int?) {

AnkiDroid/src/test/java/com/ichi2/anki/scheduling/SetDueDateDialogTest.kt

@@ -39,6 +39,7 @@ import org.junit.runner.RunWith
 
 @Ignore("selectTab(1) does not attach Ids")
 @NeedsTest("get the tests working")
+@NeedsTest("set interval to same value visibility with FSRS")
 @RunWith(AndroidJUnit4::class)
 class SetDueDateDialogTest : RobolectricTest() {
     @Test

AnkiDroid/src/test/java/com/ichi2/anki/scheduling/SetDueDateViewModelTest.kt

@@ -22,10 +22,13 @@ import com.ichi2.anki.scheduling.SetDueDateViewModel.Tab
 import com.ichi2.libanki.CardId
 import com.ichi2.libanki.sched.SetDueDateDays
 import com.ichi2.testutils.JvmTest
+import com.ichi2.testutils.common.assertThrows
 import org.hamcrest.MatcherAssert.assertThat
 import org.hamcrest.Matchers.equalTo
 import org.junit.Test
 import org.junit.runner.RunWith
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
 
 @RunWith(AndroidJUnit4::class)
 class SetDueDateViewModelTest : JvmTest() {
@@ -111,12 +114,58 @@ class SetDueDateViewModelTest : JvmTest() {
             assertThat(calculateDaysParameter(), eq("0-1!"))
         }
 
+    @Test
+    fun `if FSRS is enabled updateIntervalToMatchDueDate may not be set to false`() {
+        runViewModelTest(fsrsEnabled = true) {
+            // the value can be set to true
+            updateIntervalToMatchDueDate = true
+
+            assertFalse(
+                canSetUpdateIntervalToMatchDueDate,
+                "canSetUpdateIntervalToMatchDueDate when FSRS enabled",
+            )
+
+            val ex =
+                assertThrows<UnsupportedOperationException>("due date mismatch & FSRS enabled") {
+                    updateIntervalToMatchDueDate = false
+                }
+            assertThat(ex.message, equalTo("due date must match interval if using FSRS"))
+        }
+
+        runViewModelTest(fsrsEnabled = false) {
+            assertTrue(
+                canSetUpdateIntervalToMatchDueDate,
+                "canSetUpdateIntervalToMatchDueDate and no FSRS",
+            )
+        }
+    }
+
+    @Test
+    fun `updateIntervalToMatchDueDate default value, given scheduler`() {
+        runViewModelTest(fsrsEnabled = true) {
+            assertThat(
+                "updateIntervalToMatchDueDate default if FSRS",
+                updateIntervalToMatchDueDate,
+                equalTo(true),
+            )
+        }
+
+        runViewModelTest(fsrsEnabled = false) {
+            assertThat(
+                "updateIntervalToMatchDueDate default if no FSRS",
+                updateIntervalToMatchDueDate,
+                equalTo(false),
+            )
+        }
+    }
+
     private fun runViewModelTest(
         cardIds: List<CardId> = listOf(1, 2, 3),
+        fsrsEnabled: Boolean = false,
         testBody: suspend SetDueDateViewModel.() -> Unit,
     ) = runTest {
         val viewModel = SetDueDateViewModel()
-        viewModel.init(cardIds.toLongArray())
+        viewModel.init(cardIds.toLongArray(), fsrsEnabled = fsrsEnabled)
         testBody(viewModel)
     }
 }