[DatePicker] Add MonthAdapter helpers for finding enabled days

This change introduces helper methods to `MonthAdapter` to allow callers to identify days that are enabled according to the `CalendarConstraints`.

PiperOrigin-RevId: 826466043

Author: Material Design Team

lib/java/com/google/android/material/datepicker/MonthAdapter.java

@@ -343,4 +343,61 @@ boolean isFirstInRow(int position) {
   boolean isLastInRow(int position) {
     return (position + 1) % month.daysInWeek == 0;
   }
+
+  /**
+   * Returns true if the day at the given adapter position is within the current month, and valid
+   * according to the {@link CalendarConstraints}.
+   */
+  boolean isDayPositionValid(int position) {
+    Long day = getItem(position);
+    return day != null && calendarConstraints.getDateValidator().isValid(day);
+  }
+
+  /**
+   * Finds the closest valid day to the given position searching forward.
+   *
+   * @param position The starting position.
+   * @return The position of the next valid day, or -1 if none is found before reaching the
+   *     end of the month.
+   */
+  int findNextValidDayPosition(int position) {
+    for (int i = position + 1; i <= lastPositionInMonth(); i++) {
+      if (isDayPositionValid(i)) {
+        return i;
+      }
+    }
+    return -1;
+  }
+
+  /**
+   * Finds the closest valid day to the given position searching backward.
+   *
+   * @param position The starting position.
+   * @return The position of the previous valid day, or -1 if none is found before reaching the
+   *     start of the month.
+   */
+  int findPreviousValidDayPosition(int position) {
+    for (int i = position - 1; i >= firstPositionInMonth(); i--) {
+      if (isDayPositionValid(i)) {
+        return i;
+      }
+    }
+    return -1;
+  }
+
+  /**
+   * Returns the adapter position of the first day in the month that is valid, or -1 if no days
+   * are valid.
+   */
+  int findFirstValidDayPosition() {
+    return findNextValidDayPosition(firstPositionInMonth() - 1);
+  }
+
+  /**
+   * Returns the adapter position of the last day in the month that is valid, or -1 if no days are
+   * valid.
+   */
+  int findLastValidDayPosition() {
+    return findPreviousValidDayPosition(lastPositionInMonth() + 1);
+  }
 }

lib/javatests/com/google/android/material/datepicker/MonthAdapterTest.java

@@ -388,6 +388,90 @@ public void rangeDateSelector_isNotEndOfRange() {
     assertFalse(monthAdapter.isEndOfRange(month.getDay(9)));
   }
 
+  @Test
+  public void isDayPositionValid_withValidator() {
+    Locale.setDefault(Locale.US);
+    Month month = Month.create(2019, Calendar.FEBRUARY);
+    long dateValidFrom = month.getDay(15);
+    CalendarConstraints constraints =
+        new CalendarConstraints.Builder()
+            .setValidator(DateValidatorPointForward.from(dateValidFrom))
+            .build();
+    MonthAdapter adapter = new MonthAdapter(month, new SingleDateSelector(), constraints, null);
+
+    assertThat(adapter.isDayPositionValid(adapter.dayToPosition(14))).isFalse();
+    assertThat(adapter.isDayPositionValid(adapter.dayToPosition(15))).isTrue();
+  }
+
+  @Test
+  public void findNextValidDayPosition_withValidator() {
+    Locale.setDefault(Locale.US);
+    Month month = Month.create(2019, Calendar.FEBRUARY);
+    long dateValidFrom = month.getDay(15);
+    CalendarConstraints constraints =
+        new CalendarConstraints.Builder()
+            .setValidator(DateValidatorPointForward.from(dateValidFrom))
+            .build();
+    MonthAdapter adapter = new MonthAdapter(month, new SingleDateSelector(), constraints, null);
+
+    // Search forward from day 28 (last valid day), expecting no results.
+    assertThat(adapter.findNextValidDayPosition(adapter.dayToPosition(28))).isEqualTo(-1);
+    // Search forward from day 14 (valid) to day 15 (valid).
+    assertThat(adapter.findNextValidDayPosition(adapter.dayToPosition(14)))
+        .isEqualTo(adapter.dayToPosition(15));
+  }
+
+  @Test
+  public void findPreviousValidDayPosition_withValidator() {
+    Locale.setDefault(Locale.US);
+    Month month = Month.create(2019, Calendar.FEBRUARY);
+    long dateValidFrom = month.getDay(15);
+    CalendarConstraints constraints =
+        new CalendarConstraints.Builder()
+            .setValidator(DateValidatorPointForward.from(dateValidFrom))
+            .build();
+    MonthAdapter adapter = new MonthAdapter(month, new SingleDateSelector(), constraints, null);
+
+    // Search backward from day 15 (first valid day), expecting no results.
+    assertThat(adapter.findPreviousValidDayPosition(adapter.dayToPosition(15))).isEqualTo(-1);
+    // Search backward from day 16 (valid) to day 15 (valid).
+    assertThat(adapter.findPreviousValidDayPosition(adapter.dayToPosition(16)))
+        .isEqualTo(adapter.dayToPosition(15));
+  }
+
+  @Test
+  public void findFirstValidDayPosition_withValidator() {
+    Locale.setDefault(Locale.US);
+    Month month = Month.create(2019, Calendar.FEBRUARY);
+    long dateValidFrom = month.getDay(15);
+    CalendarConstraints constraints =
+        new CalendarConstraints.Builder()
+            .setValidator(DateValidatorPointForward.from(dateValidFrom))
+            .build();
+    MonthAdapter adapter = new MonthAdapter(month, new SingleDateSelector(), constraints, null);
+
+    assertThat(adapter.findFirstValidDayPosition()).isEqualTo(adapter.dayToPosition(15));
+  }
+
+  @Test
+  public void findLastValidDayPosition_withValidator() {
+    Locale.setDefault(Locale.US);
+    Month month = Month.create(2019, Calendar.FEBRUARY);
+    long dateValidFrom = month.getDay(15);
+    long dateValidTo = month.getDay(18);
+    CalendarConstraints constraints =
+        new CalendarConstraints.Builder()
+            .setValidator(
+                CompositeDateValidator.allOf(
+                    Arrays.asList(
+                        DateValidatorPointForward.from(dateValidFrom),
+                        DateValidatorPointBackward.before(dateValidTo))))
+            .build();
+    MonthAdapter adapter = new MonthAdapter(month, new SingleDateSelector(), constraints, null);
+
+    assertThat(adapter.findLastValidDayPosition()).isEqualTo(adapter.dayToPosition(18));
+  }
+
   private MonthAdapter createRangeMonthAdapter(Month month, Pair<Long, Long> selection) {
     DateSelector<Pair<Long, Long>> dateSelector = new RangeDateSelector();
     dateSelector.setSelection(selection);