@codecademy/gamut

Index: package/dist/DatePicker/DatePickerCalendar/Calendar/utils/dateGrid.js
===================================================================
--- package/dist/DatePicker/DatePickerCalendar/Calendar/utils/dateGrid.js
+++ package/dist/DatePicker/DatePickerCalendar/Calendar/utils/dateGrid.js
@@ -0,0 +1,188 @@
+/**
+ * Builds a grid of days for a calendar month using native Date and Intl.
+ * Each row has 7 cells; leading/trailing cells may be null (padding from adjacent months).
+ */
+
+const DAYS_PER_WEEK = 7;
+
+/**
+ * Normalize to start of day in local time for comparison.
+ */
+export const normalizeDate = date => {
+  return new Date(date.getFullYear(), date.getMonth(), date.getDate()).getTime();
+};
+
+/**
+ * Number of empty cells before the 1st of the month, for a grid whose first column is
+ * `firstWeekday` (ISO: 1 = Monday … 7 = Sunday from `Intl.Locale#getWeekInfo`).
+ */
+export const getWeekdayOffsetInGrid = ({
+  date,
+  firstWeekday
+}) => {
+  const dayOfWeek = date.getDay();
+  const iso = dayOfWeek === 0 ? 7 : dayOfWeek;
+  return (iso - firstWeekday + 14) % 7;
+};
+export const getFirstOfMonth = date => {
+  return new Date(date.getFullYear(), date.getMonth(), 1);
+};
+
+/**
+ * Returns an array of weeks for the given month. Each week is an array of 7 items:
+ * each item is either a Date (that day) or null (padding from previous/next month).
+ *
+ * @param year - Full year (e.g. 2026)
+ * @param month - Month 0-11 (0 = January)
+ * @param firstWeekday - First day of the week for the calendar row (ISO 1–7, from `getWeekInfo().firstDay`)
+ */
+export const getMonthGrid = ({
+  year,
+  month,
+  firstWeekday
+}) => {
+  const first = getFirstOfMonth(new Date(year, month, 1));
+  const last = new Date(year, month + 1, 0);
+  const firstDayOfWeek = getWeekdayOffsetInGrid({
+    date: first,
+    firstWeekday
+  });
+  const daysInMonth = last.getDate();
+  const weeks = [];
+  let currentWeek = [];
+  for (let i = 0; i < firstDayOfWeek; i += 1) {
+    currentWeek.push(null);
+  }
+  for (let day = 1; day <= daysInMonth; day += 1) {
+    currentWeek.push(new Date(year, month, day));
+    if (currentWeek.length === DAYS_PER_WEEK) {
+      weeks.push(currentWeek);
+      currentWeek = [];
+    }
+  }
+  if (currentWeek.length > 0) {
+    while (currentWeek.length < DAYS_PER_WEEK) {
+      currentWeek.push(null);
+    }
+    weeks.push(currentWeek);
+  }
+  return weeks;
+};
+
+/**
+ * Check if two dates are the same calendar day (ignoring time).
+ */
+export const isSameDay = (a, b) => {
+  if (a === null || b === null) return false;
+  return normalizeDate(a) === normalizeDate(b);
+};
+
+/**
+ * Calendar-ordered local-midnight instants for two possibly unordered `Date` values.
+ * Matches the bounds used by {@link isDateInRange} (and range selection).
+ */
+export const getOrderedCalendarEndpoints = ({
+  startDate,
+  endDate
+}) => {
+  const normalizedStartDate = new Date(startDate.getFullYear(), startDate.getMonth(), startDate.getDate());
+  const normalizedEndDate = new Date(endDate.getFullYear(), endDate.getMonth(), endDate.getDate());
+  return normalizedStartDate <= normalizedEndDate ? {
+    low: normalizedStartDate,
+    high: normalizedEndDate
+  } : {
+    low: normalizedEndDate,
+    high: normalizedStartDate
+  };
+};
+
+/**
+ * Check if `date` is between `startDate` and `endDate` (exclusive), ignoring time.
+ */
+export const isDateInRange = ({
+  date,
+  startDate,
+  endDate
+}) => {
+  if (startDate === null) return false;
+  const endBound = endDate ?? startDate;
+  const {
+    low,
+    high
+  } = getOrderedCalendarEndpoints({
+    startDate,
+    endDate: endBound
+  });
+  const normalizedDate = normalizeDate(date);
+  return normalizedDate > normalizeDate(low) && normalizedDate < normalizeDate(high);
+};
+
+/**
+ * Build a `shouldDisableDate` that disables each listed calendar day (time-of-day ignored).
+ *
+ * @example
+ * ```tsx
+ * <DatePicker
+ *   mode="single"
+ *   selectedDate={null}
+ *   onSelected={() => {}}
+ *   shouldDisableDate={matchDisabledDates([new Date(2026, 3, 14)])}
+ * />
+ * ```
+ */
+export const matchDisabledDates = (dates = []) => date => dates.some(d => isSameDay(date, d));
+
+/** True when `shouldDisableDate` returns true for this calendar day. */
+export const isDateDisabled = ({
+  date,
+  shouldDisableDate
+}) => Boolean(shouldDisableDate?.(date));
+
+/** One visible day in the month grid with its row (for Home/End and keyboard nav). */
+
+/** Flat list of dates in grid order (row-major, non-null only) with row index for Home/End */
+export const getDatesWithRow = weeks => {
+  const result = [];
+  weeks.forEach((week, rowIndex) => {
+    week.forEach(date => {
+      if (date !== null) result.push({
+        date,
+        rowIndex
+      });
+    });
+  });
+  return result;
+};
+
+/** Add `n` months to the given date. */
+export const addMonths = ({
+  date,
+  n
+}) => new Date(date.getFullYear(), date.getMonth() + n, 1);
+
+/**
+ * True if `date` falls in the left visible month, or—when `showSecondMonth`—in the
+ * month shown in the second column. Used to avoid shifting the visible month pair when
+ * the committed date is already on screen (e.g. a click in the right-hand month).
+ */
+export const isDateWithinVisibleMonths = ({
+  date,
+  startOfLeftVisibleMonth,
+  showSecondMonth
+}) => {
+  const year = date.getFullYear();
+  const month = date.getMonth();
+  const leftYear = startOfLeftVisibleMonth.getFullYear();
+  const leftMonth = startOfLeftVisibleMonth.getMonth();
+  if (year === leftYear && month === leftMonth) return true;
+  if (showSecondMonth) {
+    const rightMonthStart = addMonths({
+      date: startOfLeftVisibleMonth,
+      n: 1
+    });
+    const rightYear = rightMonthStart.getFullYear();
+    const rightMonth = rightMonthStart.getMonth();
+    return year === rightYear && month === rightMonth;
+  }
+  return false;
+};
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