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frontend/admin/js/views/academic-calendar-grid.js

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+const MILLISECONDS_PER_DAY = 24 * 60 * 60 * 1000;
+
+export function academicCalendarWeekNumber(academicYearStartIso, dateIso) {
+    const academicYearStart = parseIsoDate(academicYearStartIso);
+    const date = parseIsoDate(dateIso);
+    const daysFromYearStart = (date.timestamp - academicYearStart.timestamp) / MILLISECONDS_PER_DAY;
+
+    if (!Number.isInteger(daysFromYearStart) || daysFromYearStart < 0) {
+        throw new RangeError('Дата ячейки не может быть раньше начала учебного года');
+    }
+
+    const daysBeforeYearStartInFirstWeek = academicYearStart.dayOfWeek - 1;
+    return Math.floor((daysBeforeYearStartInFirstWeek + daysFromYearStart) / 7) + 1;
+}
+
+function parseIsoDate(value) {
+    const match = /^(\d{4})-(\d{2})-(\d{2})$/.exec(String(value));
+    if (!match) {
+        throw new TypeError('Ожидалась дата в формате ГГГГ-ММ-ДД');
+    }
+
+    const [, yearText, monthText, dayText] = match;
+    const year = Number(yearText);
+    const month = Number(monthText);
+    const day = Number(dayText);
+    const timestamp = Date.UTC(year, month - 1, day);
+    const date = new Date(timestamp);
+    if (date.getUTCFullYear() !== year
+        || date.getUTCMonth() !== month - 1
+        || date.getUTCDate() !== day) {
+        throw new TypeError('Ожидалась дата в формате ГГГГ-ММ-ДД');
+    }
+
+    const jsDayOfWeek = date.getUTCDay();
+    return {
+        timestamp,
+        dayOfWeek: jsDayOfWeek === 0 ? 7 : jsDayOfWeek
+    };
+}