working pages and app router

nextjs-pages/src/components/courses/calendar/calendarMonthUtils.ts

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+"use client";
+
+import {
+  dateToMarkdownString,
+  getDateFromStringOrThrow,
+} from "@/models/local/timeUtils";
+
+export interface CalendarMonthModel {
+  year: number;
+  month: number;
+  weeks: number[][];
+  daysByWeek: string[][]; //iso date is memo-izable
+}
+
+function weeksInMonth(year: number, month: number): number {
+  const firstDayOfMonth = new Date(year, month - 1, 1).getDay();
+  const daysInMonth = new Date(year, month, 0).getDate();
+  const longDaysInMonth = daysInMonth + firstDayOfMonth;
+  let weeks = Math.floor(longDaysInMonth / 7);
+  if (longDaysInMonth % 7 > 0) {
+    weeks += 1;
+  }
+  return weeks;
+}
+
+function createCalendarMonth(year: number, month: number): CalendarMonthModel {
+  const weeksNumber = weeksInMonth(year, month);
+  const daysInMonth = new Date(year, month, 0).getDate();
+
+  let currentDay = 1;
+  const firstDayOfMonth = new Date(year, month - 1, 1).getDay();
+
+  const daysByWeek = Array.from({ length: weeksNumber }).map((_, weekIndex) =>
+    Array.from({ length: 7 }).map((_, dayIndex) => {
+      if (weekIndex === 0 && dayIndex < firstDayOfMonth) {
+        return dateToMarkdownString(
+          new Date(year, month - 1, dayIndex - firstDayOfMonth + 1, 12, 0, 0)
+        );
+      } else if (currentDay <= daysInMonth) {
+        return dateToMarkdownString(new Date(year, month - 1, currentDay++, 12, 0, 0));
+      } else {
+        currentDay++;
+        return dateToMarkdownString(
+          new Date(year, month, currentDay - daysInMonth - 1, 12, 0, 0)
+        );
+      }
+    })
+  );
+
+  const weeks = daysByWeek.map((week) =>
+    week.map((day) =>
+      getDateFromStringOrThrow(day, "calculating weeks").getDate()
+    )
+  );
+
+  return { year, month, weeks, daysByWeek };
+}
+
+export function getMonthsBetweenDates(
+  startDate: Date,
+  endDate: Date
+): CalendarMonthModel[] {
+  const monthsInTerm =
+    1 +
+    (endDate.getFullYear() - startDate.getFullYear()) * 12 +
+    endDate.getMonth() -
+    startDate.getMonth();
+
+  return Array.from({ length: monthsInTerm }, (_, monthDiff) => {
+    const month = ((startDate.getMonth() + monthDiff) % 12) + 1;
+    const year =
+      startDate.getFullYear() +
+      Math.floor((startDate.getMonth() + monthDiff) / 12);
+    return createCalendarMonth(year, month);
+  });
+}