first commit

backend/src/services/recurrence.ts

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+import type { RecurrenceRule } from "@prisma/client";
+
+const clampDay = (year: number, monthIndex: number, day: number) => {
+  const lastDay = new Date(Date.UTC(year, monthIndex + 1, 0)).getUTCDate();
+  return Math.min(day, lastDay);
+};
+
+export const addMonthsWithClamp = (date: Date, months: number) => {
+  const year = date.getUTCFullYear();
+  const month = date.getUTCMonth();
+  const day = date.getUTCDate();
+  const hour = date.getUTCHours();
+  const minute = date.getUTCMinutes();
+  const second = date.getUTCSeconds();
+
+  const targetMonth = month + months;
+  const targetYear = year + Math.floor(targetMonth / 12);
+  const normalizedMonth = ((targetMonth % 12) + 12) % 12;
+  const clampedDay = clampDay(targetYear, normalizedMonth, day);
+
+  return new Date(Date.UTC(targetYear, normalizedMonth, clampedDay, hour, minute, second));
+};
+
+export const addYearsWithClamp = (date: Date, years: number) => {
+  const targetYear = date.getUTCFullYear() + years;
+  const month = date.getUTCMonth();
+  const day = date.getUTCDate();
+  const hour = date.getUTCHours();
+  const minute = date.getUTCMinutes();
+  const second = date.getUTCSeconds();
+  const clampedDay = clampDay(targetYear, month, day);
+  return new Date(Date.UTC(targetYear, month, clampedDay, hour, minute, second));
+};
+
+export const nextDueAt = (dueAt: Date, rule: RecurrenceRule) => {
+  const interval = rule.interval ?? 1;
+  switch (rule.type) {
+    case "hourly":
+      return new Date(dueAt.getTime() + interval * 60 * 60 * 1000);
+    case "daily":
+      return new Date(dueAt.getTime() + interval * 24 * 60 * 60 * 1000);
+    case "weekly":
+      return new Date(dueAt.getTime() + interval * 7 * 24 * 60 * 60 * 1000);
+    case "monthly":
+      return addMonthsWithClamp(dueAt, interval);
+    case "yearly":
+      return addYearsWithClamp(dueAt, interval);
+    default:
+      return dueAt;
+  }
+};