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node_modules/date-fns/differenceInYears.js

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+import { normalizeDates } from "./_lib/normalizeDates.js";
+import { compareAsc } from "./compareAsc.js";
+import { differenceInCalendarYears } from "./differenceInCalendarYears.js";
+
+/**
+ * The {@link differenceInYears} function options.
+ */
+
+/**
+ * @name differenceInYears
+ * @category Year Helpers
+ * @summary Get the number of full years between the given dates.
+ *
+ * @description
+ * Get the number of full years between the given dates.
+ *
+ * @param laterDate - The later date
+ * @param earlierDate - The earlier date
+ * @param options - An object with options
+ *
+ * @returns The number of full years
+ *
+ * @example
+ * // How many full years are between 31 December 2013 and 11 February 2015?
+ * const result = differenceInYears(new Date(2015, 1, 11), new Date(2013, 11, 31))
+ * //=> 1
+ */
+export function differenceInYears(laterDate, earlierDate, options) {
+  const [laterDate_, earlierDate_] = normalizeDates(
+    options?.in,
+    laterDate,
+    earlierDate,
+  );
+
+  // -1 if the left date is earlier than the right date
+  // 2023-12-31 - 2024-01-01 = -1
+  const sign = compareAsc(laterDate_, earlierDate_);
+
+  // First calculate the difference in calendar years
+  // 2024-01-01 - 2023-12-31 = 1 year
+  const diff = Math.abs(differenceInCalendarYears(laterDate_, earlierDate_));
+
+  // Now we need to calculate if the difference is full. To do that we set
+  // both dates to the same year and check if the both date's month and day
+  // form a full year.
+  laterDate_.setFullYear(1584);
+  earlierDate_.setFullYear(1584);
+
+  // For it to be true, when the later date is indeed later than the earlier date
+  // (2026-02-01 - 2023-12-10 = 3 years), the difference is full if
+  // the normalized later date is also later than the normalized earlier date.
+  // In our example, 1584-02-01 is earlier than 1584-12-10, so the difference
+  // is partial, hence we need to subtract 1 from the difference 3 - 1 = 2.
+  const partial = compareAsc(laterDate_, earlierDate_) === -sign;
+
+  const result = sign * (diff - +partial);
+
+  // Prevent negative zero
+  return result === 0 ? 0 : result;
+}
+
+// Fallback for modularized imports:
+export default differenceInYears;