From d37caedd64792abd8fb4026c3a6092880b83c9ef Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?L=C3=AA=20Duy=20Quang?= <leduyquang753@gmail.com>
Date: Mon, 30 Dec 2024 22:40:54 +0700
Subject: [PATCH] Make `applyDuration` reversible.

---
 src/duration.ts  | 33 +++++++++++++++++++++++++++------
 test/duration.ts | 19 +++++++++++--------
 2 files changed, 38 insertions(+), 14 deletions(-)

diff --git a/src/duration.ts b/src/duration.ts
index 74cb017..28c3bfe 100644
--- a/src/duration.ts
+++ b/src/duration.ts
@@ -84,14 +84,35 @@ export class Duration {
   }
 }
 
+const durationApplicationActionsForward = [
+  (r: Date, duration: Duration) => {
+    r.setUTCFullYear(r.getUTCFullYear() + duration.years)
+  },
+  (r: Date, duration: Duration) => {
+    r.setUTCMonth(r.getUTCMonth() + duration.months)
+  },
+  (r: Date, duration: Duration) => {
+    r.setUTCDate(r.getUTCDate() + duration.weeks * 7 + duration.days)
+  },
+  (r: Date, duration: Duration) => {
+    r.setUTCHours(r.getUTCHours() + duration.hours)
+  },
+  (r: Date, duration: Duration) => {
+    r.setUTCMinutes(r.getUTCMinutes() + duration.minutes)
+  },
+  (r: Date, duration: Duration) => {
+    r.setUTCSeconds(r.getUTCSeconds() + duration.seconds)
+  },
+]
+const durationApplicationActionsBackward = [...durationApplicationActionsForward].reverse()
+
 export function applyDuration(date: Date | number, duration: Duration): Date {
   const r = new Date(date)
-  r.setFullYear(r.getFullYear() + duration.years)
-  r.setMonth(r.getMonth() + duration.months)
-  r.setDate(r.getDate() + duration.weeks * 7 + duration.days)
-  r.setHours(r.getHours() + duration.hours)
-  r.setMinutes(r.getMinutes() + duration.minutes)
-  r.setSeconds(r.getSeconds() + duration.seconds)
+  if (duration.sign < 0) {
+    for (const action of durationApplicationActionsBackward) action(r, duration)
+  } else {
+    for (const action of durationApplicationActionsForward) action(r, duration)
+  }
   return r
 }
 
diff --git a/test/duration.ts b/test/duration.ts
index 3b35600..ecb9e68 100644
--- a/test/duration.ts
+++ b/test/duration.ts
@@ -81,16 +81,19 @@ suite('duration', function () {
   })
 
   suite('applyDuration', function () {
-    const referenceDate = '2022-10-21T16:48:44.104Z'
     const tests = new Set([
-      {input: 'P4Y', expected: '2026-10-21T16:48:44.104Z'},
-      {input: '-P4Y', expected: '2018-10-21T16:48:44.104Z'},
-      {input: '-P3MT5M', expected: '2022-07-21T16:43:44.104Z'},
-      {input: 'P1Y2M3DT4H5M6S', expected: '2023-12-24T20:53:50.104Z'},
-      {input: 'P5W', expected: '2022-11-25T16:48:44.104Z'},
-      {input: '-P5W', expected: '2022-09-16T16:48:44.104Z'},
+      {referenceDate: '2022-10-21T16:48:44.104Z', input: 'P5W', expected: '2022-11-25T16:48:44.104Z'},
+      {referenceDate: '2022-11-25T16:48:44.104Z', input: '-P5W', expected: '2022-10-21T16:48:44.104Z'},
+      {referenceDate: '2022-07-21T16:43:44.104Z', input: 'P3MT5M', expected: '2022-10-21T16:48:44.104Z'},
+      {referenceDate: '2022-10-21T16:48:44.104Z', input: '-P3MT5M', expected: '2022-07-21T16:43:44.104Z'},
+      {referenceDate: '2022-10-21T16:48:44.104Z', input: 'P4Y', expected: '2026-10-21T16:48:44.104Z'},
+      {referenceDate: '2026-10-21T16:48:44.104Z', input: '-P4Y', expected: '2022-10-21T16:48:44.104Z'},
+      {referenceDate: '2022-10-21T16:48:44.104Z', input: 'P1Y2M3DT4H5M6S', expected: '2023-12-24T20:53:50.104Z'},
+      {referenceDate: '2023-12-24T20:53:50.104Z', input: '-P1Y2M3DT4H5M6S', expected: '2022-10-21T16:48:44.104Z'},
+      {referenceDate: '2023-08-15T00:00:00.000Z', input: 'P1Y3M25D', expected: '2024-12-10T00:00:00.000Z'},
+      {referenceDate: '2024-12-10T00:00:00.000Z', input: '-P1Y3M25D', expected: '2023-08-15T00:00:00.000Z'},
     ])
-    for (const {input, expected} of tests) {
+    for (const {referenceDate, input, expected} of tests) {
       test(`${referenceDate} -> ${input} -> ${expected}`, () => {
         assert.equal(applyDuration(new Date(referenceDate), Duration.from(input))?.toISOString(), expected)
       })