Add vietnamese holidays

src/Calendars/ChineseCalendar.php

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+<?php
+
+namespace Spatie\Holidays\Calendars;
+
+use DateTimeZone;
+use IntlDateFormatter;
+
+trait ChineseCalendar
+{
+    protected $timezone = 'Asia/Shanghai';
+
+    protected function chineseToGregorianDate(string $input, int $year): string
+    {
+        $timeStamp = $this->getFormatter()->parse($year . '/' . $input);
+        $dateTime = date_create()->setTimeStamp($timeStamp)->setTimezone(new DateTimeZone($this->timezone));
+
+        return $dateTime->format('m-d');
+    }
+
+    protected function getFormatter(): IntlDateFormatter
+    {
+        return new IntlDateFormatter(
+            locale: 'zh-CN@calendar=chinese',
+            dateType: IntlDateFormatter::SHORT,
+            timeType: IntlDateFormatter::NONE,
+            timezone: $this->timezone,
+            calendar: IntlDateFormatter::TRADITIONAL
+        );
+    }
+}

src/Countries/Vietnam.php

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+<?php
+
+namespace Spatie\Holidays\Countries;
+
+use Carbon\CarbonImmutable;
+use DateTimeZone;
+use IntlDateFormatter;
+use Spatie\Holidays\Calendars\ChineseCalendar;
+
+class Vietnam extends Country
+{
+    use ChineseCalendar;
+
+    public function countryCode(): string
+    {
+        return 'vi';
+    }
+
+    protected function allHolidays(int $year): array
+    {
+        return array_merge([
+            'New Year\'s day' => '01-01',
+            'Reunification Day' => '04-30',
+            'Labour Day' => '05-01',
+            'Independence Day' => '09-02',
+        ], $this->variableHolidays($year));
+    }
+
+    /** @return array<string, CarbonImmutable> */
+    protected function variableHolidays(int $year): array
+    {
+        return [
+            'Lunar New Year\'s Eve' => $this->chineseToGregorianDate('12/30', $year - 1),
+            'Lunar New Year Day 1' => $this->chineseToGregorianDate('01/01', $year),
+            'Lunar New Year Day 2' => $this->chineseToGregorianDate('01/02', $year),
+            'Lunar New Year Day 3' => $this->chineseToGregorianDate('01/03', $year),
+            'Lunar New Year Day 4' => $this->chineseToGregorianDate('01/04', $year),
+            'Hung Kings\' Festival' => $this->chineseToGregorianDate('03/10', $year),
+        ];
+    }
+}

tests/.pest/snapshots/Countries/VietnamTest/it_can_calculate_vietnamese_holidays.snap

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+[
+    {
+        "name": "New Year's day",
+        "date": "2024-01-01"
+    },
+    {
+        "name": "Lunar New Year's Eve",
+        "date": "2024-02-09"
+    },
+    {
+        "name": "Lunar New Year Day 1",
+        "date": "2024-02-10"
+    },
+    {
+        "name": "Lunar New Year Day 2",
+        "date": "2024-02-11"
+    },
+    {
+        "name": "Lunar New Year Day 3",
+        "date": "2024-02-12"
+    },
+    {
+        "name": "Lunar New Year Day 4",
+        "date": "2024-02-13"
+    },
+    {
+        "name": "Hung Kings' Festival",
+        "date": "2024-04-18"
+    },
+    {
+        "name": "Reunification Day",
+        "date": "2024-04-30"
+    },
+    {
+        "name": "Labour Day",
+        "date": "2024-05-01"
+    },
+    {
+        "name": "Independence Day",
+        "date": "2024-09-02"
+    }
+]

tests/Countries/VietnamTest.php

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+<?php
+
+namespace Spatie\Holidays\Tests\Countries;
+
+use Carbon\CarbonImmutable;
+use Spatie\Holidays\Countries\Vietnam;
+use Spatie\Holidays\Holidays;
+
+it('can calculate vietnamese holidays', function () {
+    CarbonImmutable::setTestNowAndTimezone('2024-01-01', 'Asia/Ho_Chi_Minh');
+
+    $holidays = Holidays::for(country: 'vi')->get();
+
+    expect($holidays)
+        ->toBeArray()
+        ->not()->toBeEmpty();
+
+    expect(formatDates($holidays))->toMatchSnapshot();
+});