feat: add support for comprehensive date conversion

This commit starts using the mapping of Japanese era dates
to Gregorian calendar dates, enabling comprehensive date
conversion.

It includes accurate calculation of the time span of years and
months for periods predating the adoption of the Gregorian
calendar in Japan.
Additionally, day-to-day conversion of Japanese era dates
to Gregorian calendar dates was added.

src/content/dates.test.ts

@@ -1,8 +1,14 @@
 import { describe, expect, it } from 'vitest';
 
-import { startsWithEraName } from './dates';
+import {
+  EraInfoDate,
+  EraMeta,
+  getEraInfo,
+  parseEraDate,
+  startsWithEraName,
+} from './dates';
 
-describe('startsWithEraName', () => {
+describe('era date conversion', () => {
   it('detects strings that start with an era name', () => {
     expect(startsWithEraName('令和')).toBe(true);
     expect(startsWithEraName('令和元年')).toBe(true);
@@ -12,4 +18,196 @@ describe('startsWithEraName', () => {
     expect(startsWithEraName('㋿2年')).toBe(true);
     expect(startsWithEraName('天平神護')).toBe(true);
   });
+
+  it('parses era dates correctly', () => {
+    const cases: Array<
+      [
+        text: string,
+        matchLength?: number,
+        era?: string,
+        year?: number,
+        month?: number,
+        day?: number,
+      ]
+    > = [
+      ['  安永  2  年  閏  3  月  10  日xxx', 26, '安永', 2, -3, 10],
+
+      ['令和1年３月二十日', 9, '令和', 1, 3, 20],
+      ['令和1年３月二十', 6, '令和', 1, 3],
+      ['令和1年３', 4, '令和', 1],
+      ['令和1', 3, '令和', 1],
+      ['令和元', undefined],
+      ['令和元年', 4, '令和', 0],
+      ['令和元歳', 4, '令和', 0],
+      ['令和1歳', 4, '令和', 1],
+      ['㋿1年', 3, '令和', 1],
+
+      // Invalid parts of date
+      // Invalid era name
+      ['令1年', undefined],
+      // 安永 ends with year 10
+      ['安永11年', undefined],
+      // There is no leap month in 安永3年
+      ['安永3年閏1月1日', 4, '安永', 3, undefined],
+      // There is no 13th month
+      ['令和1年13月1日', 4, '令和', 1, undefined],
+      // Gregorian era dates don't have leap months
+      ['令和1年閏1月1日', 4, '令和', 1, undefined],
+    ];
+
+    for (const [text, matchLength, era, year, month, day] of cases) {
+      const parsed = {
+        era,
+        year,
+        month,
+        day,
+        matchLength,
+      };
+
+      const result = parseEraDate(text);
+
+      if (!matchLength) {
+        expect(result).toBeUndefined();
+      } else {
+        expect(result).toEqual(parsed);
+      }
+    }
+  });
+
+  it('gets correct era info dates', () => {
+    const cases: Array<
+      [
+        era: string,
+        year: number,
+        month?: number,
+        day?: number,
+        dateStart?: EraInfoDate,
+        dateEnd?: EraInfoDate,
+      ]
+    > = [
+      // Gregorian eras
+      ['令和', 0, 1, 1, { year: 2019, month: 1, day: 1 }],
+      ['令和', 1, 1, 1, { year: 2019, month: 1, day: 1 }],
+      ['令和', 1, 1, undefined, { year: 2019, month: 1 }],
+      ['明治', 6, undefined, undefined, { year: 1873 }],
+
+      // Full non Gregorian date
+      ['明治', 5, 6, 15, { year: 1872, month: 7, day: 20 }],
+
+      // First year of era
+      [
+        '安政',
+        0,
+        undefined,
+        undefined,
+        { year: 1855, month: 1, day: 15 },
+        { year: 1855, month: 2, day: 16 },
+      ],
+      [
+        '安政',
+        1,
+        undefined,
+        undefined,
+        { year: 1855, month: 1, day: 15 },
+        { year: 1855, month: 2, day: 16 },
+      ],
+
+      // Arbitrary year in between
+      [
+        '安政',
+        4,
+        undefined,
+        undefined,
+        { year: 1857, month: 1, day: 26 },
+        { year: 1858, month: 2, day: 13 },
+      ],
+
+      // Last year of era
+      [
+        '安政',
+        7,
+        undefined,
+        undefined,
+        { year: 1860, month: 1, day: 23 },
+        { year: 1860, month: 4, day: 8 },
+      ],
+
+      // Invalid year
+      ['安政', 8, undefined],
+
+      // Arbitrary month
+      [
+        '安政',
+        4,
+        1,
+        undefined,
+        { year: 1857, month: 1, day: 26 },
+        { year: 1857, month: 2, day: 23 },
+      ],
+
+      // Month before leap month
+      [
+        '安政',
+        4,
+        5,
+        undefined,
+        { year: 1857, month: 5, day: 23 },
+        { year: 1857, month: 6, day: 21 },
+      ],
+
+      // Leap month
+      [
+        '安政',
+        4,
+        -5,
+        undefined,
+        { year: 1857, month: 6, day: 22 },
+        { year: 1857, month: 7, day: 20 },
+      ],
+
+      // Last month of year
+      [
+        '安政',
+        4,
+        12,
+        undefined,
+        { year: 1858, month: 1, day: 15 },
+        { year: 1858, month: 2, day: 13 },
+      ],
+
+      // Last month of year is a leap month
+      [
+        '安永',
+        4,
+        -12,
+        undefined,
+        { year: 1776, month: 1, day: 21 },
+        { year: 1776, month: 2, day: 18 },
+      ],
+
+      // Invalid month
+      ['安政', 4, -12, undefined],
+    ];
+
+    for (const [era, year, month, day, dateStart, dateEnd] of cases) {
+      const meta: EraMeta = {
+        type: 'era',
+        era,
+        year,
+        month,
+        day,
+        matchLen: 0,
+      };
+
+      const result = getEraInfo(meta);
+
+      if (!dateStart && !dateEnd) {
+        expect(result).toBeUndefined();
+      } else {
+        expect(result).toBeDefined();
+        expect(result!.dateStart).toEqual(dateStart);
+        expect(result!.dateEnd).toEqual(dateEnd);
+      }
+    }
+  });
 });