Comprehensive date conversion

src/content/dates.test.ts

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+import { describe, expect, it } from 'vitest';
+
+import {
+  EraInfoDate,
+  EraMeta,
+  getEraInfo,
+  parseEraDate,
+  startsWithEraName,
+} from './dates';
+
+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);
+    expect(startsWithEraName('令元年')).toBe(false);
+    expect(startsWithEraName('令')).toBe(false);
+    expect(startsWithEraName('')).toBe(false);
+    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);
+      }
+    }
+  });
+});