Apply the simple titlecase fallback for Unicode 4 and later

Preserve historical identity mappings for titlecase digraphs while honoring the standardized uppercase fallback for null titlecase fields.

Assisted-By: GPT-6

Author: JLHwung

scripts/parse-simple-case-mapping.mjs

@@ -12,6 +12,10 @@ const parseSimpleCaseMapping = async (version) => {
 	// The column index of Simple_Uppercase_Mapping in UnicodeData.txt:
 	// https://www.unicode.org/reports/tr44/#UnicodeData.txt
 	const columnIndexOfSimpleUppercaseMapping = 12;
+	// Unicode 4.0 standardized the null titlecase fallback to uppercase.
+	// Earlier files leave the identity mapping of titlecase digraphs empty.
+	// https://www.unicode.org/L2/L2002/02345.htm (93-C12)
+	const titlecaseDefaultsToUppercase = parseInt(version, 10) >= 4;
 
 	const source = utils.readDataFile(version, 'database');
 	if (!source) {
@@ -24,6 +28,9 @@ const parseSimpleCaseMapping = async (version) => {
 			continue;
 		}
 		const data = lineTrimmed.split(';');
+		if (titlecaseDefaultsToUppercase && data[14] === '') {
+			data[14] = data[columnIndexOfSimpleUppercaseMapping];
+		}
 		const from = parseInt(data[0], 16);
 		for (let columnOffset = 0; columnOffset <= 2; columnOffset++) {
 			const raw = data[columnIndexOfSimpleUppercaseMapping + columnOffset];

tests/parser-formats.mjs

@@ -4,12 +4,29 @@ import parseEmoji from '../scripts/parse-emoji.mjs';
 import parseEmojiSequences from '../scripts/parse-emoji-sequences.mjs';
 import parseBidiBrackets from '../scripts/parse-bidi-brackets.mjs';
 import parseNameAliases from '../scripts/parse-name-aliases.mjs';
+import parseSimpleCaseMapping from '../scripts/parse-simple-case-mapping.mjs';
 import {
 	parseBlocks,
 	parseDerivedCoreProperties,
 } from '../scripts/parse-blocks-scripts-properties.mjs';
 import utils from '../scripts/utils.mjs';
 
+test('Null simple titlecase mappings default to uppercase mappings', async (t) => {
+	t.mock.method(utils, 'readDataFile', () =>
+		[
+			'0061;LATIN SMALL LETTER A;Ll;0;L;;;;;N;;;0041;;',
+			'01C6;LATIN SMALL LETTER DZ WITH CARON;Ll;0;L;<compat> 0064 017E;;;;N;;;01C4;;01C5',
+			'0030;DIGIT ZERO;Nd;0;EN;;0;0;0;N;;;;;',
+		].join('\n'),
+	);
+	for (const version of ['4.0.0', '18.0.0']) {
+		const map = await parseSimpleCaseMapping(version);
+		t.assert.equal(map.Titlecase.get(0x61), 0x41);
+		t.assert.equal(map.Titlecase.get(0x1c6), 0x1c5);
+		t.assert.ok(!map.Titlecase.has(0x30));
+	}
+});
+
 test('SpecialCasing accepts optional comments and flexible whitespace', async (t) => {
 	t.mock.method(
 		utils,

tests/parsers.mjs

@@ -7,6 +7,7 @@ import utils from '../scripts/utils.mjs';
 import parseDecompositionMapping from '../scripts/parse-decomposition-mapping.mjs';
 import parseIndicConjunctBreak from '../scripts/parse-indic-conjunct-break.mjs';
 import parseJoiningType from '../scripts/parse-joining-type.mjs';
+import parseSimpleCaseMapping from '../scripts/parse-simple-case-mapping.mjs';
 import {
 	parseBlocks,
 	parseDerivedCoreProperties,
@@ -15,6 +16,16 @@ import {
 	parseScripts,
 } from '../scripts/parse-blocks-scripts-properties.mjs';
 
+test('Historical titlecase digraphs retain their identity mapping', async (t) => {
+	for (const version of ['1.1.5', '2.0.14', '3.2.0', '4.0.0', '18.0.0']) {
+		const { Titlecase, Uppercase } = await parseSimpleCaseMapping(version);
+		for (const cp of [0x1c5, 0x1c8, 0x1cb, 0x1f2]) {
+			t.assert.equal(Titlecase.get(cp) ?? cp, cp, version);
+			t.assert.equal(Uppercase.get(cp), cp - 1, version);
+		}
+	}
+});
+
 test('Block values cover Unicode, including No_Block gaps', async (t) => {
 	for (const version of ['2.0.14', '3.1.0', '18.0.0']) {
 		const blocks = await parseBlocks(version);