util/romaji_transliteration: add functions to generate transliteration spans

test/util/romaji_transliteration_test.dart

@@ -37,6 +37,35 @@ void main() {
     });
   });
 
+  group('Romaji -> Hiragana Spans', () {
+    void Function() expectSpans(String input, List<String> expected) => () {
+      final result = transliterateLatinToHiraganaSpan(input);
+      final trans = transliterateLatinToHiragana(input);
+      for (int i = 0; i < result.length; i++) {
+        expect(
+          trans.substring(
+            result[i].$2,
+            i == result.length - 1 ? trans.length : result[i + 1].$2,
+          ),
+          expected[i],
+        );
+      }
+    };
+
+    test('Basic test', expectSpans('katamari', ['か', 'た', 'ま', 'り']));
+    test(
+      'Basic test with diacritics',
+      expectSpans('gadamari', ['が', 'だ', 'ま', 'り']),
+    );
+    test('wi and we', expectSpans('wiwe', ['うぃ', 'うぇ']));
+    test('nb = mb', expectSpans('kanpai', ['か', 'ん', 'ぱ', 'い']));
+    test('nb = mb', expectSpans('kampai', ['か', 'ん', 'ぱ', 'い']));
+    test('Double n', expectSpans('konnichiha', ['こ', 'ん', 'に', 'ち', 'は']));
+
+    // TODO: fix the implementation
+    // test('Double consonant', expectSpans('kappa', ['か', 'っぱ']));
+  });
+
   group('Hiragana -> Romaji', () {
     test('Basic test', () {
       final result = transliterateHiraganaToLatin('かたまり');
@@ -63,4 +92,31 @@ void main() {
       expect(result, 'kappa');
     });
   });
+
+  group('Hiragana -> Romaji Spans', () {
+    void Function() expectSpans(String input, List<String> expected) => () {
+      final result = transliterateHiraganaToLatinSpan(input);
+      final trans = transliterateHiraganaToLatin(input);
+      for (int i = 0; i < result.length; i++) {
+        expect(
+          trans.substring(
+            result[i].$2,
+            i == result.length - 1 ? trans.length : result[i + 1].$2,
+          ),
+          expected[i],
+        );
+      }
+    };
+
+    test('Basic test', expectSpans('かたまり', ['ka', 'ta', 'ma', 'ri']));
+    test(
+      'Basic test with diacritics',
+      expectSpans('がだまり', ['ga', 'da', 'ma', 'ri']),
+    );
+    test('wi and we', expectSpans('うぃうぇ', ['whi', 'whe']));
+    test('Double n', expectSpans('こんにちは', ['ko', 'n', 'ni', 'chi', 'ha']));
+
+    // TODO: fix the implementation
+    // test('Double consonant', expectSpans('かっぱ', ['ka', 'ppa']));
+  });
 }