public static string HiraganaToKatakana(string input)
{
if (input is null)
{
throw new ArgumentNullException(nameof(input));
}
var builder = new StringBuilder();
foreach (var character in input)
{
// Short circuit to avoid incorrect codeshift for 'ー' and '・'
if (SpecialCharacterChecker.IsLongDash(character) || SpecialCharacterChecker.IsSlashDot(character))
{
builder.Append(character);
}
else if (HiraganaChecker.IsHiragana(character))
{
// Shift charcode.
var katakanaCode = (character - CharacterRanges.HiraganaStart) + CharacterRanges.KatakanaStart;
var katakanaCharacter = (char)katakanaCode;
builder.Append(katakanaCharacter);
}
else
{
// Pass non-hiragana chars through
builder.Append(character);
}
}
return(builder.ToString());
}
public static string KatakanaToHiragana(string input, bool isDestinationRomaji = false)
{
if (input is null)
{
throw new ArgumentNullException(nameof(input));
}
char?previousKana = null;
var builder = new StringBuilder();
for (int index = 0; index < input.Length; index++)
{
var character = input[index];
// Short circuit to avoid incorrect codeshift for 'ー' and '・'
if (SpecialCharacterChecker.IsSlashDot(character) || IsInitialLongDash(character, index) || IsKanaAsSymbol(character))
{
builder.Append(character);
// Transform long vowels: 'オー' to 'おう'
}
else if (previousKana != null && IsInnerLongDash(character, index))
{
// Transform previousKana back to romaji, and slice off the vowel
var romaji = RomajiConverter.ToRomaji(previousKana.ToString()).Last();
// However, ensure 'オー' => 'おお' => 'oo' if this is a transform on the way to romaji
if (KatakanaChecker.IsKatakana(input[index - 1]) && romaji == 'o' && isDestinationRomaji)
{
builder.Append('お');
}
else
{
builder.Append(_longVowels[romaji]);
}
}
else if (!SpecialCharacterChecker.IsLongDash(character) && KatakanaChecker.IsKatakana(character))
{
// Shift charcode.
var hiraganaCode = (character - CharacterRanges.KatakanaStart) + CharacterRanges.HiraganaStart;
var hiragana = (char)hiraganaCode;
previousKana = hiragana;
builder.Append(hiragana);
}
else
{
// Pass non katakana chars through
previousKana = null;
builder.Append(character);
}
}
return(builder.ToString());
}
private static bool IsInnerLongDash(char character, int index) => SpecialCharacterChecker.IsLongDash(character) && index > 0;