From 4ca6101273d84d8d095be254fe4ef0597888567a Mon Sep 17 00:00:00 2001
From: toasted-nutbread <toasted-nutbread@users.noreply.github.com>
Date: Tue, 13 Oct 2020 21:48:21 -0400
Subject: [PATCH] Improve convertKatakanaToHiragana and
 convertHiraganaToKatakana (#916)

---
 ext/mixed/js/japanese.js | 73 ++++++++++++++++++++++++++++++++--------
 1 file changed, 59 insertions(+), 14 deletions(-)

diff --git a/ext/mixed/js/japanese.js b/ext/mixed/js/japanese.js
index 801dec84..ee0ac777 100644
--- a/ext/mixed/js/japanese.js
+++ b/ext/mixed/js/japanese.js
@@ -19,10 +19,16 @@ const jp = (() => {
     const ITERATION_MARK_CODE_POINT = 0x3005;
     const HIRAGANA_SMALL_TSU_CODE_POINT = 0x3063;
     const KATAKANA_SMALL_TSU_CODE_POINT = 0x30c3;
+    const KATAKANA_SMALL_KA_CODE_POINT = 0x30f5;
+    const KATAKANA_SMALL_KE_CODE_POINT = 0x30f6;
     const KANA_PROLONGED_SOUND_MARK_CODE_POINT = 0x30fc;
 
     const HIRAGANA_RANGE = [0x3040, 0x309f];
     const KATAKANA_RANGE = [0x30a0, 0x30ff];
+
+    const HIRAGANA_CONVERSION_RANGE = [0x3041, 0x3096];
+    const KATAKANA_CONVERSION_RANGE = [0x30a1, 0x30f6];
+
     const KANA_RANGES = [HIRAGANA_RANGE, KATAKANA_RANGE];
 
     const CJK_UNIFIED_IDEOGRAPHS_RANGE = [0x4e00, 0x9fff];
@@ -129,6 +135,29 @@ const jp = (() => {
         ['ﾝ', 'ン--']
     ]);
 
+    const VOWEL_TO_KANA_MAPPING = new Map([
+        ['a', 'ぁあかがさざただなはばぱまゃやらゎわヵァアカガサザタダナハバパマャヤラヮワヵヷ'],
+        ['i', 'ぃいきぎしじちぢにひびぴみりゐィイキギシジチヂニヒビピミリヰヸ'],
+        ['u', 'ぅうくぐすずっつづぬふぶぷむゅゆるゥウクグスズッツヅヌフブプムュユルヴ'],
+        ['e', 'ぇえけげせぜてでねへべぺめれゑヶェエケゲセゼテデネヘベペメレヱヶヹ'],
+        ['o', 'ぉおこごそぞとどのほぼぽもょよろをォオコゴソゾトドノホボポモョヨロヲヺ'],
+        ['', 'のノ']
+    ]);
+
+    const KANA_TO_VOWEL_MAPPING = (() => {
+        const map = new Map();
+        for (const [vowel, characters] of VOWEL_TO_KANA_MAPPING) {
+            for (const character of characters) {
+                map.set(character, vowel);
+            }
+        }
+        return map;
+    })();
+
+
+    function isCodePointInRange(codePoint, [min, max]) {
+        return (codePoint >= min && codePoint <= max);
+    }
 
     function isCodePointInRanges(codePoint, ranges) {
         for (const [min, max] of ranges) {
@@ -139,6 +168,17 @@ const jp = (() => {
         return false;
     }
 
+    function getProlongedHiragana(previousCharacter) {
+        switch (KANA_TO_VOWEL_MAPPING.get(previousCharacter)) {
+            case 'a': return 'あ';
+            case 'i': return 'い';
+            case 'u': return 'う';
+            case 'e': return 'え';
+            case 'o': return 'う';
+            default: return null;
+        }
+    }
+
     function getWanakana() {
         try {
             if (typeof wanakana !== 'undefined') {
@@ -219,30 +259,35 @@ const jp = (() => {
         // Conversion functions
 
         convertKatakanaToHiragana(text) {
-            const wanakana = this._getWanakana();
             let result = '';
-            for (const c of text) {
-                if (wanakana.isKatakana(c)) {
-                    result += wanakana.toHiragana(c);
-                } else {
-                    result += c;
+            const offset = (HIRAGANA_CONVERSION_RANGE[0] - KATAKANA_CONVERSION_RANGE[0]);
+            for (let char of text) {
+                const codePoint = char.codePointAt(0);
+                if (codePoint === KATAKANA_SMALL_KA_CODE_POINT || codePoint === KATAKANA_SMALL_KE_CODE_POINT) {
+                    // No change
+                } else if (codePoint === KANA_PROLONGED_SOUND_MARK_CODE_POINT) {
+                    if (result.length > 0) {
+                        const char2 = getProlongedHiragana(result[result.length - 1]);
+                        if (char2 !== null) { char = char2; }
+                    }
+                } else if (isCodePointInRange(codePoint, KATAKANA_CONVERSION_RANGE)) {
+                    char = String.fromCodePoint(codePoint + offset);
                 }
+                result += char;
             }
-
             return result;
         }
 
         convertHiraganaToKatakana(text) {
-            const wanakana = this._getWanakana();
             let result = '';
-            for (const c of text) {
-                if (wanakana.isHiragana(c)) {
-                    result += wanakana.toKatakana(c);
-                } else {
-                    result += c;
+            const offset = (KATAKANA_CONVERSION_RANGE[0] - HIRAGANA_CONVERSION_RANGE[0]);
+            for (let char of text) {
+                const codePoint = char.codePointAt(0);
+                if (isCodePointInRange(codePoint, HIRAGANA_CONVERSION_RANGE)) {
+                    char = String.fromCodePoint(codePoint + offset);
                 }
+                result += char;
             }
-
             return result;
         }