Correct tones for 不 and 一 based on context

不 is usually bù, but if the next character has the fourth tone it will become bú. Example: wǒ bú qù nǐ, wǒ bù xǐ​huan nǐ
一 is usually yī, but if the next character has the first, second or third tone it will become yì. If the next character has the fourth tone, it will become yí. Example: yī. yí gè rén. yì běn shū

src/pinyinify.js

@@ -5,6 +5,14 @@ let segment = require("./segment"),
     tag = require("./tag"),
     { isCharacterText } = require("./util");
 
+const tones = [
+    ['ā','ē','ī','ō','ū','ǖ','Ā','Ē','Ī','Ō','Ū','Ǖ'],
+    ['á','é','í','ó','ú','ǘ','Á','É','Í','Ó','Ú','Ǘ'],
+    ['ǎ','ě','ǐ','ǒ','ǔ','ǚ','Ǎ','Ě','Ǐ','Ǒ','Ǔ','Ǚ'],
+    ['à','è','ì','ò','ù','ǜ','À','È','Ì','Ò','Ù','Ǜ'],
+    ['a','e','i','o','u','ü','A','E','I','O','U','Ü']
+]; //Use tones[tone - 1] to get all possible characters with that tone
+
 function pinyinify(text, isDetailed) {
     let segments = segment(text);
     let pinyinSegments = [];
@@ -159,6 +167,19 @@ function decideAmbiguousChar(char, cuts, cutIndex) {
                 if (afterTag[0] === "v") return "chóng";
             }
             break;
+        case "不":
+            if (afterText.length > 0) {
+                const nextTone = getTone(afterText[0].charAt(0));
+                if (nextTone === 4) return "bú";
+            }
+            break;
+        case "一":
+            if (afterText.length > 0) {
+                const nextTone = getTone(afterText[0].charAt(0));
+                if (nextTone === 1 || nextTone == 2 || nextTone == 3) return "yì";
+                else if (nextTone == 4) return "yí";
+            }
+            break;
     }
 }
 
@@ -176,7 +197,16 @@ function shouldPutSpaceBetween(word1, word2) {
     if (isCharacterText(word1) && punctuationPattern.test(word2)) return false;
     if (isCharacterText(word1) !== isCharacterText(word2)) return true;
     return false;
+}
 
+function getTone(char) {
+    if(typeof pinyinDict[char] === 'undefined') return 0;
+
+    const pinyinChars = [...pinyinDict[char]]; //Getting and destructuring the pinyin string
+    for (i = 0; i < 4; i++) { //Going through the four tones and checking if there is a match
+        if (tones[i].some(c => pinyinChars.includes(c))) return i + 1;
+    }
+    return 5;
 }
 
-module.exports = pinyinify;
+module.exports = pinyinify;