fix(encode): Fix surrogate check (#562)

src/encode-trie.ts

@@ -1,14 +1,23 @@
 import htmlMap from "./maps/entities.json";
 
+const enum Surrogate {
+    Mask = 0b1111_1100_0000_0000,
+    High = 0b1101_1000_0000_0000,
+}
+
+function isHighSurrugate(c: number) {
+    return (c & Surrogate.Mask) === Surrogate.High;
+}
+
 // For compatibility with node < 4, we wrap `codePointAt`
 export const getCodePoint =
     // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
     String.prototype.codePointAt != null
         ? (str: string, index: number): number => str.codePointAt(index)!
         : // http://mathiasbynens.be/notes/javascript-encoding#surrogate-formulae
           (c: string, index: number): number =>
-              (c.charCodeAt(index) & 0xd800) === 0xd800
-                  ? (c.charCodeAt(index) - 0xd800) * 0x400 +
+              isHighSurrugate(c.charCodeAt(index))
+                  ? (c.charCodeAt(index) - Surrogate.High) * 0x400 +
                     c.charCodeAt(index + 1) -
                     0xdc00 +
                     0x10000
@@ -45,7 +54,7 @@ export function encodeHTMLTrieRe(regExp: RegExp, str: string): string {
                 i
             ).toString(16)};`;
             // Increase by 1 if we have a surrogate pair
-            lastIdx = regExp.lastIndex += Number((char & 0xd800) === 0xd800);
+            lastIdx = regExp.lastIndex += Number(isHighSurrugate(char));
         }
     }
 

src/encode.spec.ts

@@ -54,4 +54,11 @@ describe("encodeNonAsciiHTML", () => {
         expect(entities.encodeNonAsciiHTML("😄🍾🥳💥😇")).toBe(
             "😄🍾🥳💥😇"
         ));
+
+    it("should encode chars above surrogates", () =>
+        expect(
+            entities.encodeNonAsciiHTML("♒️♓️♈️♉️♊️♋️♌️♍️♎️♏️♐️♑️")
+        ).toBe(
+            "♒️♓️♈️♉️♊️♋️♌️♍️♎️♏️♐️♑️"
+        ));
 });

src/encode.ts

@@ -41,7 +41,7 @@ export function encodeXML(str: string): string {
             ).toString(16)};`;
             // Increase by 1 if we have a surrogate pair
             lastIdx = xmlReplacer.lastIndex += Number(
-                (char & 0xd800) === 0xd800
+                (char & 0b1111_1111_1000_0000) === 0xd800
             );
         }
     }