fix: preserve Unicode code points at acronym boundaries

src/index.ts

@@ -60,12 +60,14 @@ export function splitByCase<
         continue;
       }
       // Case falling edge
-      if (previousUpper === true && isUpper === false && buff.length > 1) {
-        const lastChar = buff.at(-1);
-        parts.push(buff.slice(0, Math.max(0, buff.length - 1)));
-        buff = lastChar + char;
-        previousUpper = isUpper;
-        continue;
+      if (previousUpper === true && isUpper === false) {
+        const lastChar = [...buff].at(-1)!;
+        if (buff.length > lastChar.length) {
+          parts.push(buff.slice(0, -lastChar.length));
+          buff = lastChar + char;
+          previousUpper = isUpper;
+          continue;
+        }
       }
     }
 

test/scule.test.ts

@@ -158,3 +158,14 @@ describe("flatCase", () => {
     expect(flatCase(input)).toMatchObject(expected);
   });
 });
+
+describe("splitByCase astral case boundaries", () => {
+  test.each([
+    ["𐐀foo", ["𐐀foo"]],
+    ["𐐀𐐁foo", ["𐐀", "𐐁foo"]],
+    ["A𐐀foo", ["A", "𐐀foo"]],
+    ["foo𐐀bar", ["foo", "𐐀bar"]],
+  ])("keeps Unicode code points intact in %s", (input, expected) => {
+    expect(splitByCase(input as string)).toEqual(expected);
+  });
+});