fix(infra): fix fractional indexing edge cases (#8978)

packages/common/infra/src/utils/__tests__/fractional-indexing.spec.ts

@@ -57,3 +57,19 @@ test('no postfix', () => {
     generateFractionalIndexingKeyBetween('a0', 'a01').startsWith('a00V')
   ).toBe(true);
 });
+
+test('edge cases', () => {
+  for (let i = 0; i < 100; i++) {
+    let a = generateFractionalIndexingKeyBetween(null, null);
+    let b = generateFractionalIndexingKeyBetween(null, null);
+    if (a > b) {
+      [a, b] = [b, a];
+    }
+    const c = generateFractionalIndexingKeyBetween(a, b);
+    const d = generateFractionalIndexingKeyBetween(c, b);
+    expect(a < c).toBeTruthy();
+    expect(c < b).toBeTruthy();
+    expect(c < d).toBeTruthy();
+    expect(d < b).toBeTruthy();
+  }
+});

packages/common/infra/src/utils/fractional-indexing.ts

@@ -1,5 +1,9 @@
 import { generateKeyBetween } from 'fractional-indexing';
 
+function hasSamePrefix(a: string, b: string) {
+  return a.startsWith(b) || b.startsWith(a);
+}
+
 /**
  * generate a key between a and b, the result key is always satisfied with a < result < b.
  * the key always has a random suffix, so there is no need to worry about collision.
@@ -59,7 +63,7 @@ export function generateFractionalIndexingKeyBetween(
     return generateKeyBetween(aSubkey, null) + '0' + postfix();
   } else if (aSubkey !== null && bSubkey !== null) {
     // generate a key between a and b
-    if (aSubkey === bSubkey && a !== null && b !== null) {
+    if (hasSamePrefix(aSubkey, bSubkey) && a !== null && b !== null) {
       // conflict, if the subkeys are the same, generate a key between fullkeys
       return generateKeyBetween(a, b) + '0' + postfix();
     } else {