Fix off-by-one error in CelExprFactory

PiperOrigin-RevId: 842335394

Author: Stephen Roberts

common/src/main/java/dev/cel/common/ast/CelExprFactory.java

@@ -266,7 +266,7 @@ protected long nextExprId() {
 
   /** Attempts to decrement the next expr ID if possible. */
   protected void maybeDeleteId(long id) {
-    if (id == exprId - 1) {
+    if (id == exprId) {
       exprId--;
     }
   }

common/src/test/java/dev/cel/common/ast/CelExprFactoryTest.java

@@ -37,4 +37,30 @@ public void nextExprId_startingDefaultIsOne() {
     assertThat(exprFactory.nextExprId()).isEqualTo(1L);
     assertThat(exprFactory.nextExprId()).isEqualTo(2L);
   }
+
+  @Test
+  public void maybeDeleteId_deletesLastId() {
+    CelExprFactory exprFactory = CelExprFactory.newInstance();
+    long id1 = exprFactory.nextExprId(); // 1
+    assertThat(id1).isEqualTo(1L);
+
+    exprFactory.maybeDeleteId(id1);
+
+    // Should be reused
+    assertThat(exprFactory.nextExprId()).isEqualTo(1L);
+  }
+
+  @Test
+  public void maybeDeleteId_doesNotDeletePreviouslyAllocatedId() {
+    CelExprFactory exprFactory = CelExprFactory.newInstance();
+    long id1 = exprFactory.nextExprId(); // 1
+    long id2 = exprFactory.nextExprId(); // 2
+
+    // Try to delete id1. Since id2 was allocated after, it should NOT delete id1
+    // because that would rewind the counter and cause a collision with id2.
+    exprFactory.maybeDeleteId(id1);
+
+    // Should NOT be reused. Next should be 3.
+    assertThat(exprFactory.nextExprId()).isEqualTo(3L);
+  }
 }