protected int index(Object key, Object obj, int ignoreThisSlot) {
    HashIndexSetProperties metaData = hashIndexSetProperties;
    int hash;
    int pos;
    Object[] set = metaData.set;
    int mask = metaData.mask;
    Object curr;
    hash = computeHash(key);

    /*
     * Code originated from fastutils Copyright (C) 2002-2014 Sebastiano Vigna
     *
     * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
     * except in compliance with the License. You may obtain a copy of the License at
     *
     * http://www.apache.org/licenses/LICENSE-2.0
     */
    if (!((curr = set[pos = (it.unimi.dsi.fastutil.HashCommon.mix(hash)) & mask]) == null
        || curr == REMOVED)) {
      if (((curr).equals(obj) && pos != ignoreThisSlot)) {
        return pos;
      }
      while (!((curr = set[pos = (pos + 1) & mask]) == null || curr == REMOVED)) {
        if (((curr).equals(obj)) && pos != ignoreThisSlot) {
          return pos;
        }
      }
    }
    return -1;
  }
Locates index slot of object using the provided key (in this case we are passing in old key)

@return the indexSlot of the given key/object combination
geode-core/src/main/java/org/apache/geode/cache/query/internal/index/HashIndexSet.java