public IndexEntry lookupIndex(final long logIndex) {
        mapInIfNecessary();
        this.readLock.lock();
        try {
            final ByteBuffer byteBuffer = sliceByteBuffer();
            final int slot = (int) (logIndex - this.header.getFirstLogIndex());
            if (slot < 0) {
                return IndexEntry.newInstance();
            } else {
                return parseEntry(byteBuffer, slot);
            }
        } finally {
            this.readLock.unlock();
        }
    }
Find the index entry
@param  logIndex the target log index
@return a pair holding this offset and its physical file position.
jraft-extension/java-log-storage-impl/src/main/java/com/alipay/sofa/jraft/storage/file/index/IndexFile.java