server/src/streaming/utils/head_tail_buf.rs

/* Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

#[derive(Default)]
pub struct HeadTailBuffer<T>
where
    T: Default + Eq + Copy,
{
    data: [T; 2],
    head_index: usize,
}

impl<T> HeadTailBuffer<T>
where
    T: Default + Eq + Copy,
{
    pub fn new() -> Self {
        HeadTailBuffer {
            data: [Default::default(); 2],
            head_index: 0,
        }
    }

    pub fn push(&mut self, element: T) {
        self.data[(self.head_index + 1) % 2] = element;
        self.head_index = (self.head_index + 1) % 2;
    }

    pub fn tail(&self) -> Option<T> {
        if self.head_index == 0 && self.data[1] == Default::default() {
            None
        } else {
            // If the tail is empty return None
            if self.data[self.head_index] == Default::default() {
                return None;
            }
            Some(self.data[(self.head_index + 1) % 2])
        }
    }
}