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| 1 | +// Copyright 2021-Present Datadog, Inc. |
| 2 | +// |
| 3 | +// Licensed under the Apache License, Version 2.0 (the "License"); |
| 4 | +// you may not use this file except in compliance with the License. |
| 5 | +// You may obtain a copy of the License at |
| 6 | +// |
| 7 | +// http://www.apache.org/licenses/LICENSE-2.0 |
| 8 | +// |
| 9 | +// Unless required by applicable law or agreed to in writing, software |
| 10 | +// distributed under the License is distributed on an "AS IS" BASIS, |
| 11 | +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 12 | +// See the License for the specific language governing permissions and |
| 13 | +// limitations under the License. |
| 14 | + |
| 15 | +use std::fmt::{Debug, Formatter}; |
| 16 | + |
| 17 | +/// Fixed-size buffer that keeps the last N elements pushed into it. |
| 18 | +/// |
| 19 | +/// `head` is the write cursor. It advances by one on each push and wraps |
| 20 | +/// back to 0 when it reaches N, overwriting the oldest element. |
| 21 | +/// |
| 22 | +/// ```text |
| 23 | +/// RingBuffer<u32, 4> after pushing 1, 2, 3, 4, 5, 6: |
| 24 | +/// |
| 25 | +/// buffer = [5, 6, 3, 4] head = 2 len = 4 |
| 26 | +/// ^ |
| 27 | +/// next write goes here |
| 28 | +/// |
| 29 | +/// logical view (oldest → newest): [3, 4, 5, 6] |
| 30 | +/// ``` |
| 31 | +pub struct RingBuffer<T: Copy + Default, const N: usize> { |
| 32 | + buffer: [T; N], |
| 33 | + head: usize, |
| 34 | + len: usize, |
| 35 | +} |
| 36 | + |
| 37 | +impl<T: Copy + Default, const N: usize> Default for RingBuffer<T, N> { |
| 38 | + fn default() -> Self { |
| 39 | + Self { |
| 40 | + buffer: [T::default(); N], |
| 41 | + head: 0, |
| 42 | + len: 0, |
| 43 | + } |
| 44 | + } |
| 45 | +} |
| 46 | + |
| 47 | +impl<T: Copy + Default + Debug, const N: usize> Debug for RingBuffer<T, N> { |
| 48 | + fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result { |
| 49 | + f.debug_list().entries(self.iter()).finish() |
| 50 | + } |
| 51 | +} |
| 52 | + |
| 53 | +impl<T: Copy + Default, const N: usize> RingBuffer<T, N> { |
| 54 | + pub fn push_back(&mut self, value: T) { |
| 55 | + self.buffer[self.head] = value; |
| 56 | + self.head = (self.head + 1) % N; |
| 57 | + if self.len < N { |
| 58 | + self.len += 1; |
| 59 | + } |
| 60 | + } |
| 61 | + |
| 62 | + pub fn last(&self) -> Option<T> { |
| 63 | + if self.len == 0 { |
| 64 | + return None; |
| 65 | + } |
| 66 | + Some(self.buffer[(self.head + N - 1) % N]) |
| 67 | + } |
| 68 | + |
| 69 | + pub fn front(&self) -> Option<T> { |
| 70 | + if self.len == 0 { |
| 71 | + return None; |
| 72 | + } |
| 73 | + Some(self.buffer[(self.head + N - self.len) % N]) |
| 74 | + } |
| 75 | + |
| 76 | + pub fn len(&self) -> usize { |
| 77 | + self.len |
| 78 | + } |
| 79 | + |
| 80 | + pub fn is_empty(&self) -> bool { |
| 81 | + self.len == 0 |
| 82 | + } |
| 83 | + |
| 84 | + pub fn iter(&self) -> impl Iterator<Item = &T> + '_ { |
| 85 | + let start = (self.head + N - self.len) % N; |
| 86 | + (0..self.len).map(move |i| &self.buffer[(start + i) % N]) |
| 87 | + } |
| 88 | +} |
| 89 | + |
| 90 | +#[cfg(test)] |
| 91 | +mod tests { |
| 92 | + use super::*; |
| 93 | + |
| 94 | + #[test] |
| 95 | + fn test_empty() { |
| 96 | + let rb = RingBuffer::<u32, 4>::default(); |
| 97 | + assert!(rb.is_empty()); |
| 98 | + assert_eq!(rb.len(), 0); |
| 99 | + assert_eq!(rb.last(), None); |
| 100 | + assert_eq!(rb.front(), None); |
| 101 | + assert_eq!(rb.iter().count(), 0); |
| 102 | + } |
| 103 | + |
| 104 | + #[test] |
| 105 | + fn test_single_push() { |
| 106 | + let mut rb = RingBuffer::<u32, 4>::default(); |
| 107 | + rb.push_back(10); |
| 108 | + assert_eq!(rb.len(), 1); |
| 109 | + assert!(!rb.is_empty()); |
| 110 | + assert_eq!(rb.last(), Some(10)); |
| 111 | + assert_eq!(rb.front(), Some(10)); |
| 112 | + assert_eq!(rb.iter().copied().collect::<Vec<_>>(), vec![10]); |
| 113 | + } |
| 114 | + |
| 115 | + #[test] |
| 116 | + fn test_partial_fill() { |
| 117 | + let mut rb = RingBuffer::<u32, 4>::default(); |
| 118 | + rb.push_back(1); |
| 119 | + rb.push_back(2); |
| 120 | + rb.push_back(3); |
| 121 | + assert_eq!(rb.len(), 3); |
| 122 | + assert_eq!(rb.last(), Some(3)); |
| 123 | + assert_eq!(rb.front(), Some(1)); |
| 124 | + assert_eq!(rb.iter().copied().collect::<Vec<_>>(), vec![1, 2, 3]); |
| 125 | + } |
| 126 | + |
| 127 | + #[test] |
| 128 | + fn test_exactly_full() { |
| 129 | + let mut rb = RingBuffer::<u32, 4>::default(); |
| 130 | + for i in 1..=4 { |
| 131 | + rb.push_back(i); |
| 132 | + } |
| 133 | + assert_eq!(rb.len(), 4); |
| 134 | + assert_eq!(rb.last(), Some(4)); |
| 135 | + assert_eq!(rb.front(), Some(1)); |
| 136 | + assert_eq!(rb.iter().copied().collect::<Vec<_>>(), vec![1, 2, 3, 4]); |
| 137 | + } |
| 138 | + |
| 139 | + #[test] |
| 140 | + fn test_wrap_around() { |
| 141 | + let mut rb = RingBuffer::<u32, 4>::default(); |
| 142 | + for i in 1..=6 { |
| 143 | + rb.push_back(i); |
| 144 | + } |
| 145 | + assert_eq!(rb.len(), 4); |
| 146 | + assert_eq!(rb.last(), Some(6)); |
| 147 | + assert_eq!(rb.front(), Some(3)); |
| 148 | + assert_eq!(rb.iter().copied().collect::<Vec<_>>(), vec![3, 4, 5, 6]); |
| 149 | + } |
| 150 | + |
| 151 | + #[test] |
| 152 | + fn test_many_wraps() { |
| 153 | + let mut rb = RingBuffer::<u32, 3>::default(); |
| 154 | + for i in 1..=100 { |
| 155 | + rb.push_back(i); |
| 156 | + } |
| 157 | + assert_eq!(rb.len(), 3); |
| 158 | + assert_eq!(rb.last(), Some(100)); |
| 159 | + assert_eq!(rb.front(), Some(98)); |
| 160 | + assert_eq!(rb.iter().copied().collect::<Vec<_>>(), vec![98, 99, 100]); |
| 161 | + } |
| 162 | + |
| 163 | + #[test] |
| 164 | + fn test_debug() { |
| 165 | + let mut rb = RingBuffer::<u32, 3>::default(); |
| 166 | + rb.push_back(1); |
| 167 | + rb.push_back(2); |
| 168 | + assert_eq!(format!("{:?}", rb), "[1, 2]"); |
| 169 | + } |
| 170 | +} |
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