1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
// This file is part of Substrate.

// Copyright (C) Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0

// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program. If not, see <https://www.gnu.org/licenses/>.

use chrono::{Datelike, Timelike};
use std::{cell::RefCell, fmt::Write, time::SystemTime};
use tracing_subscriber::fmt::time::FormatTime;

/// A structure which, when `Display`d, will print out the current local time.
#[derive(Debug, Clone, Copy, Eq, PartialEq, Default)]
pub struct FastLocalTime {
	/// Decides whenever the fractional timestamp with be included in the output.
	///
	/// If `false` the output will match the following `chrono` format string:
	///   `%Y-%m-%d %H:%M:%S`
	///
	/// If `true` the output will match the following `chrono` format string:
	///   `%Y-%m-%d %H:%M:%S%.3f`
	pub with_fractional: bool,
}

// This is deliberately slightly larger than we actually need, just in case.
const TIMESTAMP_MAXIMUM_LENGTH: usize = 32;

#[derive(Default)]
struct InlineString {
	buffer: [u8; TIMESTAMP_MAXIMUM_LENGTH],
	length: usize,
}

impl Write for InlineString {
	fn write_str(&mut self, s: &str) -> std::fmt::Result {
		let new_length = self.length + s.len();
		assert!(
			new_length <= TIMESTAMP_MAXIMUM_LENGTH,
			"buffer overflow when formatting the current timestamp"
		);

		self.buffer[self.length..new_length].copy_from_slice(s.as_bytes());
		self.length = new_length;
		Ok(())
	}
}

impl InlineString {
	fn as_str(&self) -> &str {
		// SAFETY: this is safe since the only place we append to the buffer
		//         is in `write_str` from an `&str`
		unsafe { std::str::from_utf8_unchecked(&self.buffer[..self.length]) }
	}
}

#[derive(Default)]
struct CachedTimestamp {
	buffer: InlineString,
	last_regenerated_at: u64,
	last_fractional: u32,
}

thread_local! {
	static TIMESTAMP: RefCell<CachedTimestamp> = Default::default();
}

impl FormatTime for FastLocalTime {
	fn format_time(&self, w: &mut dyn Write) -> std::fmt::Result {
		const TIMESTAMP_PARTIAL_LENGTH: usize = "0000-00-00 00:00:00".len();

		let elapsed = SystemTime::now()
			.duration_since(SystemTime::UNIX_EPOCH)
			.expect("system time is never before UNIX epoch; qed");
		let unix_time = elapsed.as_secs();

		TIMESTAMP.with(|cache| {
			let mut cache = cache.borrow_mut();

			// Regenerate the timestamp only at most once each second.
			if cache.last_regenerated_at != unix_time {
				let ts = chrono::Local::now();
				let fractional = (ts.nanosecond() % 1_000_000_000) / 1_000_000;
				cache.last_regenerated_at = unix_time;
				cache.last_fractional = fractional;
				cache.buffer.length = 0;

				write!(
					&mut cache.buffer,
					"{:04}-{:02}-{:02} {:02}:{:02}:{:02}.{:03}",
					ts.year(),
					ts.month(),
					ts.day(),
					ts.hour(),
					ts.minute(),
					ts.second(),
					fractional
				)?;
			} else if self.with_fractional {
				let fractional = elapsed.subsec_millis();

				// Regenerate the fractional part at most once each millisecond.
				if cache.last_fractional != fractional {
					cache.last_fractional = fractional;
					cache.buffer.length = TIMESTAMP_PARTIAL_LENGTH + 1;
					write!(&mut cache.buffer, "{:03}", fractional)?;
				}
			}

			let mut slice = cache.buffer.as_str();
			if !self.with_fractional {
				slice = &slice[..TIMESTAMP_PARTIAL_LENGTH];
			}

			w.write_str(slice)
		})
	}
}

impl std::fmt::Display for FastLocalTime {
	fn fmt(&self, w: &mut std::fmt::Formatter) -> std::fmt::Result {
		self.format_time(w)
	}
}

#[test]
fn test_format_fast_local_time() {
	assert_eq!(
		chrono::Local::now().format("%Y-%m-%d %H:%M:%S").to_string().len(),
		FastLocalTime { with_fractional: false }.to_string().len()
	);
	assert_eq!(
		chrono::Local::now().format("%Y-%m-%d %H:%M:%S%.3f").to_string().len(),
		FastLocalTime { with_fractional: true }.to_string().len()
	);

	// A simple trick to make sure this test won't randomly fail if we so happen
	// to land on the exact moment when we tick over to the next second.
	let now_1 = FastLocalTime { with_fractional: false }.to_string();
	let expected = chrono::Local::now().format("%Y-%m-%d %H:%M:%S").to_string();
	let now_2 = FastLocalTime { with_fractional: false }.to_string();

	assert!(
		now_1 == expected || now_2 == expected,
		"'{}' or '{}' should have been equal to '{}'",
		now_1,
		now_2,
		expected
	);
}