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//! A tool for use with the `hdrhistogram` crate, a rust port of HdrHistogram, that provides
//! a clean interface with sane defaults for off-thread serialization of HdrHistogram interval
//! logs to file.
//!
// there's a bunch of places we .clone() what is either a &'static str or
// a SmolStr and it would be pretty tedious to work around the warning
#![cfg_attr(not(feature = "smol_str"), allow(noop_method_call))]
use std::sync::Arc;
use std::time::{Duration, SystemTime, UNIX_EPOCH};
use std::path::{Path, PathBuf};
use std::thread::{self, JoinHandle};
use std::io;
use std::{mem, fs};
use hdrhistogram::{Histogram};
use hdrhistogram::serialization::V2DeflateSerializer;
use hdrhistogram::serialization::interval_log::{IntervalLogWriterBuilder};
use crossbeam_channel as channel;
use chrono::Utc;
#[cfg(feature = "smol_str")]
use smol_str::SmolStr;
// NOTE: `minstant::Instant` is quite a bit faster on x86/x86_64 cpus, but the
// its fallback behavior of using `MONOTONIC_COARSE` when TSC is not available
// is not a good fit for use with `histlog`. Therefore, even if the "minstant"
// feature is enabled, we only use `minstant::Instant` on linux + x86/x86_64.
#[cfg(all(feature = "minstant", target_os = "linux", any(target_arch = "x86", target_arch = "x86_64")))]
use minstant::Instant;
#[cfg(not(all(feature = "minstant", target_os = "linux", any(target_arch = "x86", target_arch = "x86_64"))))]
use std::time::Instant;
/// Type of value recorded to the hdrhistogram
pub type C = u64;
#[cfg(not(feature = "smol_str"))]
pub type SeriesName = &'static str;
#[cfg(feature = "smol_str")]
pub type SeriesName = SmolStr;
#[cfg(not(feature = "smol_str"))]
pub type Tag = &'static str;
#[cfg(feature = "smol_str")]
pub type Tag = SmolStr;
/// Significant figure passed to `hdrhistogram::Histogram::new` upon
/// construction
pub const DEFAULT_SIG_FIG: u8 = 3;
pub const DEFAULT_MAX: u64 = 1_000_000_000;
/// Capacity of `crossbeam_channel::bounded` queue used to communicate
/// between the measuring thread and the writer thread
pub const CHANNEL_SIZE: usize = 8;
/// Amount of time `HistLog::drop` will spin on a full channel to
/// the writer thread to send a terminate signal
pub const DROP_DEADLINE: Duration = Duration::from_millis(100);
/// Returns `Duration` as number of nanoseconds (`u64`)
///
/// # Examples
/// ```
/// # use std::time::*;
/// assert_eq!(histlog::nanos(Duration::from_secs(1)), 1_000_000_000u64);
/// ```
pub fn nanos(d: Duration) -> u64 {
d.as_secs() * 1_000_000_000_u64 + (d.subsec_nanos() as u64)
}
/// Provides off-thread serialization of HdrHistogram interval logs to file.
///
/// # Purpose
///
/// HdrHistogram is often used to measure latency. Generally, if something is important
/// enough to measure latency, it's unlikely you want to write to a file on the same
/// thread.
///
/// One option would be to serialize to an in-memory buffer (e.g. `Vec<u8>`). However,
/// this would still require allocating to the buffer, and would eventually require a
/// lot of memory for a long-running process.
///
/// `HistLog` allows the hot thread to pass off it's `hdrhistogram::Histogram` at regular intervals
/// to a designated writer thread that can afford to dilly dally with IO. The interval
/// log is written incrementally and can be inspected and analyzed while the program
/// is still running.
///
/// `HistLog` relies completely on the rust port of `HdrHistogram`, both for the in-memory
/// recording of values and serialization. What it does provide is off-thread writing with
/// a clean interface and sane defaults that make it relatively easy to use.
///
/// # Examples
///
/// A `HistLog` has a "series" name and a "tag." The HdrHistogram interval log format provides
/// for one tag per entry. The series name is used to name the file the interval log is written to:
///
/// ```
/// use std::time::*;
///
/// let log_dir = "/tmp/path/to/logs";
/// let series = "server-latency"; // used to name the log file
/// let tag = "xeon-e7-8891-v2"; // recorded with each entry
/// let freq = Duration::from_secs(1); // how often results sent to writer thread
///
/// // `HistLog::new` could fail creating file, `hdrhistogram::Histogram`
/// let mut server1 = histlog::HistLog::new(log_dir, series, tag, freq).unwrap();
///
/// // use `HistLog::clone_with_tag` to serialize a separate tag to same file.
/// let mut server2 = server1.clone_with_tag("xeon-e5-2670");
///
/// for i in 0..1000u64 { // dummy data
/// server1.record(i).unwrap(); // call to `hdrhistogram::Histogram::record` could fail
/// server2.record(i * 2).unwrap();
/// }
///
/// assert_eq!(server1.path(), server2.path()); // both being saved to same file, via same writer thread
/// ```
///
/// `HistLog`'s api design is built for event loops. Each iteration of the loop, new values are
/// recorded, and the current time is checked to see whether the current `Histogram` should be
/// passed off to the writer thread:
///
/// ```
/// use std::time::*;
/// #[cfg(feature = "minstant")]
/// use minstant::Instant;
///
/// let mut spintime = histlog::HistLog::new("/tmp/var/hist", "spintime", "main", Duration::from_secs(60)).unwrap();
///
/// let mut loop_time = Instant::now();
/// let mut prev: Instant;
///
/// loop {
/// prev = loop_time;
/// loop_time = Instant::now();
/// spintime.record(histlog::nanos(loop_time - prev)).unwrap(); // nanos: Duration -> u64
/// spintime.check_send(loop_time); // sends to writer thread if elapsed > freq,
/// // or...
/// spintime.check_try_send(loop_time).unwrap(); // non-blocking equivalent (can fail)
///
/// // do important stuff ...
///
/// # break
/// }
/// ```
///
/// # Logs
///
/// Logs are saved to `<log dir>/<series name>.<datetime>.hdrhistogram-interval-log.v2.gz`.
///
/// Format of log is like this:
///
/// ```console,ignore
/// #[StartTime: 1544631293.283 (seconds since epoch)]
/// #[BaseTime: 0.000 (seconds since epoch)]
/// Tag=xeon-e7-8891-v2,1544631293.283,0.003,999.000,HISTFAAAAC94Ae3GMRUAMAgD0bRI6FovNVcHmGREAgNR [...]
/// Tag=xeon-e5-2670,1544631293.283,0.003,999.000,HISTFAAAABx4AZNpmSzMwMDAxAABzFCaEUoz2X+AsQA/awK [...]
/// [...]
/// ```
///
/// Only the histogram data is compressed (deflate), so a `.gz` extension is perhaps misleading.
///
/// Log file can be viewed/analyzed [here](https://hdrhistogram.github.io/HdrHistogramJSDemo/logparser.html)
/// (javascript, runs locally) or with the Java-based [HistogramLogAnalyzer](https://github.com/HdrHistogram/HistogramLogAnalyzer).
///
/// [Full documentation](https://docs.rs/hdrhistogram/6.1.1/hdrhistogram/serialization/interval_log/index.html) of log
/// serialization available from the `hdrhistogram` crate.
///
/// # Features
///
/// - `minstant`: use `minstant::Instant` as a faster replacement for `std::time::Instant`
/// - `smol_str`: switch `&'static str` for `smol_str::SmolStr` for the `SeriesName` and `Tag`
/// types, allowing dynamic string values to be provided instead of static strings.
///
/// # Limitations
///
/// - `HistLog::check_send` and `HistLog::check_try_send` create a new `hdrhistogram::Histogram`
/// and send the current/prev one to the writer thread each interval. Internally, an
/// `hdrhistogram::Histogram` uses a `Vec` to store its counts, so there's an allocation involved.
/// - Only `u64` values can be recorded, currently.
///
pub struct HistLog {
filename: PathBuf,
series: SeriesName,
tag: Tag,
freq: Duration,
last_sent: Instant,
tx: channel::Sender<Option<Entry>>,
hist: Histogram<C>,
thread: Option<Arc<thread::JoinHandle<Result<usize, Error>>>>,
}
struct Entry {
pub tag: Tag,
pub start: SystemTime,
pub end: SystemTime,
pub hist: Histogram<C>,
}
/// Unifies all the errors that might occur from using a `HistLog` in one enum.
#[derive(Debug)]
pub enum Error {
Io(io::Error),
//HdrCreation(hdrhistogram::errors::CreationError),
HdrRecord(hdrhistogram::errors::RecordError),
TrySend(channel::TrySendError<()>),
}
impl std::fmt::Display for Error {
fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
match self {
Error::Io(e) => write!(f, "Io({e})"),
Error::HdrRecord(e) => write!(f, "HdrRecord({e})"),
Error::TrySend(e) => write!(f, "TrySend({e})"),
}
}
}
impl std::error::Error for Error {}
impl From<io::Error> for Error {
fn from(err: io::Error) -> Self {
Error::Io(err)
}
}
impl Clone for HistLog {
fn clone(&self) -> Self {
let thread = self.thread.as_ref().map(Arc::clone);
Self {
filename: self.filename.clone(),
series: self.series.clone(),
tag: self.tag.clone(),
freq: self.freq,
last_sent: Instant::now(),
tx: self.tx.clone(),
hist: self.hist.clone(),
thread,
}
}
}
impl HistLog {
/// Create a new `HistLog`.
///
/// If `save_dir` does not exist, will attempt to create it (which could
/// fail). Creating a new log file could fail. Spawning the writer thread could fail.
///
/// Default significant figures of `3` is used.
#[cfg(not(feature = "smol_str"))]
pub fn new<P>(save_dir: P, series: SeriesName, tag: Tag, freq: Duration) -> Result<Self, Error>
where P: AsRef<Path>
{
Self::new_with_sig_fig(DEFAULT_SIG_FIG, save_dir, series, tag, freq)
}
/// Create a new `HistLog`.
///
/// If `save_dir` does not exist, will attempt to create it (which could
/// fail). Creating a new log file could fail. Spawning the writer thread could fail.
///
/// Default significant figures of `3` is used.
#[cfg(feature = "smol_str")]
pub fn new<P, S, T>(save_dir: P, series: S, tag: T, freq: Duration) -> Result<Self, Error>
where P: AsRef<Path>,
S: AsRef<str>,
T: AsRef<str>
{
let series = SmolStr::new(series.as_ref());
let tag = SmolStr::new(tag.as_ref());
Self::new_with_sig_fig(DEFAULT_SIG_FIG, save_dir, series, tag, freq)
}
/// Create a new `HistLog`, specifying the number of significant digits
#[cfg(not(feature = "smol_str"))]
pub fn new_with_sig_fig<P>(
sig_fig: u8,
save_dir: P,
series: SeriesName,
tag: Tag,
freq: Duration,
) -> Result<Self, Error>
where P: AsRef<Path>
{
Self::new_with_max(DEFAULT_MAX, sig_fig, save_dir, series, tag, freq)
}
/// Create a new `HistLog`, specifying the number of significant digits
#[cfg(feature = "smol_str")]
pub fn new_with_sig_fig<P, S, T>(
sig_fig: u8,
save_dir: P,
series: S,
tag: T,
freq: Duration,
) -> Result<Self, Error>
where P: AsRef<Path>,
S: AsRef<str>,
T: AsRef<str>
{
let series = SmolStr::new(series.as_ref());
let tag = SmolStr::new(tag.as_ref());
Self::new_with_max(DEFAULT_MAX, sig_fig, save_dir, series, tag, freq)
}
/// Create a new `HistLog`, specifying the max value and number of significant digits
#[cfg(not(feature = "smol_str"))]
pub fn new_with_max<P>(
max: u64,
sig_fig: u8,
save_dir: P,
series: SeriesName,
tag: Tag,
freq: Duration,
) -> Result<Self, Error>
where P: AsRef<Path>
{
let hist = Histogram::new_with_max(max, sig_fig).expect("Histogram::new_with_max");
Self::new_from(&hist, save_dir, series, tag, freq)
}
/// Create a new `HistLog`, specifying the max value and number of significant digits
#[cfg(feature = "smol_str")]
pub fn new_with_max<P, S, T>(
max: u64,
sig_fig: u8,
save_dir: P,
series: S,
tag: T,
freq: Duration,
) -> Result<Self, Error>
where P: AsRef<Path>,
S: AsRef<str>,
T: AsRef<str>
{
let series = SmolStr::new(series.as_ref());
let tag = SmolStr::new(tag.as_ref());
let hist = Histogram::new_with_max(max, sig_fig).expect("Histogram::new_with_max");
Self::new_from(&hist, save_dir, series, tag, freq)
}
/// Create a new `HistLog` by providing a template `hdrhistogram::Histogram` for it to use in
/// recording the interval logs.
#[cfg(not(feature = "smol_str"))]
pub fn new_from<P>(
template_hist: &Histogram<C>,
save_dir: P,
series: SeriesName,
tag: Tag,
freq: Duration,
) -> Result<Self, Error>
where P: AsRef<Path>
{
let save_dir = save_dir.as_ref().to_path_buf();
let filename = Self::get_filename(&save_dir, series);
let (tx, rx) = channel::bounded(CHANNEL_SIZE);
let thread = Some(Arc::new(Self::scribe(series, rx, filename.as_path())?));
let last_sent = Instant::now();
let hist = Histogram::new_from(template_hist);
Ok(Self { filename, series, tag, freq, last_sent, tx, hist, thread })
}
/// Create a new `HistLog` by providing a template `hdrhistogram::Histogram` for it to use in
/// recording the interval logs.
#[cfg(feature = "smol_str")]
pub fn new_from<P, S, T>(
template_hist: &Histogram<C>,
save_dir: P,
series: S,
tag: T,
freq: Duration,
) -> Result<Self, Error>
where P: AsRef<Path>,
S: AsRef<str>,
T: AsRef<str>
{
let series = SmolStr::new(series.as_ref());
let tag = SmolStr::new(tag.as_ref());
let save_dir = save_dir.as_ref().to_path_buf();
let scribe_series = series.clone();
let filename = Self::get_filename(&save_dir, &series);
let (tx, rx) = channel::bounded(CHANNEL_SIZE);
let thread = Some(Arc::new(Self::scribe(scribe_series, rx, filename.as_path())?));
let last_sent = Instant::now();
let hist = Histogram::new_from(template_hist);
Ok(Self { filename, series, tag, freq, last_sent, tx, hist, thread })
}
// not sure if this is a good thing to have
//
#[doc(hidden)]
pub fn new_with_tag(&self, tag: Tag) -> Result<Self, Error> {
let mut save_dir = self.filename.clone();
if !save_dir.pop() { // `.pop` should remove the file name, leaving dir
return Err(Error::Io(io::Error::new(io::ErrorKind::Other,
"`filename.pop()` returned `false`! expected it to have a file name, return `true`.")))
}
Self::new(save_dir, self.series.clone(), tag, self.freq)
}
/// Returns the path of the log file the `HistLog` is writing to.
///
pub fn path(&self) -> &Path { self.filename.as_path() }
/// Record a new histogram with a `tag` that will serialize to the
/// same interval log file as its parent. Each cloned `HistLog`'s entries
/// will be written to their own lines in the log file, identifiable by tag.
///
/// # Limitations
///
/// No effort is made to check whether `tag` is a duplicate of a previous tag,
/// and using a duplicate may produce unexpected results.
#[cfg(not(feature = "smol_str"))]
pub fn clone_with_tag(&self, tag: Tag) -> Self {
self.inner_clone_with_tag(tag)
}
#[cfg(feature = "smol_str")]
pub fn clone_with_tag<T: AsRef<str>>(&self, tag: T) -> Self {
let tag = SmolStr::new(tag.as_ref());
self.inner_clone_with_tag(tag)
}
fn inner_clone_with_tag(&self, tag: Tag) -> Self {
assert!(self.thread.is_some(),
"self.thread cannot be `None` unless `HistLog` was already dropped");
let thread = self.thread.as_ref().map(Arc::clone).unwrap();
let tx = self.tx.clone();
Self {
filename: self.filename.clone(),
series: self.series.clone(),
tag,
freq: self.freq,
last_sent: Instant::now(),
tx,
hist: self.hist.clone(),
thread: Some(thread),
}
}
#[doc(hidden)]
pub fn clone_with_tag_and_freq(&self, tag: Tag, freq: Duration) -> Self {
let mut clone = self.clone_with_tag(tag);
clone.freq = freq;
clone
}
/// Record a single value to the histogram. This could fail if the value
/// is outside of the highest range permitted. See the
/// [`hdrhistogram` docs](https://docs.rs/hdrhistogram/6.1.1/hdrhistogram/struct.Histogram.html#method.record)
/// for further deails. The `hdrhistogram::Histogram` used by `HistLog`
/// is created with a significant figure of 3 (`histlog::SIG_FIG` const).
///
#[inline]
pub fn record(&mut self, value: u64) -> Result<(), Error> {
self.hist.record(value).map_err(Error::HdrRecord)
}
/// From `hdrhistogram` docs:
///
/// > Record value in the histogram, clamped to the range of the histogram.
///
/// > This method cannot fail, as any values that are too small or too large to be
/// > tracked will automatically be clamed to be in range. Be aware that this will
/// > hide extreme outliers from the resulting histogram without warning. Since the
/// > values are clamped, the histogram will also not be resized to accomodate the
/// > value, even if auto-resize is enabled.
#[inline]
pub fn saturating_record(&mut self, value: u64) {
self.hist.saturating_record(value)
}
/// Reset the state of the internal histogram and the last sent value.
///
/// One situation this might be used is if there was a pause in recording.
///
pub fn reset(&mut self) {
self.hist.clear();
self.last_sent = Instant::now();
}
fn send(&mut self, loop_time: Instant) {
let end = SystemTime::now();
let start = end - (loop_time - self.last_sent);
assert!(end > start, "end <= start!");
let mut next = Histogram::new_from(&self.hist);
mem::swap(&mut self.hist, &mut next);
self.tx.send(Some(Entry { tag: self.tag.clone(), start, end, hist: next })).ok(); //.expect("sending entry failed");
self.last_sent = loop_time;
}
fn try_send(&mut self, loop_time: Instant) -> Result<(), Error>{
let end = SystemTime::now();
let start = end - (loop_time - self.last_sent);
assert!(end > start, "end <= start!");
let mut next = Histogram::new_from(&self.hist);
mem::swap(&mut self.hist, &mut next);
let entry = Entry { tag: self.tag.clone(), start, end, hist: next };
match self.tx.try_send(Some(entry)) {
Ok(_) => {
self.last_sent = loop_time;
Ok(())
}
Err(channel::TrySendError::Full(Some(Entry { mut hist, .. }))) => {
// recoverable, swap rejected hist back in place
// and continue trying...
mem::swap(&mut self.hist, &mut hist);
Err(Error::TrySend(channel::TrySendError::Full(())))
}
Err(channel::TrySendError::Disconnected(_)) => {
Err(Error::TrySend(channel::TrySendError::Disconnected(())))
}
Err(channel::TrySendError::Full(None)) => {
Err(Error::TrySend(channel::TrySendError::Full(())))
}
}
}
/// Send the current histogram to the writer thread if the elapsed time
/// since the last send is greater than the interval frequency.
///
/// If the channel is disconnected, this will fail silently, instead of panicking.
///
pub fn check_send(&mut self, loop_time: Instant) -> bool {
let expired = loop_time > self.last_sent && loop_time - self.last_sent >= self.freq;
if expired { self.send(loop_time); }
expired
}
/// Non-blocking variant of `HistLog::check_send`, which will also return any errors,
/// including a disconnected channel, encountered while trying to send to the
/// writer thread.
///
#[inline]
pub fn check_try_send(&mut self, loop_time: Instant) -> Result<bool, Error> {
let elapsed = loop_time.saturating_duration_since(self.last_sent);
let expired = elapsed >= self.freq;
if expired { self.try_send(loop_time)?; }
Ok(expired)
}
fn get_filename<S: AsRef<str>>(save_dir: &Path, series: S) -> PathBuf {
use rand::prelude::*;
let now = Utc::now();
let id: u32 = thread_rng().gen();
let series: &str = series.as_ref();
let filename =
format!("{series}.{time}.{id:x}.hlog",
series = series,
time = now.format("%Y-%m-%d-%H%M%SZ"));
save_dir.join(filename)
}
fn ensure_parent_dir_exists<P: AsRef<Path>>(path: P) -> Result<(), io::Error> {
let path: &Path = path.as_ref();
match path.parent() {
Some(parent) if parent.exists() => Ok(()),
Some(parent) => {
std::fs::create_dir_all(parent)?;
Ok(())
}
None => {
Err(io::Error::new(
io::ErrorKind::InvalidInput,
format!("parent path (of {}) is not a directory", path.display()),
))
}
}
}
fn scribe(
series: SeriesName,
rx: channel::Receiver<Option<Entry>>,
filename: &Path,
) -> Result<JoinHandle<Result<usize, Error>>, Error> {
let mut ser = V2DeflateSerializer::new();
let start_time = SystemTime::now();
Self::ensure_parent_dir_exists(filename)?;
let file = fs::File::create(filename).map_err(Error::Io)?;
thread::Builder::new().name(format!("histlog:{}", series)).spawn(move || {
let mut buf = io::LineWriter::new(file);
let mut wtr =
IntervalLogWriterBuilder::new()
.with_base_time(UNIX_EPOCH)
.with_start_time(start_time)
.begin_log_with(&mut buf, &mut ser)
.map_err(Error::Io)?; // unrecoverable, so exit early
let mut n_rcvd = 0;
loop {
match rx.recv() {
Ok(Some(Entry { tag, start, end, hist })) => {
// TODO: this currently fails silently
//
// improved implementation might include a logger so there
// is some record that it failed.
//
// alternatively, this could panic, so at least you know at
// the end it didn't work.
//
// the fact that `file` is created before the thread is spawned
// is mitigating, because typically if you can create the file,
// you can write to it, too.
//
#[cfg(feature = "smol_str")]
let tag: &str = tag.as_ref();
wtr.write_histogram(&hist, start.duration_since(UNIX_EPOCH).unwrap(),
end.duration_since(start).unwrap(), hdrhistogram::serialization::interval_log::Tag::new(tag))
.ok();
n_rcvd += 1;
}
Ok(None) => break, // terminate signal sent by `Drop`
_ => thread::sleep(Duration::new(0, 1)), // nothing new, yield thread
}
}
Ok(n_rcvd)
}).map_err(Error::Io)
}
}
impl Drop for HistLog {
/// Checks if the current instance is the last remaining instance with a reference
/// to the underlying writer thread, and, if so, sends a terminate signal to the
/// writer thread and attempts to join it.
///
/// # May Pause Up To 100ms
///
/// In the event the channel to the writer thread is full, will continue trying
/// to send a terminate command (busy polling the channel) until `DROP_DEADLINE`
/// has expired (currently 5ms), upon which it will abort.
///
/// If channel is disconnected, will simply abort without trying to join the
/// writer thread.
///
fn drop(&mut self) {
// don't remember why this was added now ... presumably to
// prepare the internal/queue state in some way.
if !self.hist.is_empty() { self.send(Instant::now()) }
if let Some(arc) = self.thread.take() {
if let Ok(thread) = Arc::try_unwrap(arc) {
let start = Instant::now();
while Instant::now() - start < DROP_DEADLINE {
match self.tx.try_send(None) {
Ok(_) => {
let _ = thread.join();
break
}
Err(channel::TrySendError::Full(_)) => {}
Err(_) => {
break
}
}
}
}
}
}
}
#[allow(unused)]
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn create_histlog_record_one_and_drop() {
let mut hist = HistLog::new("/tmp/histlog", "test", "red", Duration::from_millis(3)).unwrap();
for i in 0..1000u64 {
hist.record(i).unwrap();
}
assert_eq!(hist.check_send(Instant::now()), false);
assert!(hist.check_try_send(Instant::now()).is_ok());
assert_eq!(hist.check_try_send(Instant::now()).unwrap(), false);
thread::sleep(Duration::from_millis(3));
assert_eq!(hist.check_send(Instant::now()), true);
let path = hist.filename.clone();
drop(hist);
assert!(path.exists());
}
#[test]
fn clone_it() {
let mut hist = HistLog::new("/tmp/histlog", "test", "red", Duration::from_millis(1)).unwrap();
let tx = hist.tx.clone();
let mut a = hist.clone_with_tag("blue");
for i in 0..1000u64 {
hist.record(i).unwrap();
a.record(i * 2).unwrap();
}
drop(hist);
drop(a);
match tx.try_send(None) {
Err(channel::TrySendError::Disconnected(None)) => {},
other => panic!("unexpected variant: {:?}", other)
}
}
#[test]
fn generated_hlog_filenames_are_unique() {
let save_dir = Path::new("a/b/c");
let series = "d-e-f";
let filenames: Vec<PathBuf> = (0..1000)
.map(|_| HistLog::get_filename(&save_dir, &series))
.collect();
let unique = filenames.iter().cloned().collect::<std::collections::HashSet<_>>();
assert_eq!(filenames.len(), unique.len());
}
}