TL;博士
在某些情况下,功能实现可能比最初的程序实现更快.
为什么功能性风格比命令式风格慢得多?功能实现是否存在导致如此巨大减速的问题?
作为Matthieu M. already pointed out,需要注意的重要一点是algorithm很重要.算法的表达方式(过程性、命令性、面向对象、功能性、声明性)通常并不重要.
我发现功能代码有两个主要问题:
使用more个itertools(batching
take_while_ref
format_with
)使这两个实现更加接近:
pub fn encode_slim(data: &str) -> String {
data.chars()
.batching(|it| {
it.next()
.map(|v| (v, it.take_while_ref(|&v2| v2 == v).count() + 1))
})
.format_with("", |(c, count), f| match count {
1 => f(&c),
n => f(&format_args!("{}{}", n, c)),
})
.to_string()
}
4MiB随机字母数字数据的基准,用RUSTFLAGS='-C target-cpu=native'
:
encode (procedural) time: [21.082 ms 21.620 ms 22.211 ms]
encode (fast) time: [26.457 ms 27.104 ms 27.882 ms]
Found 7 outliers among 100 measurements (7.00%)
4 (4.00%) high mild
3 (3.00%) high severe
如果您对创建自己的迭代器感兴趣,可以将过程代码与更多功能代码混合匹配:
struct RunLength<I> {
iter: I,
saved: Option<char>,
}
impl<I> RunLength<I>
where
I: Iterator<Item = char>,
{
fn new(mut iter: I) -> Self {
let saved = iter.next(); // See footnote 1
Self { iter, saved }
}
}
impl<I> Iterator for RunLength<I>
where
I: Iterator<Item = char>,
{
type Item = (char, usize);
fn next(&mut self) -> Option<Self::Item> {
let c = self.saved.take().or_else(|| self.iter.next())?;
let mut count = 1;
while let Some(n) = self.iter.next() {
if n == c {
count += 1
} else {
self.saved = Some(n);
break;
}
}
Some((c, count))
}
}
pub fn encode_tiny(data: &str) -> String {
use std::fmt::Write;
RunLength::new(data.chars()).fold(String::new(), |mut s, (c, count)| {
match count {
1 => s.push(c),
n => write!(&mut s, "{}{}", n, c).unwrap(),
}
s
})
}
1-感谢Stargateur for pointing out,Eager 地获得第一个值有助于分支预测.
4MiB随机字母数字数据的基准,用RUSTFLAGS='-C target-cpu=native'
:
encode (procedural) time: [19.888 ms 20.301 ms 20.794 ms]
Found 4 outliers among 100 measurements (4.00%)
3 (3.00%) high mild
1 (1.00%) high severe
encode (tiny) time: [19.150 ms 19.262 ms 19.399 ms]
Found 11 outliers among 100 measurements (11.00%)
5 (5.00%) high mild
6 (6.00%) high severe
我认为这更清楚地显示了这两种实现之间的主要区别:基于迭代器的解决方案是resumable.每次我们拨打next
,我们都需要看看是否有之前读过的字符(self.saved
).这会向程序代码中没有的代码添加一个分支.
另一方面,基于迭代器的解决方案更灵活——我们现在可以对数据进行各种转换,或者直接写入文件而不是String
,等等.自定义迭代器可以扩展为对泛型类型而不是char
进行操作,从而使其更灵活.
另见:
如果我想写高性能代码,我应该使用这种函数式风格吗?
我会的,直到基准测试显示这是瓶颈.然后判断why,这是瓶颈.
支持代码
总是要展示你的作品,对吗?
benchmark.rs
use criterion::{criterion_group, criterion_main, Criterion}; // 0.2.11
use rle::*;
fn criterion_benchmark(c: &mut Criterion) {
let data = rand_data(4 * 1024 * 1024);
c.bench_function("encode (procedural)", {
let data = data.clone();
move |b| b.iter(|| encode_proc(&data))
});
c.bench_function("encode (functional)", {
let data = data.clone();
move |b| b.iter(|| encode_iter(&data))
});
c.bench_function("encode (fast)", {
let data = data.clone();
move |b| b.iter(|| encode_slim(&data))
});
c.bench_function("encode (tiny)", {
let data = data.clone();
move |b| b.iter(|| encode_tiny(&data))
});
}
criterion_group!(benches, criterion_benchmark);
criterion_main!(benches);
lib.rs
use itertools::Itertools; // 0.8.0
use rand; // 0.6.5
pub fn rand_data(len: usize) -> String {
use rand::distributions::{Alphanumeric, Distribution};
let mut rng = rand::thread_rng();
Alphanumeric.sample_iter(&mut rng).take(len).collect()
}
pub fn encode_proc(source: &str) -> String {
let mut retval = String::new();
let firstchar = source.chars().next();
let mut currentchar = match firstchar {
Some(x) => x,
None => return retval,
};
let mut currentcharcount: u32 = 0;
for c in source.chars() {
if c == currentchar {
currentcharcount += 1;
} else {
if currentcharcount > 1 {
retval.push_str(¤tcharcount.to_string());
}
retval.push(currentchar);
currentchar = c;
currentcharcount = 1;
}
}
if currentcharcount > 1 {
retval.push_str(¤tcharcount.to_string());
}
retval.push(currentchar);
retval
}
pub fn encode_iter(data: &str) -> String {
data.chars()
.group_by(|&c| c)
.into_iter()
.map(|(c, group)| match group.count() {
1 => c.to_string(),
n => format!("{}{}", n, c),
})
.collect()
}
pub fn encode_slim(data: &str) -> String {
data.chars()
.batching(|it| {
it.next()
.map(|v| (v, it.take_while_ref(|&v2| v2 == v).count() + 1))
})
.format_with("", |(c, count), f| match count {
1 => f(&c),
n => f(&format_args!("{}{}", n, c)),
})
.to_string()
}
struct RunLength<I> {
iter: I,
saved: Option<char>,
}
impl<I> RunLength<I>
where
I: Iterator<Item = char>,
{
fn new(mut iter: I) -> Self {
let saved = iter.next();
Self { iter, saved }
}
}
impl<I> Iterator for RunLength<I>
where
I: Iterator<Item = char>,
{
type Item = (char, usize);
fn next(&mut self) -> Option<Self::Item> {
let c = self.saved.take().or_else(|| self.iter.next())?;
let mut count = 1;
while let Some(n) = self.iter.next() {
if n == c {
count += 1
} else {
self.saved = Some(n);
break;
}
}
Some((c, count))
}
}
pub fn encode_tiny(data: &str) -> String {
use std::fmt::Write;
RunLength::new(data.chars()).fold(String::new(), |mut s, (c, count)| {
match count {
1 => s.push(c),
n => write!(&mut s, "{}{}", n, c).unwrap(),
}
s
})
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn all_the_same() {
let data = rand_data(1024);
let a = encode_proc(&data);
let b = encode_iter(&data);
let c = encode_slim(&data);
let d = encode_tiny(&data);
assert_eq!(a, b);
assert_eq!(a, c);
assert_eq!(a, d);
}
}