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performance.rst

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Rust's Performance

類別 狀況
Compiler rustc 1.3.0-nightly (69ca01256 2015-07-23)
Options -C opt-level-3

Interesting Topic

Iterator

已知數字做 filter

這份 code 可以在優化的時候事先算出 "499999999500000000" 後直接放入：

#![feature(iter_arith)]

fn main() {
    let result: u64 = (1..1000000000).sum();
    println!("{}", result);
}

但是這個卻不行，優化開下去之後還是需要經過 runtime 的計算：

#![feature(iter_arith)]

fn main() {
    let result: u64 = (1..1000000000).filter(|&x| x > 0).sum();
    println!("{}", result);
}

感覺應該要可以優化掉？

str::contains & pattern matching

#![feature(convert)]

use std::io::Read;

fn tokenize1(text: &str) -> Vec<&str> {
    let delims = [' ', ',', '.', '!', '?', ';', '\'', '"', ':', '\t', '\n', '(', ')', '-'];
    text.split(|c| {
            for &x in delims.iter() {
                if x == c {
                    return true;
                }
            }
            return false;
        })
        .filter(|data| !data.is_empty())
        .collect()
}

fn tokenize2(text: &str) -> Vec<&str> {
    text.split(|c| {
            match c {
                ' '  => true,
                ','  => true,
                '.'  => true,
                '!'  => true,
                '?'  => true,
                ';'  => true,
                '\'' => true,
                '"'  => true,
                ':'  => true,
                '\t' => true,
                '\n' => true,
                '('  => true,
                ')'  => true,
                '-'  => true,
                _    => false
            }
        })
        .filter(|data| !data.is_empty())
        .collect()
}

fn tokenize3(text: &str) -> Vec<&str> {
    let delims = " ,.!?;'\":\t\n()-";
    text.split(|c| str::contains(&delims, c))
        .filter(|data| !data.is_empty())
        .collect()
}

fn main() {
    let mut stdin = std::io::stdin();
    let mut tmp = String::new();
    stdin.read_to_string(&mut tmp);
    let result1 = tokenize1(tmp.as_str());
    let result2 = tokenize2(tmp.as_str());
    let result3 = tokenize3(tmp.as_str());
}

隨便生一個 73 MB 的檔案下去測， 結果發現 pattern matching 的版本最快， str::contains 的版本最慢， 覺得可以做優化 ...

function type speed
tokenize1 for loop 1.30 s
tokenize2 pattern matching 0.80 s
tokenize3 str::contains 2.45 s

Benchmarks Game