Commits on Jul 23, 2020

1. Rearrange the pipeline of pow to gain efficiency …

   The check of the `exp` parameter seems useless if we execute the while-loop more than once.
   The original implementation of `pow` function using one more comparison if the `exp==0` and may break the pipeline of the cpu, which may generate a slower code.
   The performance gap between the old and the new implementation may be small, but IMO, at least the newer one looks more beautiful.
   
   ---
   
   bench prog:
   ```
   extern crate test;
   ($a:expr)=>{let time=std::time::Instant::now();{$a;}print!("{:?} ",time.elapsed())};
   ($a:expr,$b:literal)=>{let time=std::time::Instant::now();let mut a=0;for _ in 0..$b{a^=$a;}print!("{:?} {} ",time.elapsed(),a)}
   }
   pub fn pow_rust(x:i64, mut exp: u32) -> i64 {
       let mut base = x;
       let mut acc = 1;
       while exp > 1 {
           if (exp & 1) == 1 {
               acc = acc * base;
           }
           exp /= 2;
           base = base * base;
       }
       if exp == 1 {
           acc = acc * base;
       }
       acc
   }
   pub fn pow_new(x:i64, mut exp: u32) -> i64 {
       if exp==0{
           1
       }else{
           let mut base = x;
           let mut acc = 1;
           while exp > 1 {
               if (exp & 1) == 1 {
                   acc = acc * base;
               }
               exp >>= 1;
               base = base * base;
           }
           acc * base
       }
   }
   
   fn main(){
   let a=2i64;
   let b=1_u32;
   println!();
   timing!(test::black_box(a).pow(test::black_box(b)),100000000);
   timing!(pow_new(test::black_box(a),test::black_box(b)),100000000);
   timing!(pow_rust(test::black_box(a),test::black_box(b)),100000000);
   println!();
   timing!(test::black_box(a).pow(test::black_box(b)),100000000);
   timing!(pow_new(test::black_box(a),test::black_box(b)),100000000);
   timing!(pow_rust(test::black_box(a),test::black_box(b)),100000000);
   println!();
   timing!(test::black_box(a).pow(test::black_box(b)),100000000);
   timing!(pow_new(test::black_box(a),test::black_box(b)),100000000);
   timing!(pow_rust(test::black_box(a),test::black_box(b)),100000000);
   println!();
   timing!(test::black_box(a).pow(test::black_box(b)),100000000);
   timing!(pow_new(test::black_box(a),test::black_box(b)),100000000);
   timing!(pow_rust(test::black_box(a),test::black_box(b)),100000000);
   println!();
   timing!(test::black_box(a).pow(test::black_box(b)),100000000);
   timing!(pow_new(test::black_box(a),test::black_box(b)),100000000);
   timing!(pow_rust(test::black_box(a),test::black_box(b)),100000000);
   println!();
   timing!(test::black_box(a).pow(test::black_box(b)),100000000);
   timing!(pow_new(test::black_box(a),test::black_box(b)),100000000);
   timing!(pow_rust(test::black_box(a),test::black_box(b)),100000000);
   println!();
   timing!(test::black_box(a).pow(test::black_box(b)),100000000);
   timing!(pow_new(test::black_box(a),test::black_box(b)),100000000);
   timing!(pow_rust(test::black_box(a),test::black_box(b)),100000000);
   println!();
   timing!(test::black_box(a).pow(test::black_box(b)),100000000);
   timing!(pow_new(test::black_box(a),test::black_box(b)),100000000);
   timing!(pow_rust(test::black_box(a),test::black_box(b)),100000000);
   println!();
   }
   ```
   bench in my laptop:
   ```
   neutron@Neutron:/me/rust$ rc commit.rs
   rustc commit.rs  && ./commit
   
   3.978419716s 0 4.079765171s 0 3.964630622s 0
   3.997127013s 0 4.260304804s 0 3.997638211s 0
   3.963195544s 0 4.11657718s 0 4.176054164s 0
   3.830128579s 0 3.980396122s 0 3.937258567s 0
   3.986055948s 0 4.127804162s 0 4.018943411s 0
   4.185568857s 0 4.217512517s 0 3.98313603s 0
   3.863018225s 0 4.030447988s 0 3.694878237s 0
   4.206987927s 0 4.137608047s 0 4.115564664s 0
   neutron@Neutron:/me/rust$ rc commit.rs -O
   rustc commit.rs -O && ./commit
   
   162.111993ms 0 165.107125ms 0 166.26924ms 0
   175.20479ms 0 205.062565ms 0 176.278791ms 0
   174.408975ms 0 166.526899ms 0 201.857604ms 0
   146.190062ms 0 168.592821ms 0 154.61411ms 0
   199.678912ms 0 168.411598ms 0 162.129996ms 0
   147.420765ms 0 209.759326ms 0 154.807907ms 0
   165.507134ms 0 188.476239ms 0 157.351524ms 0
   121.320123ms 0 126.401229ms 0 114.86428ms 0
   ```
   
   delete an unnecessary semicolon...
   
   Sorry for the typo.
   
   delete trailing whitespace
   
   Sorry, too..
   
   Sorry for the missing...
   
   I checked all the implementations, and finally found that there is one function that does not check whether `exp == 0`
   
   add extra tests
   
   add extra tests.
   
   finished adding the extra tests to prevent further typo
   
   add pow(2) to negative exp
   
   add whitespace.
   
   add whitespace
   
   add whitespace
   
   delete extra line

   

   Neutron3529 authored and Neutron3529 committed Jul 23, 2020
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