/*
* threadpi.rs
* Neil Gershenfeld 12/22/24
* Rust threads parallel pi calculation benchmark
* pi = 3.14159265358979323846
*/

use std::thread;
use std::env;
use std::process;
use std::time::SystemTime;

const NPTS:u64 = 1e9 as u64;

fn main() {
   let args:Vec<String> = env::args().collect();
   if args.len() != 2 {
      println!("command line: threadpi.rs number_of_threads");
      process::exit(-1);
      }
   let num_threads:u64 = args[1].parse().unwrap();
   let a:f64 = 0.5;
   let b:f64 = 0.75;
   let c:f64 = 0.25;
   let start = SystemTime::now().duration_since(SystemTime::UNIX_EPOCH).unwrap().as_micros();
   let mut handles = Vec::new();
   for i in 0..num_threads {
      handles.push(
         thread::spawn(move || {
            let start:u64 = 1+NPTS*i;
            let end:u64 = NPTS*(i+1);
            let mut partial_pi:f64 =0.0;
            for i in start..=end {
               partial_pi += a/(((i as f64)-b)*((i as f64)-c));
               }
            return partial_pi;
            }));
         }
   let mut pi:f64 = 0.0;
   for handle in handles {
      pi += handle.join().unwrap();
      }
   let end = SystemTime::now().duration_since(SystemTime::UNIX_EPOCH).unwrap().as_micros();
   let dt = ((end-start) as f64)/1e6;
   let mflops = (num_threads as f64)*(NPTS as f64)*5.0/((dt as f64)*1e6);
   println!("NPTS = {NPTS}, threads = {num_threads}, pi = {pi}");
   println!("time = {dt:.3}, estimated MFlops = {mflops:.0}");
   }