/* author Bill Wood */

use rayon::prelude::*;
use std::io::Write;

fn main() {
    let mut args = std::env::args()
        .skip(1)
        .map(|a| a
            .split('_')
            .fold("".to_string(), |a, s| a + s)
            .parse::<usize>());

    if args.len() <= 3 {
        if let (Ok(mut low), Ok(mut high), Ok(seg_size_kb), ) = (
                args.next().unwrap_or(Ok(1_000_000_000)),
                args.next().unwrap_or(Ok(2_000_000_000)),
                args.next().unwrap_or(Ok(128)),) {
            println!("S15: Finding sexy primes with args {} {} {}", low, high, seg_size_kb, );

            low = if low < 3 { 3 } else { low | 1 };   // lowest odd >= low
            high = (high - 1) | 1;  // highest odd <= high

            // find sieving primes
            let sieve_primes = simple_sieve(high);

            // prepare for parallel segment sieve
            let mut candidates_per_seg = seg_size_kb*1024*2;
            while (high - low)/candidates_per_seg > 100_000_000 {   // increase seg size to reduce mem usage
                candidates_per_seg *= 2;
            }
            dbg!(candidates_per_seg/1024/2);
            let segs: Vec<(usize, usize)> = { low..=high }
                    .step_by(candidates_per_seg)
                    .map(|seg_low| {
                            let seg_high = seg_low + candidates_per_seg - 2;
                            (seg_low, if seg_high > high { high } else { seg_high } + 6)
                        })
                    .collect();

            // do parallel segment sieve
            let (sexy_count, nprimes) =
                segs.into_par_iter()
                    .map(|(low, high)| {
                        // sieve this segment
                        let sieve_seg = seg_sieve(low, high, &sieve_primes);
                        // find sexy primes in this segment
                        let (mut sexy_count, mut nprimes) = (0_usize, 0_usize);
                        for p in 0..sieve_seg.len() - 6/2 {
                            if unsafe { !sieve_seg.get_unchecked(p) } {
                                nprimes += 1;
                                if unsafe { !sieve_seg.get_unchecked(p + 6/2) } {
                                    sexy_count += 1;
                                }
                            }
                        }
                        (sexy_count, nprimes)
                    })
                    .reduce(|| (0, 0),
                            |a, b| (a.0 + b.0, a.1 + b.1));

            println!("{} of {} primes are sexy", sexy_count, nprimes);
            return;
        }
    }

    writeln!(std::io::stderr(), "Usage: sexy_primes [low [high [seg_size_kb]]]").unwrap();
    std::process::exit(1);
}

fn simple_sieve(mut high: usize) -> Vec<u32> {
    // x where x: odd and (x+2)^2 > high and x^2 < high
    high = (sqrt(high) - 1) | 1;
    let mut pvec = vec![false; (high + 1)/2];
    let mut primes = vec![];
    let n = high >> 1;
    let mut i = 3;
    while i*i <= high {
        if unsafe { !pvec.get_unchecked(i >> 1) } {
            primes.push(i as u32);
            let mut j = (i*i) >> 1;
            while j <= n {
                unsafe { *pvec.get_unchecked_mut(j) = true };
                j += i;
            }
        }
        i += 2;
    }
    i >>= 1;
    while i <= n {
        if unsafe { !pvec.get_unchecked(i) } {
            primes.push(((i << 1) | 1) as u32);
        }
        i += 1;
    }
    primes
}

fn seg_sieve(low: usize, high: usize, sieve_primes: &Vec<u32>) -> Vec<bool> {
    let n = (high + 2 - low) >> 1;
    let mut sieve_seg = vec![false; n];
    for i in 0..sieve_primes.len() {
        let p = unsafe { *sieve_primes.get_unchecked(i) as usize };
        let mut j = p*p;
        if j > high {
            break;
        }
        if j < low {
            j += (low - 1 - j + p*2)/2/p*p*2;
        }
        j = (j - low) >> 1;
        while j < n {
            unsafe { *sieve_seg.get_unchecked_mut(j) = true };
            j += p;
        }
    }
    sieve_seg
}

fn sqrt(n: usize) -> usize {
    let (mut r1, mut r) = (n, n + 1);
    while r1 < r {
        r = r1;
        r1 = (r1 + n/r1) >> 1
    }
    r
}