fannkuch-redux Rust #4 program

// The Computer Language Benchmarks Game
// https://salsa.debian.org/benchmarksgame-team/benchmarksgame/
//
// contributed by the Rust Project Developers
// contributed by TeXitoi
// contributed by Cristi Cobzarenco

extern crate rayon;

use std::{cmp, mem};
use rayon::prelude::*;

// This value controls the preferred maximum number of  blocks the workload is
// broken up into. The actual value may be one higher (if the number of
// permutations doesn't divide exactly by this value) or might be set to 1 if
// the number of permutations is lower than this value.
const NUM_BLOCKS: u32 = 24;

fn fannkuch(n: i32) -> (i32, i32) {
    // Precompute a table a factorials to reuse all over the place.
    let mut factorials = [1; 16];
    for i in 1..n as usize + 1 {
        factorials[i] = factorials[i - 1] * i as u32;
    }
    let perm_max = factorials[n as usize];

    // Compute the number of blocks and their size. If n! is less than
    // NUM_BLOCKS then use a single block (perform the work serially for small
    // values of n). If n! doesn't divide exactly by NUM_BLOCKS, then add one
    // extra block to compute the remainder.
    let (num_blocks, block_size) = if perm_max < NUM_BLOCKS {
        (1, perm_max)
    } else {
        (NUM_BLOCKS + if perm_max % NUM_BLOCKS == 0 { 0 } else { 1 },
         perm_max / NUM_BLOCKS)
    };

    // Compute the `checksum` and `maxflips` for each block in parallel.
    (0..num_blocks).into_par_iter().map(|i_block| {
        let initial = i_block * block_size;
        let mut count = [0i32; 16];
        let mut temp = [0i32; 16];
        let mut current = [0i32; 16];

        // Initialise `count` and the current permutation (`current`)
        for (i, value) in current.iter_mut().enumerate() {
            *value = i as i32;
        }

        let mut permutation_index = initial as i32;
        for i in (1..n as usize).rev() {
            let factorial = factorials[i] as i32;
            let d = permutation_index / factorial;
            permutation_index %= factorial;
            count[i] = d;

            temp.copy_from_slice(&current);
            let d = d as usize;
            for j in 0..i + 1 {
                current[j] = if j + d <= i {
                    temp[j + d]
                } else {
                    temp[j + d - i - 1]
                };
            }
        }

        // Iterate over each permutation in the block.
        let last_permutation_in_block = cmp::min(initial + block_size,
                                                 perm_max) - 1;
        let mut permutation_index = initial;
        let (mut checksum, mut maxflips) = (0, 0);
        loop {
            // If the first value in the current permutation is not 1 (0) then
            // we will need to do at least one flip for `current`.
            if current[0] > 0 {
                // Copy the current permutation to work on it.
                temp.copy_from_slice(&current);

                // Flip `temp` (the copy of the current permutation) until its
                // first element is 1 (0).
                let mut flip_count = 1;
                let mut first_value = current[0] as usize;
                while temp[first_value] != 0 {
                    let new_first_value = mem::replace(&mut temp[first_value],
                                                       first_value as i32);
                    // If the first value is greater than 3 (2), then we are
                    // flipping a series of four or more values so we will need
                    // to flip additional elements in the middle of `temp`.
                    if first_value > 2 {
                        temp[1..first_value].reverse();
                    }

                    // Update `first_value` to the value we saved earlier and
                    // record a flip in `flip_count`.
                    first_value = new_first_value as usize;
                    flip_count += 1;
                }

                // Update the `checksum` and `maxflips` of this block.
                checksum += if permutation_index % 2 == 0 {
                    flip_count
                } else {
                    -flip_count
                };
                maxflips = cmp::max(maxflips, flip_count);
            }

            // If this was the last permutation in the block, we're done: return
            // the `checksum` and `maxflips` values which get reduced across
            // blocks in parallel by `rayon`.
            if permutation_index >= last_permutation_in_block {
                return (checksum, maxflips);
            }
            permutation_index += 1;

            // Generate the next permutation.
            let mut first_value = current[1];
            current[1] = current[0];
            current[0] = first_value;
            let mut i = 1;
            while count[i] >= i as i32 {
                count[i] = 0;
                i += 1;
                let new_first_value = current[1];
                current[0] = new_first_value;
                for j in 1..i {
                    current[j] = current[j + 1];
                }
                current[i] = mem::replace(&mut first_value, new_first_value);
            }
            count[i] += 1;
        }
    }).reduce(|| (0, 0),
              |(cs1, mf1), (cs2, mf2)| (cs1 + cs2, cmp::max(mf1, mf2)))
}

fn main() {
    let n = std::env::args().nth(1)
        .and_then(|n| n.parse().ok())
        .unwrap_or(7);

    let (checksum, maxflips) = fannkuch(n);
    println!("{}\nPfannkuchen({}) = {}", checksum, n, maxflips);
}
    

NOTES:
64-bit Ubuntu quad core
rustc 1.44.0 (49cae5576 2020-06-01)
LLVM version: 9.0


Fri, 05 Jun 2020 20:08:54 GMT

MAKE:
/opt/src/rust-1.44.0/bin/rustc -C opt-level=3 -C target-cpu=core2 -C lto -C codegen-units=1 -L /opt/src/rust-libs -C llvm-args='-unroll-threshold=500' fannkuchredux.rs -o fannkuchredux.rust-4.rust_run

18.28s to complete and log all make actions

COMMAND LINE:
./fannkuchredux.rust-4.rust_run 12

PROGRAM OUTPUT:
3968050
Pfannkuchen(12) = 65