// ./src/std_misc/threads/testcase_mapreduce.md

use std::thread;

// This is the `main` thread
fn part0() {

    // This is our data to process.
    // We will calculate the sum of all digits via a threaded  map-reduce algorithm.
    // Each whitespace separated chunk will be handled in a different thread.
    //
    // TODO: see what happens to the output if you insert spaces!
    let data = "86967897737416471853297327050364959
11861322575564723963297542624962850
70856234701860851907960690014725639
38397966707106094172783238747669219
52380795257888236525459303330302837
58495327135744041048897885734297812
69920216438980873548808413720956532
16278424637452589860345374828574668";

    // Make a vector to hold the child-threads which we will spawn.
    let mut children = vec![];

    /*************************************************************************
     * "Map" phase
     *
     * Divide our data into segments, and apply initial processing
     ************************************************************************/

    // split our data into segments for individual calculation
    // each chunk will be a reference (&str) into the actual data
    let chunked_data = data.split_whitespace();

    // Iterate over the data segments.
    // .enumerate() adds the current loop index to whatever is iterated
    // the resulting tuple "(index, element)" is then immediately
    // "destructured" into two variables, "i" and "data_segment" with a
    // "destructuring assignment"
    for (i, data_segment) in chunked_data.enumerate() {
        println!("data segment {} is \"{}\"", i, data_segment);

        // Process each data segment in a separate thread
        //
        // spawn() returns a handle to the new thread,
        // which we MUST keep to access the returned value
        //
        // 'move || -> u32' is syntax for a closure that:
        // * takes no arguments ('||')
        // * takes ownership of its captured variables ('move') and
        // * returns an unsigned 32-bit integer ('-> u32')
        //
        // Rust is smart enough to infer the '-> u32' from
        // the closure itself so we could have left that out.
        //
        // TODO: try removing the 'move' and see what happens
        children.push(thread::spawn(move || -> u32 {
            // Calculate the intermediate sum of this segment:
            let result = data_segment
                        // iterate over the characters of our segment..
                        .chars()
                        // .. convert text-characters to their number value..
                        .map(|c| c.to_digit(10).expect("should be a digit"))
                        // .. and sum the resulting iterator of numbers
                        .sum();

            // println! locks stdout, so no text-interleaving occurs
            println!("processed segment {}, result={}", i, result);

            // "return" not needed, because Rust is an "expression language", the
            // last evaluated expression in each block is automatically its value.
            result

        }));
    }


    /*************************************************************************
     * "Reduce" phase
     *
     * Collect our intermediate results, and combine them into a final result
     ************************************************************************/

    // combine each thread's intermediate results into a single final sum.
    //
    // we use the "turbofish" ::<> to provide sum() with a type hint.
    //
    // TODO: try without the turbofish, by instead explicitly
    // specifying the type of final_result
    let final_result = children.into_iter().map(|c| c.join().unwrap()).sum::<u32>();

    println!("Final sum result: {}", final_result);
}


pub fn main() {
	part0();
}