# L-13 MCS 572 Wed 8 Feb 2023 : mergesortmt.jl

# Example from https://julialang.org/blog/2019/07/multithreading
 
# At the command line, type
# for n in 1 2 4 8; do JULIA_NUM_THREADS=$n julia mergesortmt.jl; done
# to run with 1, 2, 4, and 8 threads.

import Base.Threads.@spawn

# sort the elements of `v` in place, from indices `lo` to `hi` inclusive

"""
Sorts the elements of v in place, from hi to lo.
"""
function psort!(v, lo::Int=1, hi::Int=length(v))
    if lo >= hi
        return v
    end
    if hi - lo < 100000            # no multithreading
        sort!(view(v, lo:hi), alg = MergeSort)
        return v
    end

    mid = (lo+hi)>>>1              # find the midpoint

    # task to sort the first half starts
    half = @spawn psort!(v, lo, mid)
    # runs with the current call below
    psort!(v, mid+1, hi)
    # wait for the lower half to finish
    wait(half)

    temp = v[lo:mid] # workspace for merging
    i, k, j = 1, lo, mid+1 # merge the two sorted sub-arrays

    @inbounds while k < j <= hi 
         if v[j] < temp[i] 
             v[k] = v[j]
             j += 1
         else
             v[k] = temp[i]
             i += 1
         end
         k += 1
    end
    @inbounds while k < j
         v[i] = temp[i]
         k += 1
         i += 1
    end

    return v
end

"""
Calls the psort! once 
to avoid compilation overhead.
"""
function main()
    a = rand(100)
    b = copy(a)
    psort!(b)
    a = rand(20000000)
    b = copy(a)
    @time psort!(b)
end

main()